steffen

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Author name code: steffen
ADS astronomy entries on 2022-09-14
author:"Steffen, Matthias"

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Title: Barium lines in high-quality spectra of two metal-poor giants
in the Galactic halo
Authors: Cescutti, G.; Morossi, C.; Franchini, M.; Di Marcantonio, P.;
Chiappini, C.; Steffen, M.; Valentini, M.; François, P.; Christlieb,
N.; Cortés, C.; Kobayashi, C.; Depagne, E.
2021A&A...654A.164C Altcode: 2021arXiv210900277C
Context. Theoretical results showed the possibility that neutron capture
elements were produced in the early Universe by two different sources:
a frequent s-process source hosted by rotating massive stars, and a
rare r-process source hosted most likely by neutron star mergers. The
two sources produce barium with different isotopic compositions. Aims: We aim to investigate the lines of barium in two halo stars,
HD 6268 and HD 4306. The spectra present an exquisite quality, both in
terms of resolution (R &gt; 100 000) and signal-to-noise (~400). Due
to hyperfine splitting (hfs) effects, barium lines are expected to
show slightly different profiles depending on the barium isotopic
fraction. Methods: We applied a standard local thermodynamic
equilibrium synthesis of the barium lines. We compared the synthetic
results assuming an s-process isotopic pattern or an r-process isotopic
pattern for the two barium lines for each star that exhibited hfs. We
also applied a methodology, less dependent on the accuracy of the
theoretical Ba hfs structure, that transforms the lines of HD 4306
into those we would observe if its atmospheric parameter values
(i.e. Teff, log g, micro- and macro-turbulence, V sin i,
and Ba abundance) were the same as those of HD 6268. Results:
With both methods, our results show that the barium lines with hfs
effects of HD 4306 are in agreement with an s-process composition
and the lines in HD 6268 have a different profile, which is most
likely linked to the presence of an r-process isotopic pattern. Conclusions: Two lines of barium of HD 6268 and HD 4306 seem to
confirm the theoretical expectation that both r-process events and
also s-process contribution by rotating massive stars have polluted
the ancient halo of our Galaxy. Based on observations made with
the UVES at the ESO Very Large Telescope, Paranal Observatory, Chile
(ID 098.B-0094(A); P.I. G. Cescutti).

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Title: The metal-poor end of the Spite plateau. II. Chemical and
dynamical investigation
Authors: Matas Pinto, A. M.; Spite, M.; Caffau, E.; Bonifacio, P.;
Sbordone, L.; Sivarani, T.; Steffen, M.; Spite, F.; François, P.;
Di Matteo, P.
2021A&A...654A.170M Altcode: 2021arXiv211000243M
Context. The study of old, metal-poor stars deepens our knowledge on
the early stages of the universe. In particular, the study of these
stars gives us a valuable insight into the masses of the first massive
stars and their emission of ionising photons. Aims: We present
a detailed chemical analysis and determination of the kinematic and
orbital properties of a sample of 11 dwarf stars. These are metal-poor
stars, and a few of them present a low lithium content. We inspected
whether the other elements also present anomalies. Methods:
We analysed the high-resolution UVES spectra of a few metal-poor stars
using the Turbospectrum code to synthesise spectral lines profiles. This
allowed us to derive a detailed chemical analysis of Fe, C, Li, Na,
Mg, Al, Si, CaI, CaII, ScII, TiII, Cr, Mn, Co, Ni, Sr, and Ba. 
Results: We find excellent coherence with the reference metal-poor First
Stars sample. The lithium-poor stars do not present any anomaly of the
abundance of the elements other than lithium. Among the Li-poor stars,
we show that CS 22882-027 is very probably a blue-straggler. The star
CS 30302-145, which has a Li abundance compatible with the plateau,
has a very low Si abundance and a high Mn abundance. In many aspects,
it is similar to the α-poor star HE 1424-0241, but it is less
extreme. It could have been formed in a satellite galaxy and later
been accreted by our Galaxy. This hypothesis is also supported by
its kinematics. The table with equivalent widths discussed in
this paper is only available at the CDS via anonymous ftp to <A
href="http://cdsarc.u-strasbg.fr/">cdsarc.u-strasbg.fr</A>
(ftp://130.79.128.5) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/654/A170">http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/654/A170</A>
Based on observations collected at the European Organisation for
Astronomical Research in the Southern Hemisphere (Programmes 076.A-0463
PI(Lopez), 077.D-0299 PI(Bonifacio)), 086.D-0871(A) (PI Meléndez).

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Title: VizieR Online Data Catalog: Abundances of metal-poor stars
(Matas Pinto+, 2021)
Authors: Matas Pinto, A. M.; Spite, M.; Caffau, E.; Bonifacio, P.;
Sbordone, L.; Sivarani, T.; Steffen, M.; Spite, F.; Francois, P.;
Di Matteo, P.
2021yCat..36540170M Altcode:
The observations of the stars we studied are described in detail in
Paper I (Sbordone et al., 2010A&amp;A...522A..26S, Cat. J/A+A/522/A26)
(see their Table 1). Briefly, observations were performed with
the high-resolution spectrograph UVES at the ESO-VLT. The spectra have
a resolving power R=~40000 and were centred at 390nm (spectral range:
330-451nm) and 580nm (spectral range: 479-680nm). For two stars (BS
17572-100 and HE 1413-1954) that were previously studied in the frame
of the HERES program (Christlieb et al., 2004A&amp;A...428.1027C;
Barklem et al., 2005A&amp;A...439..129B, Cat. J/A+A/439/129) from
UVES spectra centred at 437nm (spectral range: 376-497nm), the blue
spectra were centred at 346 nm (spectral range: 320-386nm). The S/N of
the spectra at 400nm is only about half of the S/N measured at 670nm
(see Table 1 in Paper I) and thus generally does not exceed 50. For
two stars, CS 22188-033 and HE 0148-2611, new UVES spectra from the
ESO archives, centred at 390 and 580nm, were also used, increasing
the S/N ratio of the mean spectrum. The data were reduced using the
standard UVES pipeline with the same procedures as used in Bonifacio
et al. (2007A&amp;A...462..851B). Here we present the table with
equivalent widths discussed in the paper. (2 data files).

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Title: The Gaia RVS benchmark stars. I. Chemical inventory of the
first sample of evolved stars and its Rb NLTE investigation
Authors: Caffau, E.; Bonifacio, P.; Korotin, S. A.; François,
P.; Lallement, R.; Matas Pinto, A. M.; Di Matteo, P.; Steffen, M.;
Mucciarelli, A.; Katz, D.; Haywood, M.; Chemin, L.; Sartoretti, P.;
Sbordone, L.; Andrievsky, S. M.; Kovtyukh, V. V.; Spite, M.; Spite,
F.; Panuzzo, P.; Royer, F.; Thévenin, F.; Ludwig, H. -G.; Marchal,
O.; Plum, G.
2021A&A...651A..20C Altcode:
Context. The Radial Velocity Spectrometer (RVS) on board the Gaia
satellite is not provided with a wavelength calibration lamp. It uses
its observations of stars with known radial velocity to derive the
dispersion relation. To derive an accurate radial velocity calibration,
a precise knowledge of the line spread function (LSF) of the RVS is
necessary. Good-quality ground-based observations in the wavelength
range of the RVS are highly desired to determine the LSF. Aims:
Several radial velocity standard stars are available to the Gaia
community. The highest possible number of calibrators will surely
allow us to improve the accuracy of the radial velocity. Because
the LSF may vary across the focal plane of the RVS, a large number
of high-quality spectra for the LSF calibration may allow us to
better sample the properties of the focal plane. Methods:
We selected a sample of stars to be observed with UVES at the Very
Large Telescope, in a setting including the wavelength range of RVS,
that are bright enough to allow obtaining high-quality spectra in a
short time. We also selected stars that lack chemical investigation in
order to increase the sample of bright, close by stars with a complete
chemical inventory. Results: We here present the chemical
analysis of the first sample of 80 evolved stars. The quality of the
spectra is very good, therefore we were able to derive abundances for
20 elements. The metallicity range spanned by the sample is about 1
dex, from slightly metal-poor to solar metallicity. We derived the
Rb abundance for all stars and investigated departures from local
thermodynamical equilibrium (NLTE) in the formation of its lines. 
Conclusions: The sample of spectra is of good quality, which is useful
for a Gaia radial velocity calibration. The Rb NLTE effects in this
stellar parameters range are small but sometimes non-negligible,
especially for spectra of this good quality. Tables B.3
and C.1 are only available at the CDS via anonymous ftp to <A
href="http://cdsarc.u-strasbg.fr">cdsarc.u-strasbg.fr</A>
(ftp://130.79.128.5) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/651/A20">http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/651/A20</A>
Based on observations made with UVES at VLT 104.D.0325.

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Title: VizieR Online Data Catalog: Gaia RVS benchmark
stars. I. (Caffau+, 2021)
Authors: Caffau, E.; Bonifacio, P.; Korotin, S. A.; Francois, P.;
Lallement, R.; Matas Pinto, A. M.; Di Matteo, P.; Steffen, M.;
Mucciarelli, A.; Katz, D.; Haywood, M.; Chemin, L.; Sartoretti, P.;
Sbordone, L.; Andrievsky, S. M.; Kovtyukh, V. V.; Spite, M.; Spite,
F.; Panuzzo, P.; Royer, F.; Thevenin, F.; Ludwig, H. -G.; Marchal,
O.; Plum, G.
2021yCat..36510020C Altcode:
For this project on the UVES spectrograph, we selected the setting
437+760. The choices on the setting were that (i) the 760 range
completely covers the RVS range without any gaps, and (ii) the 437 range
is the reddest setting that can be coupled with the 760 setting. For
metal-rich stars (the majority of our targets), observations in
blue settings provide very crowded spectra, and a higher S/N can be
achieved in this selected setting than in bluer settings such as the
390 setting. We chose the highest UVES resolution (slit 0.4"
in the blue arm and 0.3" in the red arm). For all observations, the
DIC2 437+760 setting was used. For the stars brighter than V magnitude
8.5, an observing block comprises ten observations of 77.5s to avoid
detector saturation. For the stars fainter than 8.5, five exposures of
202 s allow avoiding detector saturation. In this program, 90 stars
have been observed, 80 of which are evolved stars and are analysed
here. The 10 unevolved stars will be analysed with stars of similar
stellar parameters that are observed or are scheduled to be observed
for the following two ESO periods (P105 and P106). (2 data files).

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Title: 3D spectroscopic analysis of helium-line white dwarfs
Authors: Cukanovaite, Elena; Tremblay, Pier-Emmanuel; Bergeron,
Pierre; Freytag, Bernd; Ludwig, Hans-Günter; Steffen, Matthias
2021MNRAS.501.5274C Altcode: 2020arXiv201112693C; 2020MNRAS.tmp.3465C
In this paper, we present corrections to the spectroscopic parameters
of DB and DBA white dwarfs with -10.0 ≤ log (H/He) ≤ -2.0, 7.5
≤ log g ≤ 9.0, and $12\, 000$ ≲ Teff $\lesssim 34\,
000\,\mathrm{ K}$ , based on 282 3D atmospheric models calculated with
the CO5BOLD radiation-hydrodynamics code. These corrections
arise due to a better physical treatment of convective energy transport
in 3D models when compared to the previously available 1D model
atmospheres. By applying the corrections to an existing Sloan Digital
Sky Survey (SDSS) sample of DB and DBA white dwarfs, we find significant
corrections both for effective temperature and surface gravity. The
3D log g corrections are most significant for Teff ≲
18 000 K, reaching up to -0.20 dex at log g = 8.0. However, in this
low effective temperature range, the surface gravity determined from
the spectroscopic technique can also be significantly affected by
the treatment of the neutral van der Waals line broadening of helium
and by non-ideal effects due to the perturbation of helium by neutral
atoms. Thus, by removing uncertainties due to 1D convection, our work
showcases the need for improved description of microphysics for DB
and DBA model atmospheres. Overall, we find that our 3D spectroscopic
parameters for the SDSS sample are generally in agreement with Gaia
Data Release 2 absolute fluxes within 1σ-3σ for individual white
dwarfs. By comparing our results to DA white dwarfs, we determine
that the precision and accuracy of DB/DBA atmospheric models are
similar. For ease of user application of the correction functions,
we provide an example PYTHON code.

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Title: The solar gravitational redshift from HARPS-LFC Moon
spectra⋆. A test of the general theory of relativity
Authors: González Hernández, J. I.; Rebolo, R.; Pasquini, L.;
Lo Curto, G.; Molaro, P.; Caffau, E.; Ludwig, H. -G.; Steffen, M.;
Esposito, M.; Suárez Mascareño, A.; Toledo-Padrón, B.; Probst,
R. A.; Hänsch, T. W.; Holzwarth, R.; Manescau, A.; Steinmetz, T.;
Udem, Th.; Wilken, T.
2020A&A...643A.146G Altcode: 2020arXiv200910558G
Context. The general theory of relativity predicts the redshift of
spectral lines in the solar photosphere as a consequence of the
gravitational potential of the Sun. This effect can be measured
from a solar disk-integrated flux spectrum of the Sun's reflected
light on Solar System bodies. Aims: The laser frequency comb
(LFC) calibration system attached to the HARPS spectrograph offers
the possibility of performing an accurate measurement of the solar
gravitational redshift (GRS) by observing the Moon or other Solar System
bodies. Here, we analyse the line shift observed in Fe absorption lines
from five high-quality HARPS-LFC spectra of the Moon. Methods:
We selected an initial sample of 326 photospheric Fe lines in the
spectral range between 476-585 nm and measured their line positions
and equivalent widths (EWs). Accurate line shifts were derived from
the wavelength position of the core of the lines compared with the
laboratory wavelengths of Fe lines. We also used a CO5BOLD
3D hydrodynamical model atmosphere of the Sun to compute 3D synthetic
line profiles of a subsample of about 200 spectral Fe lines centred
at their laboratory wavelengths. We fit the observed relatively
weak spectral Fe lines (with EW&lt; 180 mÅ) with the 3D synthetic
profiles. Results: Convective motions in the solar photosphere
do not affect the line cores of Fe lines stronger than about ∼150
mÅ. In our sample, only 15 Fe I lines have EWs in the range 150&lt;
EW(mÅ) &lt; 550, providing a measurement of the solar GRS at 639 ±
14 m s-1, which is consistent with the expected theoretical
value on Earth of ∼633.1 m s-1. A final sample of about 97
weak Fe lines with EW &lt; 180 mÅ allows us to derive a mean global
line shift of 638 ± 6 m s-1, which is in agreement with
the theoretical solar GRS. Conclusions: These are the most
accurate measurements of the solar GRS obtained thus far. Ultrastable
spectrographs calibrated with the LFC over a larger spectral range,
such as HARPS or ESPRESSO, together with a further improvement on the
laboratory wavelengths, could provide a more robust measurement of the
solar GRS and further testing of 3D hydrodynamical models. Tables
A.1 and A.2 are only available at the CDS via anonymous ftp to <A
href="http://cdsarc.u-strasbg.fr/">http://cdsarc.u-strasbg.fr</A>
(ftp://130.79.128.5) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/643/A146">http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/643/A146</A>
Based on observations taken with the ESO 3.6 m telescope at La
Silla Observatory, Chile.

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Title: Probing the atmosphere of HD189733b with the Na I and K I lines
Authors: Keles, E.; Kitzmann, D.; Mallonn, M.; Alexoudi, X.; Fossati,
L.; Pino, L.; Seidel, J. V.; Carroll, T. A.; Steffen, M.; Ilyin, I.;
Poppenhäger, K.; Strassmeier, K. G.; von Essen, C.; Nascimbeni, V.;
Turner, J. D.
2020MNRAS.498.1023K Altcode: 2020MNRAS.tmp.2443K; 2020arXiv200804044K
High spectral resolution transmission spectroscopy is a powerful tool to
characterize exoplanet atmospheres. Especially for hot Jupiters, this
technique is highly relevant, due to their high-altitude absorption,
e.g. from resonant sodium (Na I) and potassium (K I) lines. We resolve
the atmospheric K I absorption on HD189733b with the aim to compare
the resolved K I line and previously obtained high-resolution Na
I-D line observations with synthetic transmission spectra. The line
profiles suggest atmospheric processes leading to a line broadening of
the order of ∼10 km/s for the Na I-D lines and only a few km/s for
the K I line. The investigation hints that either the atmosphere of
HD189733b lacks a significant amount of K I or the alkali lines probe
different atmospheric regions with different temperature, which could
explain the differences we see in the resolved absorption lines.

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Title: VizieR Online Data Catalog: The solar gravitational redshift
(Gonzalez Hernandez+, 2020)
Authors: Gonzalez Hernandez, J. I.; Rebolo, R.; Pasquini, L.; Lo Curto,
G.; Molaro, P.; Caffau, E.; Ludwig, H. -G.; Steffen, M.; Esposito,
M.; Suarez Mascarenno, A.; Toledo-Padron, B.; Probst, R. A.; Hansch,
T. W.; Holzwarth, R.; Manescau, A.; Steinmetz, T.; Udem, T.; Wilken, T.
2020yCat..36430146G Altcode:
Line data and velocity shifts of the FeI and FeII lines,
with laboratory wavelengths, λlab, from Nave et
al. (1994ApJS...94..221N, 2013ApJS..204....1N) and excitation
potentials, oscillator strengths from the VALD database (Piskunov et
al. 1995A&amp;AS..112..525P). In Table A.1 we provide the mean line
core shifts, vcoreobs, measured on the spectral lines from
the observed HARPS-LFC spectra of the MOON and computed with respect to
the original laboratory wavelengths (Nave et al. 1994ApJS...94..221N,
2013ApJS..204....1N). We also give the recalibrated wavelengths,
lambda_nist, computed from recalibrated wavenumber measurements and
Ritz wavelengths, lambda_ritz, computed from recalibrated energy levels,
with their corresponding wavelengths uncertainties, extracted from the
NIST database (Kramida et al. 2019APS..DMPN09004K). In Table A.2,
we give the line core shifts measured on the observed spectral lines,
vcoreobs_n, estimated using the recalibrated wavelengths,
lambda_nist, as reference laboratory wavelengths, the 3D profiles,
vcore,3D, and the global line shifts, vfit3Dn,
from fitting the observed spectral lines using 3D profiles, and
corrected using the recalibrated wavelengths lambda_nist as reference
laboratory wavelengths. Wavelengths are given in Angstroms,
wavelength uncertainties in miliAngstroems, excitation potentials in
eV, equivalent widths (EW) in miliAngstroems, and velocity shifts in
m/s. (2 data files).

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Title: 16th Potsdam Thinkshop: The rotation periods of cool stars:
Measurements, uses, connections and prospects
Authors: Barnes, S. A.; Steffen, M.; Strassmeier, K. G.
2020AN....341..485B Altcode:
No abstract at ADS

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Title: Improving spectroscopic lithium abundances. Fitting functions
for 3D non-LTE corrections in FGK stars of different metallicity
Authors: Mott, A.; Steffen, M.; Caffau, E.; Strassmeier, K. G.
2020A&A...638A..58M Altcode: 2020arXiv200410803M
Context. Accurate spectroscopic lithium abundances are essential in
addressing a variety of open questions, such as the origin of a uniform
lithium content in the atmospheres of metal-poor stars (Spite plateau)
or the existence of a correlation between the properties of extrasolar
planetary systems and the lithium abundance in the atmosphere of
their host stars. Aims: We have developed a tool that allows
the user to improve the accuracy of standard lithium abundance
determinations based on 1D model atmospheres and the assumption of
local thermodynamic equilibrium (LTE) by applying corrections that
account for hydrodynamic (3D) and non-LTE (NLTE) effects in FGK stars
of different metallicity. Methods: Based on a grid of CO5BOLD
3D models and associated 1D hydrostatic atmospheres, we computed
three libraries of synthetic spectra of the lithium λ 670.8 nm line
for a wide range of lithium abundances, accounting for detailed line
formation in 3D NLTE, 1D NLTE, and 1D LTE, respectively. The resulting
curves-of-growth were then used to derive 3D NLTE and 1D NLTE lithium
abundance corrections. Results: For all metallicities, the
largest corrections are found at the coolest effective temperature,
Teff = 5000 K. They are mostly positive, up to + 0.2 dex,
for the weakest lines (lithium abundance A(Li)1DLTE = 1.0),
whereas they become more negative towards lower metallicities, where
they can reach - 0.4 dex for the strongest lines (A(Li)1DLTE
= 3.0) at [Fe/H] = - 2.0. We demonstrate that 3D and NLTE effects are
small for metal-poor stars on the Spite plateau, leading to errors of at
most ± 0.05 dex if ignored. We present analytical functions evaluating
the 3D NLTE and 1D NLTE corrections as a function of Teff
[5000…6500 K], log g [3.5…4.5], and LTE lithium abundance A(Li)
[1.0…3.0] for a fixed grid of metallicities [Fe/H] [ - 3.0…0.0]. In
addition, we also provide analytical fitting functions for directly
converting a given lithium abundance into an equivalent width, or vice
versa, a given equivalent width (EW) into a lithium abundance. For
convenience, a Python script is made available that evaluates all
fitting functions for given Teff, log g, [Fe/H], and A(Li)
or EW. Conclusions: By means of the fitting functions developed
in this work, the results of complex 3D and NLTE calculations are
made readily accessible and quickly applicable to large samples of
stars across a wide range of metallicities. Improving the accuracy
of spectroscopic lithium abundance determinations will contribute to
a better understanding of the open questions related to the lithium
content in metal-poor and solar-like stellar atmospheres.

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Title: The 6Li/7Li isotopic ratio in the
metal-poor binary CS22876-032
Authors: González Hernández, J. I.; Bonifacio, P.; Caffau, E.;
Ludwig, H. -G.; Steffen, M.; Monaco, L.; Cayrel, R.
2019A&A...628A.111G Altcode: 2019arXiv190705109G
 Aims: We present high-resolution and high-quality UVES
spectroscopic data of the metal-poor double-lined spectroscopic binary
CS 22876-032 ([Fe/H] approximately -3.7 dex). Our goal is to derive
the 6Li/7Li isotopic ratio by analysing the
Li I λ 670.8 nm doublet. Methods: We co-added all 28 useful
spectra normalised and corrected for radial velocity to the rest frame
of the primary star. We fitted the Li profile with a grid of the 3D
non-local thermodynamic equilibrium (NLTE) synthetic spectra to take
into account the line profile asymmetries induced by stellar convection,
and performed Monte Carlo simulations to evaluate the uncertainty of
the fit of the Li line profile. Results: We checked that the
veiling factor does not affect the derived isotopic ratio, 6
Li/7Li, and only modifies the Li abundance, A(Li), by
about 0.15 dex. The best fit of the Li profile of the primary star
provides A(Li) = 2.17 ± 0.01 dex and 6 Li/7Li =
8-5+2% at 68% confidence level. In addition, we
improved the Li abundance of the secondary star at A(Li) = 1.55 ± 0.04
dex, which is about 0.6 dex lower than that of the primary star. Conclusions: The analysis of the Li profile of the primary star is
consistent with no detection of 6 Li and provides an upper
limit to the isotopic ratio of 6 Li/7Li &lt;
10% at this very low metallicity, about 0.5 dex lower in metallicity
than previous attempts for detection of 6 Li in extremely
metal poor stars. These results do not solve or worsen the cosmological
7 Li problem, nor do they support the need for non-standard
6Li production in the early Universe. The two averaged
spectra are only available at the CDS via anonymous ftp to <A
href="http://cdsarc.u-strasbg.fr">http://cdsarc.u-strasbg.fr</A>
(ftp://130.79.128.5) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/628/A111">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/628/A111</A>Based
on observations made with the Very Large Telescope (VLT) at ESO Paranal
Observatory, Chile, Programme 080.D-0333.

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Title: VizieR Online Data Catalog: Li in BPS CS22876-032 spectrum
(Gonzalez Hernandez+, 2019)
Authors: Gonzalez Hernandez, J. I.; Bonifacio, P.; Caffau, E.; Ludwig,
H. -G.; Steffen, M.; Monaco, L.; Cayrel, R.
2019yCat..36280111G Altcode:
Average co-added, rebinned spectra in the region around the LiI 670.8nm
resonance line of the two stellar components of the metal-poor binary
CS 22876-032 A and CS 22876-032 B. For each star, wavelength, normalised
flux and flux error are given. (2 data files).

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Title: Confronting expansion distances of planetary nebulae with
Gaia DR2 measurements
Authors: Schönberner, D.; Steffen, M.
2019A&A...625A.137S Altcode: 2019arXiv190410721S
Context. Individual distances to planetary nebulae are of the utmost
relevance for our understanding of post-asymptotic giant-branch
evolution because they allow a precise determination of stellar and
nebular properties. Also, objects with individual distances serve
as calibrators for the so-called statistical distances based on
secondary nebular properties. Aims: With independently known
distances, it is possible to check empirically our understanding
of the formation and evolution of planetary nebulae as suggested by
existing hydrodynamical simulations. Methods: We compared the
expansion parallaxes that have recently been determined for a number
of planetary nebulae with the trigonometric parallaxes provided by the
Gaia Data Release 2. Results: Except for two out of 11 nebulae,
we found good agreement between the expansion and the Gaia trigonometric
parallaxes without any systematic trend with distance. Therefore, the
Gaia measurements also prove that the correction factors necessary
to convert proper motions of shocks into Doppler velocities cannot
be ignored. Rather, the size of these correction factors and their
evolution with time as predicted by 1D hydrodynamical models of
planetary nebulae is basically validated. These correction factors
are generally greater than unity and are different for the outer
shell and the inner bright rim of a planetary nebula. The Gaia
measurements also confirm earlier findings that spectroscopic methods
often lead to an overestimation of the distance. They also show that
even modelling of the entire system of star and nebula by means
of sophisticated photoionisation modelling may not always provide
reliable results. Conclusions: The Gaia measurements confirm
the basic correctness of the present radiation-hydrodynamics models,
which predict that both the shell and the rim of a planetary nebula are
two independently expanding entities, created and driven by different
physical processes, namely thermal pressure (shell) or wind interaction
(rim), both of which vary differently with time. This work has
made use of data from the European Space Agency (ESA) mission Gaia (<A
href="https://www.cosmos.esa.int/gaia">https://www.cosmos.esa.int/gaia</A>),
processed by the Gaia Data Processing and Analysis Consortium (DPAC, <A
href="https://www.cosmos.esa.int/web/gaia/dpac/consortium">https://www.cosmos.esa.int/web/gaia/dpac/consortium</A>).
Funding for the DPAC has been provided by national institutions, in
particular the institutions participating in the Gaia Multilateral
Agreement.

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Title: TOPoS. V. Abundance ratios in a sample of very metal-poor
turn-off stars
Authors: François, P.; Caffau, E.; Bonifacio, P.; Spite, M.; Spite,
F.; Cayrel, R.; Christlieb, N.; Gallagher, A. J.; Klessen, R.; Koch,
A.; Ludwig, H. -G.; Monaco, L.; Plez, B.; Steffen, M.; Zaggia, S.
2018A&A...620A.187F Altcode: 2018arXiv181100035F
Context. Extremely metal-poor stars are keys to understand the early
evolution of our Galaxy. The ESO large programme TOPoS has been tailored
to analyse a new set of metal-poor turn-off stars, whereas most of
the previously known extremely metal-poor stars are giant stars. Aims: Sixty five turn-off stars (preselected from SDSS spectra)
have been observed with the X-shooter spectrograph at the ESO VLT Unit
Telescope 2, to derive accurate and detailed abundances of magnesium,
silicon, calcium, iron, strontium and barium. Methods: We
analysed medium-resolution spectra (R ≃ 10 000) obtained with the
ESO X-shooter spectrograph and computed the abundances of several
α and neutron-capture elements using standard one-dimensional local
thermodynamic equilibrium (1D LTE) model atmospheres. Results:
Our results confirms the super-solar [Mg/Fe] and [Ca/Fe] ratios in
metal-poor turn-off stars as observed in metal-poor giant stars. We
found a significant spread of the [α/Fe] ratios with several stars
showing subsolar [Ca/Fe] ratios. We could measure the abundance of
strontium in 12 stars of the sample, leading to abundance ratios
[Sr/Fe] around the Solar value. We detected barium in two stars
of the sample. One of the stars (SDSS J114424-004658) shows both
very high [Ba/Fe] and [Sr/Fe] abundance ratios (&gt;1 dex). Based on observations collected at the European Organisation for
Astronomical Research in the Southern Hemisphere under ESO programme ID
189.D-0165. Equivalent widths of the Fe lines are only, and Tables
A.1 and A.2 are also available at the CDS via anonymous ftp to <A
href="http://cdsarc.u-strasbg.fr">http://cdsarc.u-strasbg.fr</A>
(ftp://130.79.128.5) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/620/A187">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/620/A187</A>

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Title: Hot bubbles of planetary nebulae with hydrogen-deficient
winds. II. Analytical approximations with application to BD + 30°3639
Authors: Heller, R.; Jacob, R.; Schönberner, D.; Steffen, M.
2018A&A...620A..98H Altcode: 2018arXiv180908305H
Context. The first high-resolution X-ray spectroscopy of a planetary
nebula, BD +30° 3639, opened the possibility to study plasma conditions
and chemical compositions of X-ray emitting "hot" bubbles of planetary
nebulae in much greater detail than before. Aims: We investigate
(i) how diagnostic line ratios are influenced by the bubble's thermal
structure and chemical profile, (ii) whether the chemical composition
inside the bubble of BD +30° 3639 is consistent with the hydrogen-poor
composition of the stellar photosphere and wind, and (iii) whether
hydrogen-rich nebular matter has already been added to the bubble of BD
+30° 3639 by evaporation. Methods: We applied an analytical,
one-dimensional (1D) model for wind-blown bubbles with temperature
and density profiles based on self-similar solutions including
thermal conduction. We also constructed heat-conduction bubbles with
a chemical stratification. The X-ray emission was computed using the
well-documented CHIANTI code. These bubble models are used to re-analyse
the high-resolution X-ray spectrum from the hot bubble of BD +30°
3639. Results: We found that our 1D heat-conducting bubble
models reproduce the observed line ratios much better than plasmas
with single electron temperatures. In particular, all the temperature-
and abundance-sensitive line ratios are consistent with BD +30° 3639
X-ray observations for (i) an intervening column density of neutral
hydrogen, NH = 0.20-0.10+0.05 ×
1022cm-2, (ii) a characteristic bubble X-ray
temperature of TX = 1.8 ± 0.1 MK together with (iii)
a very high neon mass fraction of about 0.05, virtually as high as
that of oxygen. For lower values of NH, we cannot exclude
the possibility that the hot bubble of BD +30° 3639 contains a small
amount of "evaporated" (or mixed) hydrogen-rich nebular matter. Given
the possible range of NH, the fraction of evaporated
hydrogen-rich matter cannot exceed 3% of the bubble mass. 
Conclusions: The diffuse X-ray emission from BD +30° 3639 can be well
explained by models of wind-blown bubbles with thermal conduction and
a chemical composition equal to that of the hydrogen-poor and carbon-,
oxygen-, and neon-rich stellar surface.

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Title: VizieR Online Data Catalog: Very metal-poor turn-off stars
abundances (Francois+, 2018)
Authors: Francois, P.; Caffau, E.; Bonifacio, P.; Spite, M.; Spite,
F.; Cayrel, R.; Christlieb, N.; Gallagher, A.; Klessen, R.; Koch,
A.; Ludwig, H. -G.; Monaco, L.; Plez, B.; Steffen, M.; Zaggia, S.
2018yCat..36200187F Altcode:
Sixty five turn-off stars (preselected from SDSS spectra) have been
observed with the X-Shooter spectrograph at the ESO VLT Unit Telescope
2, to derive accurate and detailed abundances of magnesium, silicon,
calcium, iron, strontium and barium. We analysed medium-resolution
spectra (R~10000) obtained with the ESO X-Shooter spectrograph and
computed the abundances of several alpha and neutron-capture elements
using standard one-dimensional local thermodynamic equilibrium (1D LTE)
model atmospheres. (3 data files).

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Title: 3D non-LTE corrections for Li abundance and
6Li/7Li isotopic ratio in solar-type
stars. I. Application to HD 207129 and HD 95456
Authors: Harutyunyan, G.; Steffen, M.; Mott, A.; Caffau, E.; Israelian,
G.; González Hernández, J. I.; Strassmeier, K. G.
2018A&A...618A..16H Altcode: 2018arXiv180704089H
Context. Convective motions in solar-type stellar atmospheres
induce Doppler shifts that affect the strengths and shapes of
spectral absorption lines and create slightly asymmetric line
profiles. One-dimensional (1D) local thermodynamic equilibrium
(LTE) studies of elemental abundances are not able to reproduce this
phenomenon, which becomes particularly important when modeling the
impact of isotopic fine structure, like the subtle depression created by
the 6Li isotope on the red wing of the Li I resonance doublet
line. Aims: The purpose of this work is to provide corrections
for the lithium abundance, A(Li), and the 6Li/7Li
isotopic ratio that can easily be applied to correct 1D LTE lithium
abundances in G and F dwarf stars of approximately solar mass and
metallicity for three-dimensional (3D) and non-LTE (NLTE) effects. Methods: The corrections for A(Li) and 6Li/7Li
are computed using grids of 3D NLTE and 1D LTE synthetic lithium
line profiles, generated from 3D hydro-dynamical CO5BOLD
and 1D hydrostatic model atmospheres, respectively. For comparative
purposes, all calculations are performed for three different line
lists representing the Li I λ670.8 nm spectral region. The 3D NLTE
corrections are then approximated by analytical expressions as a
function of the stellar parameters (Teff, log ℊ, [Fe/H],
ν sin i, A(Li), 6Li/7Li). These are applied to
adjust the 1D LTE isotopic lithium abundances in two solar-type stars,
<ASTROBJ>HD 207129</ASTROBJ> and <ASTROBJ>HD 95456</ASTROBJ>, for
which high-quality HARPS observations are available. Results:
The derived 3D NLTE corrections range between -0.01 and +0.11 dex for
A(Li), and between -4.9 and -0.4% for 6Li/7Li,
depending on the adopted stellar parameters. We confirm that the
inferred 6Li abundance depends critically on the strength of
the Si I 670.8025 nm line. Our findings show a general consistency with
recent works on lithium abundance corrections. After the application of
such corrections, we do not find a significant amount of 6Li
in any of the two target stars. Conclusions: In the case of
6Li/7Li, our corrections are always negative,
showing that 1D LTE analysis can significantly overestimate the
presence of 6Li (up to 4.9% points) in the atmospheres
of solar-like dwarf stars. These results emphasize the importance
of reliable 3D model atmospheres combined with NLTE line formation
for deriving precise isotopic lithium abundances. Although 3D NLTE
spectral synthesis implies an extensive computational effort,
the results can be made accessible with parametric tools like
the ones presented in this work. The table with the 3D NLTE
corrections is only available at the CDS via anonymous ftp to <A
href="http://cdsarc.u-strasbg.fr">http://cdsarc.u-strasbg.fr</A>
(ftp://130.79.128.5) or via <A
href="http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/618/A16">http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/618/A16</A>

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Title: Abundance of zinc in the red giants of Galactic globular
cluster 47 Tucanae
Authors: Černiauskas, A.; Kučinskas, A.; Klevas, J.; Bonifacio,
P.; Ludwig, H. -G.; Caffau, E.; Steffen, M.
2018A&A...616A.142C Altcode: 2018arXiv180603132C
 Aims: We investigate possible relations between the abundances
of zinc and the light elements sodium, magnesium, and potassium
in the atmospheres of red giant branch (RGB) stars of the Galactic
globular cluster 47 Tuc and study connections between the chemical
composition and dynamical properties of the cluster RGB stars. Methods: The abundance of zinc was determined in 27 RGB stars
of 47 Tuc using 1D local thermal equilibrium (LTE) synthetic line
profile fitting to the high-resolution 2dF/HERMES spectra obtained
with the Anglo-Australian Telescope (AAT). Synthetic spectra used
in the fitting procedure were computed with the SYNTHE code and
1D ATLAS9 stellar model atmospheres. Results: The average
1D LTE zinc-to-iron abundance ratio and its RMS variations due to
star-to-star abundance spread determined in the sample of 27 RGB stars
is &lt;[Zn/Fe]&gt;1D LTE = 0.11 ± 0.09. We did not detect
any statistically significant relations between the abundances of
zinc and those of light elements. Neither did we find any significant
correlation or anticorrelation between the zinc abundance in individual
stars and their projected distance from the cluster center. Finally,
no statistically significant relation between the absolute radial
velocities of individual stars and the abundance of zinc in their
atmospheres was detected. The obtained average [Zn/Fe]1DLTE
ratio agrees well with those determined in this cluster in earlier
studies and nearly coincides with that of Galactic field stars at this
metallicity. All these results suggest that nucleosynthesis of zinc
and light elements proceeded in separate, unrelated pathways in 47 Tuc.

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Title: Carbon-enhanced metal-poor 3D model atmospheres
Authors: Steffen, M.; Gallagher, A. J.; Caffau, E.; Bonifacio, P.;
Ludwig, H. -G.
2018IAUS..334..364S Altcode: 2017arXiv170805686S
We present our latest 3D model atmospheres for carbon-enhanced
metal-poor (CEMP) stars computed with the CO5BOLD code. The stellar
parameters are representative of hot turn-off objects (Teff
~ 6250 K, log g = 4.0, [Fe/H]=-3). The main purpose of these models
is to investigate the role of 3D effects on synthetic spectra of the
CH G-band (4140-4400 Å), the CN BX-band (3870-3890 Å), and several
UV OH transitions (3122-3128 Å). By comparison with the synthetic
spectra from standard 1D model atmospheres (assuming local thermodynamic
equilibrium, LTE), we derive 3D abundance corrections for carbon and
oxygen of up to -0.5 and -0.7 dex, respectively.

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Title: Abundances of Mg and K in the atmospheres of turn-off starsin
Galactic globular cluster 47 Tucanae
Authors: Černiauskas, A.; Kučinskas, A.; Klevas, J.; Dobrovolskas,
V.; Korotin, S.; Bonifacio, P.; Ludwig, H. -G.; Caffau, E.; Steffen, M.
2018A&A...615A.173C Altcode: 2018arXiv180410033C
 Aims: We determined abundances of Mg and K in the atmospheres
of 53 (Mg) and 75 (K) turn-off (TO) stars of the Galactic globular
cluster 47 Tuc. The obtained abundances, together with those of Li,
O, and Na that we had earlier determined for the same sample of stars,
were used to search for possible relations between the abundances of K
and other light elements, Li, O, Na, and Mg, as well as the connections
between the chemical composition of TO stars and their kinematical
properties. Methods: Abundances of Mg and K were determined using
archival high resolution VLT FLAMES/GIRAFFE spectra, in combination
with the one-dimensional (1D) non-local thermodynamic equilibrium
(NLTE) spectral synthesis methodology. Spectral line profiles were
computed with the MULTI code, using 1D hydrostatic ATLAS9 stellar model
atmospheres. We also utilized three-dimensional (3D) hydrodynamical
CO5BOLD and 1D hydrostatic LHD model atmospheres for
computing 3D-1D LTE abundance corrections for the spectral lines
of Mg and K, in order to assess the influence of convection on their
formation in the atmospheres of TO stars. Results: The determined
average abundance-to-iron ratios and their root mean square variations
due to star-to-star abundance spreads were &lt;[Mg/Fe]&gt;1D
NLTE = 0.47 ± 0.12, and &lt;[K/Fe]&gt;1D NLTE = 0.39
± 0.09. Although the data suggest the possible existence of a weak
correlation in the [K/Fe]-[Na/Fe] plane, its statistical significance
is low. No statistically significant relations between the abundance
of K and other light elements were detected. Also, we did not find any
significant correlations or anti-correlations between the [Mg/Fe] and
[K/Fe] ratios and projected distance from the cluster center. Similarly,
no relations between the absolute radial velocities of individual stars
and abundances of Mg and K in their atmospheres were detected. The
3D-1D abundance corrections were found to be small (≤0.1 dex) for
the lines of Mg and K used in this study, thus indicating that the
influence of convection on their formation is small.

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Title: VizieR Online Data Catalog: A(Li) and 6Li/7Li 3D NLTE
corrections (Harutyunyan+, 2018)
Authors: Harutyunyan, G.; Steffen, M.; Mott, A.; Caffau, E.; Israelian,
G.; Gonzalez Hernandez, J. I.; Strassmeier, K. G.
2018yCat..36180016H Altcode:
A grid of 3D non-LTE (NLTE) corrections for the lithium abundance,
A(Li), and the 6Li/7Li isotopic ratio are
presented. These corrections can be easily applied to correct 1D
LTE lithium abundances in G and F dwarf stars of approximately
solar mass and metallicity for 3D and NLTE effects. The stellar
parameters defining the grid are effective temperatures, Teff (5900,
6300 and 6500K), surface gravity, logg (4.0 and 4.5), metallicity,
[Fe/H] (-1.0, -0.5, 0.0, +0.5), 1D LTE lithium abundance, A(Li)
(1.5, 2.0, 2.5), 1D LTE 6Li/7Li isotopic ratio
(0, 5 and 10%), as well as projected rotational velocity, vsini (0,
2, 4 and 6km/s). Based on this table, a web page calculator was
created that allows to compute the 3D NLTE corrections of A(Li)
and 6Li/7Li ratio for a given combination of stellar parameters
(https://pages.aip.de/li67nlte3d/). (1 data file).

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Title: Simulation of the small-scale magnetism in main-sequence
stellar atmospheres
Authors: Salhab, R. G.; Steiner, O.; Berdyugina, S. V.; Freytag, B.;
Rajaguru, S. P.; Steffen, M.
2018A&A...614A..78S Altcode:
Context. Observations of the Sun tell us that its granular and
subgranular small-scale magnetism has significant consequences for
global quantities such as the total solar irradiance or convective
blueshift of spectral lines. Aims: In this paper, properties
of the small-scale magnetism of four cool stellar atmospheres,
including the Sun, are investigated, and in particular its effects
on the radiative intensity and flux. Methods: We carried out
three-dimensional radiation magnetohydrodynamic simulations with the
CO5BOLD code in two different settings: with and without
a magnetic field. These are thought to represent states of high and
low small-scale magnetic activity of a stellar magnetic cycle. Results: We find that the presence of small-scale magnetism
increases the bolometric intensity and flux in all investigated
models. The surplus in radiative flux of the magnetic over the magnetic
field-free atmosphere increases with increasing effective temperature,
Teff, from 0.47% for spectral type K8V to 1.05% for the solar
model, but decreases for higher effective temperatures than solar. The
degree of evacuation of the magnetic flux concentrations monotonically
increases with Teff as does their depression of the visible
optical surface, that is the Wilson depression. Nevertheless, the
strength of the field concentrations on this surface stays remarkably
unchanged at ≈1560 G throughout the considered range of spectral
types. With respect to the surrounding gas pressure, the field strength
is close to (thermal) equipartition for the Sun and spectral type F5V
but is clearly sub-equipartition for K2V and more so for K8V. The
magnetic flux concentrations appear most conspicuous for model K2V
owing to their high brightness contrast. Conclusions: For mean
magnetic flux densities of approximately 50 G, we expect the small-scale
magnetism of stars in the spectral range from F5V to K8V to produce a
positive contribution to their bolometric luminosity. The modulation
seems to be most effective for early G-type stars.

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Title: Using the CIFIST grid of CO5BOLD 3D model
atmospheres to study the effects of stellar granulation on photometric
colours. II. The role of convection across the H-R diagram
Authors: Kučinskas, A.; Klevas, J.; Ludwig, H. -G.; Bonifacio, P.;
Steffen, M.; Caffau, E.
2018A&A...613A..24K Altcode: 2018arXiv180200073K
 Aims: We studied the influence of convection on the spectral
energy distributions (SEDs), photometric magnitudes, and colour
indices of different types of stars across the H-R diagram. 
Methods: The 3D hydrodynamical CO5BOLD, averaged ⟨3D⟩,
and 1D hydrostatic LHD model atmospheres were used to compute SEDs
of stars on the main sequence (MS), main sequence turn-off (TO),
subgiant branch (SGB), and red giant branch (RGB), in each case at
two different effective temperatures and two metallicities, [M/H] =
0.0 and - 2.0. Using the obtained SEDs, we calculated photometric
magnitudes and colour indices in the broad-band Johnson-Cousins
UBVRI and 2MASS JHKs, and the medium-band Strömgren
uvby photometric systems. Results: The 3D-1D differences in
photometric magnitudes and colour indices are small in both photometric
systems and typically do not exceed ± 0.03 mag. Only in the case of the
coolest giants located on the upper RGB are the differences in the U and
u bands able reach ≈-0.2 mag at [M/H] = 0.0 and ≈-0.1 mag at [M/H]
= -2.0. Generally, the 3D-1D differences are largest in the blue-UV
part of the spectrum and decrease towards longer wavelengths. They
are also sensitive to the effective temperature and are significantly
smaller in hotter stars. Metallicity also plays a role and leads to
slightly larger 3D-1D differences at [M/H] = 0.0. All these patterns are
caused by a complex interplay between the radiation field, opacities,
and horizontal temperature fluctuations that occur due to convective
motions in stellar atmospheres. Although small, the 3D-1D differences
in the magnitudes and colour indices are nevertheless comparable to
or larger than typical photometric uncertainties and may therefore
cause non-negligible systematic differences in the estimated effective
temperatures.

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Title: TOPoS. IV. Chemical abundances from high-resolution
observations of seven extremely metal-poor stars
Authors: Bonifacio, P.; Caffau, E.; Spite, M.; Spite, F.; Sbordone,
L.; Monaco, L.; François, P.; Plez, B.; Molaro, P.; Gallagher, A. J.;
Cayrel, R.; Christlieb, N.; Klessen, R. S.; Koch, A.; Ludwig, H. -G.;
Steffen, M.; Zaggia, S.; Abate, C.
2018A&A...612A..65B Altcode: 2018arXiv180103935B
Context. Extremely metal-poor (EMP) stars provide us with indirect
information on the first generations of massive stars. The TOPoS
survey has been designed to increase the census of these stars and to
provide a chemical inventory that is as detailed as possible. 
Aims: Seven of the most iron-poor stars have been observed with the
UVES spectrograph at the ESO VLT Kueyen 8.2 m telescope to refine
their chemical composition. Methods: We analysed the spectra
based on 1D LTE model atmospheres, but also used 3D hydrodynamical
simulations of stellar atmospheres. Results: We measured carbon
in six of the seven stars: all are carbon-enhanced and belong to the
low-carbon band, defined in the TOPoS II paper. We measured lithium
(A(Li) = 1.9) in the most iron-poor star (SDSS J1035+0641, [Fe/H]
&lt;-5.2). We were also able to measure Li in three stars at [Fe/H]
-4.0, two of which lie on the Spite plateau. We confirm that SDSS
J1349+1407 is extremely rich in Mg, but not in Ca. It is also very
rich in Na. Several of our stars are characterised by low α-to-iron
ratios. Conclusions: The lack of high-carbon band stars at low
metallicity can be understood in terms of evolutionary timescales
of binary systems. The detection of Li in SDSS J1035+0641 places a
strong constraint on theories that aim at solving the cosmological
lithium problem. The Li abundance of the two warmer stars at [Fe/H]
-4.0 places them on the Spite plateau, while the third, cooler star,
lies below. We argue that this suggests that the temperature at which
Li depletion begins increases with decreasing [Fe/H]. SDSS J1349+1407
may belong to a class of Mg-rich EMP stars. We cannot assess if there
is a scatter in α-to-iron ratios among the EMP stars or if there are
several discrete populations. However, the existence of stars with
low α-to-iron ratios is supported by our observations. Based
on observations obtained at ESO Paranal Observatory, Programmes
189.D-0165,090.D-0306, 093.D-0136, and 096.D-0468.

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Title: PEPSI deep spectra. I. The Sun-as-a-star
Authors: Strassmeier, K. G.; Ilyin, I.; Steffen, M.
2018A&A...612A..44S Altcode: 2017arXiv171206960S
Context. Full-disk solar flux spectra can be directly compared to
stellar spectra and thereby serve as our most important reference
source for, for example stellar chemical abundances, magnetic activity
phenomena, radial-velocity signatures or global pulsations. Aim. As part
of the first Potsdam Echelle Polarimetric and Spectroscopic Instrument
(PEPSI) key-science project, we aim to provide well-exposed and
average-combined (viz. deep) high-resolution spectra of representative
stellar targets. Such deep spectra contain an overwhelming amount
of information, typically much more than what could be analyzed and
discussed within a single publication. Therefore, these spectra will
be made available in form of (electronic) atlases. The first star in
this series of papers is our Sun. It also acts as a system-performance
cornerstone. Methods: The Sun was monitored with PEPSI at the
Large Binocular Telescope (LBT). Instead of the LBT we used a small
robotic solar disk integration (SDI) telescope. The deep spectra
in this paper are the results of combining up to ≈100 consecutive
exposures per wavelength setting and are compared with other solar
flux atlases. Results: Our software for the optimal data
extraction and reduction of PEPSI spectra is described and verified
with the solar data. Three deep solar flux spectra with a spectral
resolution of up to 270 000, a continuous wavelength coverage from
383 nm to 914 nm, and a photon signal to noise ratio (S/N) of between
2000-8000:1 depending on wavelength are presented. Additionally,
a time-series of 996 high-cadence spectra in one cross disperser
is used to search for intrinsic solar modulations. The wavelength
calibration based on Th-Ar exposures and simultaneous Fabry-Pérot
combs enables an absolute wavelength solution within 10 m s-1
(rms) with respect to the HARPS laser-comb solar atlas and a relative
rms of 1.2 m s-1 for one day. For science demonstration,
we redetermined the disk-average solar Li abundance to 1.09 ±
0.04 dex on the basis of 3D NLTE model atmospheres. We detected
disk-averaged p-mode RV oscillations with a full amplitude of
47 cm s-1 at 5.5 min. Conclusions: Comparisons
with two solar FTS atlases, as well as with the HARPS solar atlas,
validate the PEPSI data product. Now, PEPSI/SDI solar-flux spectra
are being taken with a sampling of one deep spectrum per day, and
are supposed to continue a full magnetic cycle of the Sun. Based
on data acquired with PEPSI fed by the solar disk integration (SDI)
telescope operated by AIP at the Large Binocular Telescope Observatory
(LBTO). The LBT is an international collaboration among institutions
in the United States, Italy and Germany. LBT Corporation partners are:
The University of Arizona on behalf of the Arizona Board of Regents;
Istituto Nazionale di Astrofisica, Italy; LBT Beteiligungsgesellschaft,
Germany, representing the Max-Planck Society, The Leibniz Institute for
Astrophysics Potsdam (AIP), and Heidelberg University; The Ohio State
University, and The Research Corporation, on behalf of The University
of Notre Dame, University of Minnesota and University of Virginia.

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Title: Using the CIFIST grid of CO5BOLD 3D model
atmospheres to study the effects of stellar granulation on photometric
colours. I. Grids of 3D corrections in the UBVRI, 2MASS, HIPPARCOS,
Gaia, and SDSS systems
Authors: Bonifacio, P.; Caffau, E.; Ludwig, H. -G.; Steffen, M.;
Castelli, F.; Gallagher, A. J.; Kučinskas, A.; Prakapavičius, D.;
Cayrel, R.; Freytag, B.; Plez, B.; Homeier, D.
2018A&A...611A..68B Altcode: 2017arXiv171200024B
Context. The atmospheres of cool stars are temporally and spatially
inhomogeneous due to the effects of convection. The influence of
this inhomogeneity, referred to as granulation, on colours has never
been investigated over a large range of effective temperatures and
gravities. Aim. We aim to study, in a quantitative way, the impact of
granulation on colours. Methods: We use the CIFIST (Cosmological
Impact of the FIrst Stars) grid of CO5BOLD (COnservative COde for the
COmputation of COmpressible COnvection in a BOx of L Dimensions, L = 2,
3) hydrodynamical models to compute emerging fluxes. These in turn are
used to compute theoretical colours in the UBV RI, 2MASS, HIPPARCOS,
Gaia and SDSS systems. Every CO5BOLD model has a corresponding one
dimensional (1D) plane-parallel LHD (Lagrangian HydroDynamics) model
computed for the same atmospheric parameters, which we used to define
a "3D correction" that can be applied to colours computed from fluxes
computed from any 1D model atmosphere code. As an example, we illustrate
these corrections applied to colours computed from ATLAS models. 
Results: The 3D corrections on colours are generally small, of the order
of a few hundredths of a magnitude, yet they are far from negligible. We
find that ignoring granulation effects can lead to underestimation of
Teff by up to 200 K and overestimation of gravity by up to 0.5 dex, when
using colours as diagnostics. We have identified a major shortcoming in
how scattering is treated in the current version of the CIFIST grid,
which could lead to offsets of the order 0.01 mag, especially for
colours involving blue and UV bands. We have investigated the Gaia and
HIPPARCOS photometric systems and found that the (G - Hp),
(BP - RP) diagram is immune to the effects of granulation. In addition,
we point to the potential of the RVS photometry as a metallicity
diagnostic. Conclusions: Our investigation shows that the
effects of granulation should not be neglected if one wants to use
colours as diagnostics of the stellar parameters of F, G, K stars. A
limitation is that scattering is treated as true absorption in our
current computations, thus our 3D corrections are likely an upper
limit to the true effect. We are already computing the next generation
of the CIFIST grid, using an approximate treatment of scattering. The appendix tables are only available at the CDS via anonymous ftp
to <A href="http://cdsarc.u-strasbg.fr">http://cdsarc.u-strasbg.fr</A>
(<A href="http://cdsarc.u-strasbg.fr">http://130.79.128.5</A>) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/611/A68">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/611/A68</A>

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Title: VizieR Online Data Catalog: 3D correction in 5 photometric
systems (Bonifacio+, 2018)
Authors: Bonifacio, P.; Caffau, E.; Ludwig, H. -G.; Steffen, M.;
Castelli, F.; Gallagher, A. J.; Kucinskas, A.; Prakapavicius, D.;
Cayrel, R.; Freytag, B.; Plez, B.; Homeier, D.
2018yCat..36110068B Altcode:
We have used the CIFIST grid of CO5BOLD models to investigate the
effects of granulation on fluxes and colours of stars of spectral
type F, G, and K. We publish tables with 3D corrections that
can be applied to colours computed from any 1D model atmosphere. For
Teff&gt;=5000K, the corrections are smooth enough, as a function
of atmospheric parameters, that it is possible to interpolate the
corrections between grid points; thus the coarseness of the CIFIST
grid should not be a major limitation. However at the cool end there
are still far too few models to allow a reliable interpolation. (20 data files).

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Title: Analysis of the X-ray spectrum of the hot bubble of BD+30°3639
Authors: Schönberner, Detlef; Jacob, Ralf; Heller, René; Steffen,
Matthias
2017IAUS..323..109S Altcode: 2016arXiv161201389S
We developed a model for wind-blown bubbles with temperature
and density profiles based on self-similar solutions including
thermal conduction. We constructed also heat-conduction bubbles with
chemical discontinuities. The X-ray emission is computed using the
well-documented CHIANTI code (v6.0.1). These bubble models are used
to (re)analyse the high-resolution X-ray spectrum of the hot bubble
of BD+30°3639, and they appeared to be much superior to constant
temperature approaches. We found for the X-ray emission of
BD+30°3639 that temperature-sensitive and abundance-sensitive line
ratios computed on the basis of heat-conducting wind-blown bubbles and
with abundances as found in the stellar photosphere/wind can only be
reconciled with the observations if the hot bubble of BD+30°3639 is
chemically stratified, i.e. if it contains also a small mass fraction
(~= 3 %) of hydrogen-rich matter immediately behind the conduction
front. Neon appears to be strongly enriched, with a mass fraction of
at least about 0.06.

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Title: The Gaia-ESO Survey: Galactic evolution of sulphur and zinc
Authors: Duffau, S.; Caffau, E.; Sbordone, L.; Bonifacio, P.;
Andrievsky, S.; Korotin, S.; Babusiaux, C.; Salvadori, S.; Monaco, L.;
François, P.; Skúladóttir, Á.; Bragaglia, A.; Donati, P.; Spina,
L.; Gallagher, A. J.; Ludwig, H. -G.; Christlieb, N.; Hansen, C. J.;
Mott, A.; Steffen, M.; Zaggia, S.; Blanco-Cuaresma, S.; Calura, F.;
Friel, E.; Jiménez-Esteban, F. M.; Koch, A.; Magrini, L.; Pancino,
E.; Tang, B.; Tautvaišienė, G.; Vallenari, A.; Hawkins, K.; Gilmore,
G.; Randich, S.; Feltzing, S.; Bensby, T.; Flaccomio, E.; Smiljanic,
R.; Bayo, A.; Carraro, G.; Casey, A. R.; Costado, M. T.; Damiani,
F.; Franciosini, E.; Hourihane, A.; Jofré, P.; Lardo, C.; Lewis,
J.; Morbidelli, L.; Sousa, S. G.; Worley, C. C.
2017A&A...604A.128D Altcode: 2017arXiv170402981D
Context. Due to their volatile nature, when sulphur and zinc are
observed in external galaxies, their determined abundances represent
the gas-phase abundances in the interstellar medium. This implies
that they can be used as tracers of the chemical enrichment of matter
in the Universe at high redshift. Comparable observations in stars
are more difficult and, until recently, plagued by small number
statistics. Aims: We wish to exploit the Gaia-ESO Survey
(GES) data to study the behaviour of sulphur and zinc abundances
of a large number of Galactic stars, in a homogeneous way. 
Methods: By using the UVES spectra of the GES sample, we are able to
assemble a sample of 1301 Galactic stars, including stars in open and
globular clusters in which both sulphur and zinc were measured. Results: We confirm the results from the literature that sulphur
behaves as an α-element. We find a large scatter in [Zn/Fe] ratios
among giant stars around solar metallicity. The lower ratios are
observed in giant stars at Galactocentric distances less than 7.5
kpc. No such effect is observed among dwarf stars, since they do not
extend to that radius. Conclusions: Given the sample selection,
giants and dwarfs are observed at different Galactic locations, and it
is plausible, and compatible with simple calculations, that Zn-poor
giants trace a younger population more polluted by SN Ia yields. It
is necessary to extend observations in order to observe both giants
and dwarfs at the same Galactic location. Further theoretical work on
the evolution of zinc is also necessary. Based on observations
collected at the European Organisation for Astronomical Research in the
Southern Hemisphere under ESO programmes 188.B-3002, 193.B-0936.The full
table of S abundances is only available at the CDS via anonymous ftp
to <A href="http://cdsarc.u-strasbg.fr">http://cdsarc.u-strasbg.fr</A>
(<A href="http://130.79.128.5">http://130.79.128.5</A>) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/604/A128">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/604/A128</A>

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Title: Lithium abundance and 6Li/7Li ratio in
the active giant HD 123351. I. A comparative analysis of 3D and 1D
NLTE line-profile fits
Authors: Mott, A.; Steffen, M.; Caffau, E.; Spada, F.; Strassmeier,
K. G.
2017A&A...604A..44M Altcode: 2017arXiv170406460M
Context. Current three-dimensional (3D) hydrodynamical model
atmospheres together with detailed spectrum synthesis, accounting
for departures from local thermodynamic equilibrium (LTE), permit
to derive reliable atomic and isotopic chemical abundances from
high-resolution stellar spectra. Not much is known about the presence
of the fragile 6Li isotope in evolved solar-metallicity red
giant branch (RGB) stars, not to mention its production in magnetically
active targets like HD 123351. Aims: A detailed spectroscopic
investigation of the lithium resonance doublet in HD 123351 in terms
of both abundance and isotopic ratio is presented. From fits of
the observed spectrum, taken at the Canada-France-Hawaii telescope,
with synthetic line profiles based on 1D and 3D model atmospheres,
we seek to estimate the abundance of the 6Li isotope and
to place constraints on its origin. Methods: We derive the
lithium abundance A(Li) and the 6Li/7Li isotopic
ratio by fitting different synthetic spectra to the Li-line region
of a high-resolution CFHT spectrum (R = 120 000, S/N = 400). The
synthetic spectra are computed with four different line lists,
using in parallel 3D hydrodynamical CO5BOLD and 1D LHD
model atmospheres and treating the line formation of the lithium
components in non-LTE (NLTE). The fitting procedure is repeated with
different assumptions and wavelength ranges to obtain a reasonable
estimate of the involved uncertainties. Results: We find A(Li)
= 1.69 ± 0.11 dex and 6Li/7Li = 8.0 ± 4.4%
in 3D-NLTE, using the line list of Meléndez et al. (2012, A&amp;A,
543, A29), updated with new atomic data for V I, which results in
the best fit of the lithium line profile of HD 123351. Two other line
lists lead to similar results but with inferior fit qualities. 
Conclusions: Our 2σ detection of the 6Li isotope is the
result of a careful statistical analysis and the visual inspection
of each achieved fit. Since the presence of a significant amount of
6Li in the atmosphere of a cool evolved star is not expected
in the framework of standard stellar evolution theory, non-standard,
external lithium production mechanisms, possibly related to stellar
activity or a recent accretion of rocky material, need to be invoked
to explain the detection of 6Li in HD 123351.

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Title: Abundances of Na, Mg, and K in the atmospheres of red giant
branch stars of Galactic globular cluster 47 Tucanae
Authors: Černiauskas, A.; Kučinskas, A.; Klevas, J.; Prakapavičius,
D.; Korotin, S.; Bonifacio, P.; Ludwig, H. -G.; Caffau, E.; Steffen, M.
2017A&A...604A..35C Altcode: 2017arXiv170402751C
 Aims: We study the abundances of Na, Mg, and K in the atmospheres
of 32 red giant branch (RGB) stars in the Galactic globular cluster
(GGC) 47 Tuc, with the goal to investigate the possible existence of
Na-K and Mg-K correlations/anti-correlations, similar to those that
were recently discovered in two other GGCs, NGC 2419 and 2808. Methods: The abundances of K, Na, and Mg were determined using
high-resolution 2dF/HERMES spectra obtained with the Anglo-Australian
Telescope (AAT). The one-dimensional (1D) NLTE abundance estimates
were obtained using 1D hydrostatic ATLAS9 model atmospheres and
spectral line profiles synthesized with the MULTI package. We also
used three-dimensional (3D) hydrodynamical CO5BOLD and
1D hydrostatic LHD model atmospheres to compute 3D-1D LTE abundance
corrections, Δ3D - 1D LTE, for the spectral lines of Na,
Mg, and K used in our study. These abundance corrections were used to
understand the role of convection in the formation of spectral lines,
as well as to estimate the differences in the abundances obtained with
the 3D hydrodynamical and 1D hydrostatic model atmospheres. 
Results: The average element-to-iron abundance ratios and their RMS
variations due to star-to-star abundance spreads determined in our
sample of RGB stars were ⟨ [ Na / Fe ] ⟩ 1D NLTE =
0.42 ± 0.13, ⟨ [ Mg / Fe ] ⟩ 1D NLTE = 0.41 ± 0.11,
and ⟨ [ K / Fe ] ⟩ 1D NLTE = 0.05 ± 0.14. We found no
statistically significant relations between the abundances of the three
elements studied here. Also, there were no abundance trends with the
distance from the cluster center, nor any statistically significant
relations between the abundance/abundance ratios and absolute radial
velocities of individual stars. All these facts suggest the similarity
of K abundance in stars that belong to different generations in 47
Tuc which, in turn, may hint that evolution of K in this particular
cluster was unrelated to the nucleosynthesis of Na and/or Mg.

---------------------------------------------------------
Title: VizieR Online Data Catalog: S abundances for 1301 stars from
GES (Duffau+, 2017)
Authors: Duffau, S.; Caffau, E.; Sbordone, L.; Bonifacio, P.;
Andrievsky, S.; Korotin, S.; Babusiaux, C.; Salvadori, S.; Monaco,
L.; Francois, P.; Skuladottir, A.; Bragaglia, A.; Donati, P.; Spina,
L.; Gallagher, A. J.; Ludwig, H. -G.; Christlieb, N.; Hansen, C. J.;
Mott, A.; Steffen, M.; Zaggia, S.; Blanco-Cuaresma, S.; Calura, F.;
Friel, E.; Jimenez-Esteban, F. M.; Koch, A.; Magrini, L.; Pancino,
E.; Tang, B.; Tautvaisiene, G.; Vallenari, A.; Hawkins, K.; Gilmore,
G.; Randich, S.; Feltzing, S.; Bensby, T.; Flaccomio, E.; Smiljanic,
R.; Bayo, A.; Carraro, G.; Casey, A. R.; Costado, M. T.; Damiani,
F.; Franciosini, E.; Hourihane, A.; Jofre, P.; Lardo, C.; Lewis, J.;
Morbidelli, L.; Sousa, S. G.; Worley, C. C.
2017yCat..36040128D Altcode:
GES internal star identifier (CNAME), Sulphur abundances and NLTE
corrections to the Sulphur abundances for 1301 stars. Sulphur
abundances are expressed in the customary logarithmic form:
A(S)=log_10(N(S)/N(H))+12. The abundances delivered are the LTE
ones. NLTEabundances can be determined by directly summing the NLTE
correction delivered: A(S)NLTE = A(S) + NLTEC. So that a
negative NLTE correction indicates that the NLTE abundance is lower
than the LTE one. (1 data file).

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Title: Three-dimensional hydrodynamical CO5BOLD model
atmospheres of red giant stars. V. Oxygen abundance in the metal-poor
giant HD 122563 from OH UV lines
Authors: Prakapavičius, D.; Kučinskas, A.; Dobrovolskas, V.; Klevas,
J.; Steffen, M.; Bonifacio, P.; Ludwig, H. -G.; Spite, M.
2017A&A...599A.128P Altcode: 2016arXiv161103283P
Context. Although oxygen is an important tracer of the early Galactic
evolution, its abundance trends with metallicity are still relatively
poorly known at [Fe/H] ≲ -2.5. This is in part due to a lack of
reliable oxygen abundance indicators in the metal-poor stars, and in
part due to shortcomings in 1D LTE abundance analyses where different
abundance indicators, such as OH lines located in the UV and IR or
the forbidden [O I] line at 630 nm, frequently provide inconsistent
results. Aims: In this study, we determined the oxygen abundance
in the metal-poor halo giant HD 122563 using a 3D hydrodynamical
CO5BOLD model atmosphere. Our main goal was to understand
whether a 3D LTE analysis can help to improve the reliability of oxygen
abundances that are determined from OH UV lines in comparison to those
obtained using standard 1D LTE methodology. Methods: The oxygen
abundance in HD 122563 was determined using 71 OH UV lines located in
the wavelength range between 308-330 nm. The analysis was performed
using a high-resolution VLT UVES spectrum with a 1D LTE spectral line
synthesis performed using the SYNTHE package and classical ATLAS9
model atmosphere. Subsequently, a 3D hydrodynamical CO5BOLD
and 1D hydrostatic LHD model atmospheres were used to compute 3D-1D
abundance corrections. For this, the microturbulence velocity used
with the 1D LHD model atmosphere was derived from the hydrodynamical
CO5BOLD model atmosphere of HD 122563. The obtained abundance
corrections were then applied to determine 3D LTE oxygen abundances from
each individual OH UV line. Results: As in previous studies,
we found trends in the 1D LTE oxygen abundances determined from OH UV
lines with line parameters, such as the line excitation potential, χ,
and the line equivalent width, W. These trends become significantly less
pronounced in 3D LTE. Using OH UV lines, we determined a 3D LTE oxygen
abundance in HD 122563 of A(O)3D LTE = 6.23 ± 0.13 ([O/Fe]
= 0.07 ± 0.13). This is in fair agreement with the oxygen abundance
obtained from OH IR lines, A(O)3D LTE = 6.39 ± 0.11 ([O/Fe]
= 0.23 ± 0.11), but it is noticeably lower than that determined when
using the forbidden [O I] line, A(O)3D LTE = 6.53 ± 0.15
([O/Fe] = 0.37 ± 0.15). While the exact cause of this discrepancy
remains unclear, it is very likely that non-LTE effects may play a
decisive role here. Oxygen-to-iron ratios determined in HD 122563
using OH UV/IR lines and the forbidden [O I] line fall on the lower
boundary of the [O/Fe] distribution as observed in the Galactic field
stars at this metallicity and suggest a very mild oxygen overabundance
with respect to iron, [O/Fe] ≲ 0.4.

---------------------------------------------------------
Title: VizieR Online Data Catalog: NGC104 RGB Na, Mg, and K abundances
(Cerniauskas+, 2017)
Authors: Cerniauskas, A.; Kucinskas, A.; Klevas, J.; Prakapavicius,
D.; Korotin, S.; Bonifacio, P.; Ludwig, H. -G.; Caffau, E.; Steffen, M.
2017yCat..36040035C Altcode:
We used 2dF/HERMES spectra obtained in two wavelength regions,
564.9-587.3nm (GREEN) and 758.5-788.7nm (IR), using the spectral
resolution of R~28000 and exposure time of 1200s. The observations
were carried out during the period of Oct 22 - Dec 20, 2013 (1
data file).

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Title: An in-depth spectroscopic examination of molecular bands from
3D hydrodynamical model atmospheres. II. Carbon-enhanced metal-poor
3D model atmospheres
Authors: Gallagher, A. J.; Caffau, E.; Bonifacio, P.; Ludwig, H. -G.;
Steffen, M.; Homeier, D.; Plez, B.
2017A&A...598L..10G Altcode: 2017arXiv170109102G
Context. Tighter constraints on metal-poor stars we observe are
needed to better understand the chemical processes of the early
Universe. Computing a stellar spectrum in 3D allows one to model complex
stellar behaviours, which cannot be replicated in 1D. Aims:
We examine the effect that the intrinsic CNO abundances have on a 3D
model structure and the resulting 3D spectrum synthesis. Methods:
Model atmospheres were computed in 3D for three distinct CNO chemical
compositions using the CO5BOLD model atmosphere code,
and their internal structures were examined. Synthetic spectra were
computed from these models using Linfor3D and they were compared. New
3D abundance corrections for the G-band and a selection of UV OH lines
were also computed. Results: The varying CNO abundances change
the metal content of the 3D models. This had an effect on the model
structure and the resulting synthesis. However, it was found that
the C/O ratio had a larger effect than the overall metal content of
a model. Conclusions: Our results suggest that varying the
C/O ratio has a substantial impact on the internal structure of the
3D model, even in the hot turn-off star models explored here. This
suggests that bespoke 3D models, for specific CNO abundances should
be sought. Such effects are not seen in 1D at these temperature regimes.

---------------------------------------------------------
Title: 3D non-LTE corrections for the 6Li/7Li
isotopic ratio in solar-type stars
Authors: Harutyunyan, G.; Steffen, M.; Mott, A.; Caffau, E.; Israelian,
G.; González Hernández, J. I.; Strassmeier, K. G.
2017MmSAI..88...61H Altcode:
Doppler shifts induced by convective motions in stellar atmospheres
affect the shape of spectral absorption lines and create slightly
asymmetric line profiles. It is important to take this effect into
account in modeling the subtle depression created by the 6Li
isotope which lies on the red wing of the Li I 670.8 nm resonance
doublet line, since convective motions in stellar atmospheres can mimic
a presence of 6Li when intrinsically symmetric theoretical
line profiles are presumed for the analysis of the 7Li
doublet \citep{cayrel2007}. Based on CO5BOLD hydrodynamical model
atmospheres, we compute 3D non-local thermodynamic equilibrium
(NLTE) corrections for the 6Li/7Li isotopic
ratio by using a grid of 3D NLTE and 1D LTE synthetic spectra. These
corrections must be added to the results of the 1D LTE analysis to
correct them for the combined 3D non-LTE effects. As one would expect,
the resulting corrections are always negative and they range between
0 and -5 %, depending on effective temperature, surface gravity, and
metallicity. For each metallicity we derive an analytic expression
approximating the 3D NLTE corrections as a function of effective
temperature, surface gravity and projected rotational velocity.

---------------------------------------------------------
Title: Radiation transport in CO5BOLD. A short-characteristics module
for local box models
Authors: Steffen, M.
2017MmSAI..88...22S Altcode:
We give a description of a new radiative transfer package based on an
energy-conserving short-characteristics (SC) approach, developed for
both CO5BOLD 3D radiation hydrodynamics local box simulations and NLTE3D
line formation post processing. First test results for a realistic 3D
solar atmosphere model indicate that the thermal structure obtained
with the new radiation transport scheme closely matches the one found
with the standard long-characteristics Feautrier (LCF) method that has
served in CO5BOLD for many years. This is a very reassuring result,
corroborating the validity of the CO5BOLD models computed previously
with the LCF radiation transport.

---------------------------------------------------------
Title: Using CO5BOLD models to predict the effects of granulation
on colours .
Authors: Bonifacio, P.; Caffau, E.; Ludwig, H. -G.; Steffen, M.;
Castelli, F.; Gallagher, A. J.; Prakapavičius, D.; Kučinskas, A.;
Cayrel, R.; Freytag, B.; Plez, B.; Homeier, D.
2017MmSAI..88...90B Altcode:
In order to investigate the effects of granulation on fluxes and
colours, we computed the emerging fluxes from the models in the
CO5BOLD grid with metallicities [M/H]=0.0,-1.0,-2.0 and -3.0. These
fluxes have been used to compute colours in different photometric
systems. We explain here how our computations have been performed and
provide some results.

---------------------------------------------------------
Title: Enhanced methods for computing spectra from CO5BOLD models
using Linfor3D. Molecular bands in metal-poor stars
Authors: Gallagher, A. J.; Steffen, M.; Caffau, E.; Bonifacio, P.;
Ludwig, H. -G.; Freytag, B.
2017MmSAI..88...82G Altcode: 2016arXiv161004427G
Molecular features such as the G-band, CN-band and NH-band are important
diagnostics for measuring a star's carbon and nitrogen abundances,
especially in metal-poor stars where atomic lines are no longer visible
in stellar spectra. Unlike atomic transitions, molecular features
tend to form in bands, which cover large wavelength regions in a
spectrum. While it is a trivial matter to compute carbon and nitrogen
molecular bands under the assumption of 1D, it is extremely time
consuming in 3D. In this contribution to the 2016 COBOLD workshop we
review the improvements made to the 3D spectral synthesis code Linfor3D,
and discuss the new challenges found when computing molecular features
in 3D.

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Title: Lithium in the active sub-giant HD123351. A quantitative
analysis with 3D and 1D model atmospheres using different observed
spectra
Authors: Mott, A.; Steffen, M.; Caffau, E.; Strassmeier, K. G.
2017MmSAI..88...68M Altcode:
Current 3D hydrodynamical model atmosphere simulations together with
non-LTE spectrum synthesis calculations permit to determine reliable
atomic and in particular isotopic chemical abundances. Although this
approach is computationally time demanding, it became feasible in
studying lithium in stellar spectra. In the literature not much is
known about the presence of the more fragile {6Li} isotope
in evolved metal-rich objects. In this case the analysis is complicated
by the lack of a suitable list of atomic and molecular lines in the
spectral region of the lithium resonance line at 670.8 nm. Here we
present a spectroscopic comparative analysis of the Li doublet region
of HD 123351, an active sub-giant star of solar metallicity. We fit
the Li profile in three observed spectra characterized by different
qualities: two very-high resolution spectra (Gecko@CFHT, R=120 000,
SNR=400 and PEPSI@LBT, R=150 000, SNR=663) and a high-resolution
SOPHIE@OHP spectrum (R=40 000, SNR=300). We adopt a set of model
atmospheres, both 3D and 1D, having different stellar parameters
(T_{eff} and log g). The 3D models are taken from the CIFIST grid of
COBOLD model atmospheres and departures from LTE are considered for
the lithium components. For the blends other than the lithium in this
wavelength region we adopt the linelist of \citet{melendez12}. We find
consistent results for all three observations and an overall good fit
with the selected list of atomic and molecular lines, indicating a
high {6Li} content. The presence of {6Li}
is not expected in cool stellar atmospheres. Its detection is of
crucial importance for understanding mixing processes in stars and
external lithium production mechanisms, possibly related to stellar
activity or planetray accretion of {6Li}-rich material.

---------------------------------------------------------
Title: The influence of convection on OH UV line formation in the
atmosphere of the metal-poor red giant HD 122563.
Authors: Prakapavičius, D.; Kučinskas, A.; Dobrovolskas, V.; Klevas,
J.; Steffen, M.; Bonifacio, P.; Ludwig, H. -G.; Spite, M.
2017MmSAI..88...77P Altcode:
We utilized high-resolution spectra of the metal-poor red giant star
HD 122563 and classical 1D hydrostatic ATLAS9 model atmosphere to
derive the 1D LTE oxygen abundance from OH UV lines. The obtained
average 1D LTE oxygen abundance is xtmean {ensuremath {A(O)_{1D
LTE}} = 6.41 ± 0.16. We also used 3D hydrodynamical COBOLD and 1D
hydrostatic LHD model atmospheres to correct the 1D LTE abundances for
convection-related effects and to determine average 3D LTE abundance,
xtmean {ensuremath {A(O)_{3D LTE}} = 6.23 ± 0.13. We found that while
the oxygen abundances determined using 1D hydrostatic model atmospheres
showed trends both with the line strength and excitation potential,
these trends essentially disappeared in the 3D LTE case. The average 3D
LTE oxygen abundance obtained from the OH UV lines agrees reasonably
well with the oxygen abundances obtained in the earlier analyses from
the OH IR lines and the 630.0 nm [O I] line, while the remaining small
discrepancies may possibly be attributed to NLTE effects.

---------------------------------------------------------
Title: Investigation of the solar centre-to-limb variation of oxygen
and lithium spectral features
Authors: Caffau, E.; Malherbe, J. -M.; Steffen, M.; Ludwig, H. -G.;
Mott, A.
2017MmSAI..88...45C Altcode:
We compare intensity spectra of the Sun observed at different limb
angles in the wavelength range covering the forbidden oxygen lines and
the lithium resonance feature with line formation computations performed
on a CO5BOLD 3D hydrodynamical simulation of the solar atmosphere. Among
the prime oxygen abundance indicators, the forbidden line at 630 nm
is contaminated with a significant Ni I blend. The availability of
observations at different positions on the solar disc allows us to
disentangle the contributions of oxygen and nickel and to derive their
individual abundances. We derived in the past, from the [OI] line,
A(O)=8.73± 0.05 with a nickel abundance of A(Ni)=6.1± 0.04. From
the observations here presented, we obtain A(O)=8.71 and A(Ni)=6.09,
in excellent agreement with the previous result. For lithium, we
investigated the Li doublet at 670.7 nm and compared synthetic spectra
of the Li spectra range based on different line-lists available in the
literature to the observed data. With these observations, we are still
unable to conclude on which is the best line-list to be used for the
blending lines.

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Title: TOPoS. III. An ultra iron-poor multiple CEMP system
Authors: Caffau, E.; Bonifacio, P.; Spite, M.; Spite, F.; Monaco, L.;
Sbordone, L.; François, P.; Gallagher, A. J.; Plez, B.; Zaggia, S.;
Ludwig, H. -G.; Cayrel, R.; Koch, A.; Steffen, M.; Salvadori, S.;
Klessen, R.; Glover, S.; Christlieb, N.
2016A&A...595L...6C Altcode: 2016arXiv161004106C
 Aims: One of the primary objectives of the TOPoS survey
is to search for the most metal-poor stars. Our search has led
to the discovery of one of the most iron-poor objects known, SDSS
J092912.32+023817.0. This object is a multiple system, in which two
components are clearly detected in the spectrum. Methods:
We have analysed 16 high-resolution spectra obtained using the UVES
spectrograph at the ESO 8.2 m VLT telescope to measure radial velocities
and determine the chemical composition of the system. Results:
Cross correlation of the spectra with a synthetic template yields a
double-peaked cross-correlation function (CCF) for eight spectra, and
in one case there is evidence for the presence of a third peak. Chemical
analysis of the spectrum obtained by averaging all the spectra for which
the CCF showed a single peak found that the iron abundance is [Fe/H] =
-4.97. The system is also carbon enhanced with [C/Fe] = +3.91 (A(C) =
7.44). From the permitted oxygen triplet we determined an upper limit
for oxygen of [O/Fe] &lt; +3.52 such that C/O &gt; 1.3. We are also
able to provide more stringent upper limits on the Sr and Ba abundances
([Sr/Fe] &lt; +0.70, and [Ba/Fe] &lt; +1.46, respectively). Based
on observations made with ESO Telescopes at the La Silla Paranal
Observatory under programme ID 094.D-0488 and 096.D-0616.

---------------------------------------------------------
Title: An in-depth spectroscopic examination of molecular bands from
3D hydrodynamical model atmospheres. I. Formation of the G-band in
metal-poor dwarf stars
Authors: Gallagher, A. J.; Caffau, E.; Bonifacio, P.; Ludwig, H. -G.;
Steffen, M.; Spite, M.
2016A&A...593A..48G Altcode: 2016arXiv160507215G
Context. Recent developments in the three-dimensional (3D) spectral
synthesis code Linfor3D have meant that for the first time, large
spectral wavelength regions, such as molecular bands, can be synthesised
with it in a short amount of time. Aims: A detailed spectral
analysis of the synthetic G-band for several dwarf turn-off-type 3D
atmospheres (5850 ≲ Teff [ K ] ≲ 6550, 4.0 ≤ log g
≤ 4.5, - 3.0 ≤ [Fe/H] ≤-1.0) was conducted, under the assumption
of local thermodynamic equilibrium. We also examine carbon and oxygen
molecule formation at various metallicity regimes and discuss the impact
it has on the G-band. Methods: Using a qualitative approach,
we describe the different behaviours between the 3D atmospheres and
the traditional one-dimensional (1D) atmospheres and how the different
physics involved inevitably leads to abundance corrections, which
differ over varying metallicities. Spectra computed in 1D were fit to
every 3D spectrum to determine the 3D abundance correction. 
Results: Early analysis revealed that the CH molecules that make up
the G-band exhibited an oxygen abundance dependency; a higher oxygen
abundance leads to weaker CH features. Nitrogen abundances showed zero
impact to CH formation. The 3D corrections are also stronger at lower
metallicity. Analysis of the 3D corrections to the G-band allows us to
assign estimations of the 3D abundance correction to most dwarf stars
presented in the literature. Conclusions: The 3D corrections
suggest that A(C) in carbon-enhanced metal-poor (CEMP) stars with high
A(C) would remain unchanged, but would decrease in CEMP stars with lower
A(C). It was found that the C/O ratio is an important parameter to the
G-band in 3D. Additional testing confirmed that the C/O ratio is an
equally important parameter for OH transitions under 3D. This presents
a clear interrelation between the carbon and oxygen abundances in 3D
atmospheres through their molecular species, which is not seen in 1D.

---------------------------------------------------------
Title: Hydrodynamical model atmospheres: Their impact on stellar
spectroscopy and asteroseismology of late-type stars
Authors: Ludwig, H. -G.; Steffen, M.
2016AN....337..844L Altcode: 2016arXiv160900219L
Hydrodynamical, i.e. multi-dimensional and time-dependent,
model atmospheres of late-type stars have reached a high level
of realism. They are commonly applied in high-fidelity work on
stellar abundances but also allow the study of processes that
are not modelled in standard, one-dimensional hydrostatic model
atmospheres. Here, we discuss two observational aspects that emerge
from such processes, the photometric granulation background and the
spectroscopic microturbulence. We use CO5BOLD hydrodynamical model
atmospheres to characterize the total granular brightness fluctuations
and characteristic time scale for FGK stars. Emphasis is put on the
diagnostic potential of the granulation background for constraining
the fundamental atmospheric parameters. We find a clear metallicity
dependence of the granulation background. The comparison between
the model predictions and available observational constraints at
solar metallicity shows significant differences, that need further
clarification. Concerning microturbulence, we report on the derivation
of a theoretical calibration based on CO5BOLD models, which shows
good correspondence with the measurements for stars in the Hyades. We
emphasize the importance of a consistent procedure when determining
the microturbulence, and point to limitations of the commonly applied
description of microturbulence in hydrostatic model atmospheres.

---------------------------------------------------------
Title: Reconstructing the Milky Way's history: spectroscopic surveys,
asteroseismology, and chemodynamical models
Authors: Chiappini, Cristina; Montalbán, Josefina; Steffen, Matthias
2016AN....337..773C Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Investigation of the lithium 670.7 nm wavelength range in
the solar spectrum
Authors: Caffau, Elisabetta; Mott, Alessandro; Harutyunyan, Gohar;
Malherbe, Jean-Marie; Steffen, Matthias
2016cosp...41E.281C Altcode:
Lithium is a key chemical element, with a chemical evolution that is
different from that of most other elements. It is also very fragile,
as it is destroyed by nuclear reactions with protons at temperatures
higher than about 2.5 million K. According to standard Big Bang
nucleosynthesis, only the isotope 7Li is produced in significant
amounts, while the primordial abundance of the lighter isotope 6Li
is negligible. Lithium is not produced by nucleosynthesis in normal
stars, except in peculiar phases of stellar evolution (e.g. in AGB
stars and Novae). Lithium may also be formed as a result of flares in
the atmospheres of young, active stars. To investigate the history
of Li production and depletion in the Galaxy, it is necessary to
analyse stars of all ages, including those at solar metallicity. In
this case, the spectroscopic determination of the Li abundance is
complicated by the presence of other spectral lines overlapping
with the Li doublet at 670.7 nm. The correct identification and
knowledge of the atomic parameters of these blend lines is critical,
especially if the 6LI/7Li isotopic ratio is to be derived. In this
investigation, we consider several line lists of the blending components
available in the literature and use them to compute synthetic spectra,
performing the line formation computations both for the classical 1D
Holweger-Mueller model and a CO5BOLD 3D hydrodynamical simulation of the
solar atmosphere. The synthetic spectra are then compared to the solar
spectrum observed at different limb angles. This allows us to check the
quality of existing line lists, to find potentially misidentified blend
lines, and to construct an optimized line list for solar-type stars.

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Title: The Central Role of FORS1/2 Spectropolarimetric Observations
for the Progress of Stellar Magnetism Studies
Authors: Schöller, M.; Hubrig, S.; Ilyin, I.; Steffen, M.; Briquet,
M.; Kholtygin, A. F.
2016Msngr.163...21S Altcode:
The spectropolarimetric mode of the FOcal Reducer and low dispersion
Spectrographs (FORS), which was first implemented in FORS1, and then
moved to FORS2 seven years ago, has made it possible to probe the
presence of magnetic fields in stars of different spectral classes
at almost all stages of stellar evolution. While in the early days of
FORS1, many of the observations were related to magnetic Ap/Bp stars
and their progenitor Herbig Ae/Be stars, recent spectropolarimetric
studies with FORS2 have involved more challenging targets, such as
massive O- and B-type stars in clusters and in the field, very fast
rotating massive stars with magnetospheres, Wolf-Rayet stars and
central stars of planetary nebulae. The role of FORS observations for
stellar magnetic field measurements is summarised and improvements in
the measurement technique are described.

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Title: Hot bubbles of planetary nebulae with hydrogen-deficient
winds. I. Heat conduction in a chemically stratified plasma
Authors: Sandin, C.; Steffen, M.; Schönberner, D.; Rühling, U.
2016A&A...586A..57S Altcode: 2015arXiv151204986S
Heat conduction has been found a plausible solution to explain
discrepancies between expected and measured temperatures in hot bubbles
of planetary nebulae (PNe). While the heat conduction process depends
on the chemical composition, to date it has been exclusively studied
for pure hydrogen plasmas in PNe. A smaller population of PNe show
hydrogen-deficient and helium- and carbon-enriched surfaces surrounded
by bubbles of the same composition; considerable differences are
expected in physical properties of these objects in comparison to
the pure hydrogen case. The aim of this study is to explore how
a chemistry-dependent formulation of the heat conduction affects
physical properties and how it affects the X-ray emission from PN
bubbles of hydrogen-deficient stars. We extend the description of
heat conduction in our radiation hydrodynamics code to work with
any chemical composition. We then compare the bubble-formation
process with a representative PN model using both the new and the
old descriptions. We also compare differences in the resulting X-ray
temperature and luminosity observables of the two descriptions. The
improved equations show that the heat conduction in our representative
model of a hydrogen-deficient PN is nearly as efficient with the
chemistry-dependent description; a lower value on the diffusion
coefficient is compensated by a slightly steeper temperature
gradient. The bubble becomes somewhat hotter with the improved
equations, but differences are otherwise minute. The observable
properties of the bubble in terms of the X-ray temperature and
luminosity are seemingly unaffected.

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Title: Lithium spectral line formation in stellar atmospheres. The
impact of convection and NLTE effects
Authors: Klevas, J.; Kučinskas, A.; Steffen, M.; Caffau, E.; Ludwig,
H. -G.
2016A&A...586A.156K Altcode: 2015arXiv151208999K
 Aims: Because of the complexities involved in treating
spectral line formation in full 3D and non-local thermodynamic
equilibrium (NLTE), different simplified approaches are sometimes
used to account for the NLTE effects with 3D hydrodynamical model
atmospheres. In certain cases, chemical abundances are derived in
1D NLTE and then corrected for the 3D effects by adding 3D-1D LTE
(Local Thermodynamic Equilibrium, LTE) abundance corrections (3D+NLTE
approach). Alternatively, average ⟨3D⟩ model atmospheres are
sometimes used to substitute for the full 3D hydrodynamical models. Methods: In this work we tested whether the results obtained using
these simplified schemes (3D+NLTE, ⟨3D⟩ NLTE) may reproduce those
derived using the full 3D NLTE computations. The tests were made using
3D hydrodynamical CO5BOLD model atmospheres of the main
sequence (MS), main sequence turn-off (TO), subgiant (SGB), and red
giant branch (RGB) stars, all at two metallicities, [ M / H ] = 0.0
and -2.0. Our goal was to investigate the role of 3D and NLTE effects
on the formation of the 670.8 nm lithium resonance line. This was done
by assessing differences in the strengths of synthetic 670.8 nm line
profiles, which were computed using 3D/1D NLTE/LTE approaches. 
Results: Our results show that Li 670.8 nm line strengths obtained
using different methodologies differ only slightly in most of the
models at solar metallicity studied here. However, the line strengths
predicted with the 3D NLTE and 3D+NLTE approaches become significantly
different at subsolar metallicities. At [ M / H ] = -2.0, this may lead
to (3D NLTE) - (3D+NLTE) differences in the predicted lithium abundance
of ~0.46 and ~0.31 dex in the TO and RGB stars respectively. On the
other hand, NLTE line strengths computed with the average ⟨3D⟩ and
1D model atmospheres are similar to those obtained with the full 3D
NLTE approach for MS, TO, SGB, and RGB stars, at all metallicities;
3D - ⟨3D⟩ and 3D - 1D differences in the predicted abundances
are always less than ~0.04 dex and ~0.08 dex, respectively. However,
neither of the simplified approaches can reliably substitute 3D NLTE
spectral synthesis when precision is required.

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Title: Consistent metallicity scale for cool dwarfs and giants. A
benchmark test using the Hyades
Authors: Dutra-Ferreira, L.; Pasquini, L.; Smiljanic, R.; Porto de
Mello, G. F.; Steffen, M.
2016A&A...585A..75D Altcode: 2015arXiv150907725D
Context. In several instances chemical abundances of main-sequence
and giant stars are used simultaneously under the assumption that they
share the same abundance scale. This assumption, if wrong, might have
important implications in different astrophysical contexts. 
Aims: It is therefore crucial to understand whether the metallicity
or abundance differences among dwarfs and giants are real or are
produced by systematic errors in the analysis. We aim to ascertain
a methodology capable of producing a consistent metallicity scale
for giants and dwarfs. Methods: To achieve that, we analyzed
giants and dwarfs in the Hyades open cluster, under the assumption
that they share the same chemical composition. All the stars in this
cluster have archival high-resolution spectroscopic data obtained
with HARPS and UVES. In addition, the giants have interferometric
measurements of the angular diameters. We analyzed the sample with
two methods. The first method constrains the atmospheric parameters
independently from spectroscopic method. For that we present a novel
calibration of microturbulence based on 3D model atmospheres. The
second method is the classical spectroscopic analysis based on Fe
lines. We also tested two different line lists in an attempt to
minimize possible non-LTE effects and to optimize the treatment of
the giants. Results: We show that it is possible to obtain a
consistent metallicity scale between dwarfs and giants. The preferred
method should constrain the three parameters Teff, log g,
and ξ independent of spectroscopy. A careful selection of Fe lines is
also important. In particular, the lines should not be chosen based
on the Sun or other dwarfs, but specifically to be free of blends in
the spectra of giants. When attention is paid to the line list, the
classical spectroscopic method can also produce consistent results. In
our test, the metallicities derived with the well-constrained set
of stellar parameters are consistent independent of the line list
used. Therefore, for this cluster we favor the metallicity of +0.18
± 0.03 dex obtained with this method. The classical spectroscopic
analysis, using the line list optimized for the giants, provides a
metallicity of +0.14 ± 0.03 dex, in agreement with previous works. Based on data obtained from the ESO Science Archive Facility. The
observations were made with ESO Telescopes at the La Silla and Paranal
Observatories under programmes ID 070.D-0356, 088.C-0513 and 070.D-0421.

---------------------------------------------------------
Title: SSALMON - The Solar Simulations for the Atacama Large
Millimeter Observatory Network
Authors: Wedemeyer, S.; Bastian, T.; Brajša, R.; Barta, M.; Hudson,
H.; Fleishman, G.; Loukitcheva, M.; Fleck, B.; Kontar, E.; De Pontieu,
B.; Tiwari, S.; Kato, Y.; Soler, R.; Yagoubov, P.; Black, J. H.;
Antolin, P.; Gunár, S.; Labrosse, N.; Benz, A. O.; Nindos, A.;
Steffen, M.; Scullion, E.; Doyle, J. G.; Zaqarashvili, T.; Hanslmeier,
A.; Nakariakov, V. M.; Heinzel, P.; Ayres, T.; Karlicky, M.
2015AdSpR..56.2679W Altcode: 2015arXiv150205601W
The Solar Simulations for the Atacama Large Millimeter Observatory
Network (SSALMON) was initiated in 2014 in connection with two ALMA
development studies. The Atacama Large Millimeter/submillimeter Array
(ALMA) is a powerful new tool, which can also observe the Sun at
high spatial, temporal, and spectral resolution. The international
SSALMONetwork aims at co-ordinating the further development of solar
observing modes for ALMA and at promoting scientific opportunities
for solar physics with particular focus on numerical simulations,
which can provide important constraints for the observing modes and
can aid the interpretation of future observations. The radiation
detected by ALMA originates mostly in the solar chromosphere - a
complex and dynamic layer between the photosphere and corona, which
plays an important role in the transport of energy and matter and the
heating of the outer layers of the solar atmosphere. Potential targets
include active regions, prominences, quiet Sun regions, flares. Here,
we give a brief overview over the network and potential science cases
for future solar observations with ALMA.

---------------------------------------------------------
Title: Chemical composition of a sample of bright solar-metallicity
stars
Authors: Caffau, E.; Mott, A.; Steffen, M.; Bonifacio, P.; Strassmeier,
K. G.; Gallagher, A.; Faraggiana, R.; Sbordone, L.
2015AN....336..968C Altcode: 2015arXiv151004269C
We present a detailed analysis of seven young stars observed with the
spectrograph SOPHIE at the Observatoire de Haute-Provence for which the
chemical composition was incomplete or absent in the literature. For
five stars, we derived the stellar parameters and chemical compositions
using our automatic pipeline optimized for F, G, and K stars, while
for the other two stars with high rotational velocity, we derived the
stellar parameters by using other information (parallax), and performed
a line-by-line analysis. Chromospheric emission-line fluxes from Ca
II are obtained for all targets. The stellar parameters we derive are
generally in good agreement with what is available in the literature. We
provide a chemical analysis of two of the stars for the first time. The
star HIP 80124 shows a strong Li feature at 670.8 nm implying a high
lithium abundance. Its chemical pattern is not consistent with it
being a solar sibling, as has been suggested. Data obtained at
Observatoire de Haute Provence, with the SOPHIE spectrograph.

---------------------------------------------------------
Title: The photospheric solar oxygen project. IV. 3D-NLTE
investigation of the 777 nm triplet lines
Authors: Steffen, M.; Prakapavičius, D.; Caffau, E.; Ludwig, H. -G.;
Bonifacio, P.; Cayrel, R.; Kučinskas, A.; Livingston, W. C.
2015A&A...583A..57S Altcode: 2015arXiv150803487S
Context. The solar photospheric oxygen abundance is still widely
debated. Adopting the solar chemical composition based on the "low"
oxygen abundance, as determined with the use of three-dimensional (3D)
hydrodynamical model atmospheres, results in a well-known mismatch
between theoretical solar models and helioseismic measurements
that is so far unresolved. Aims: We carry out an independent
redetermination of the solar oxygen abundance by investigating the
center-to-limb variation of the O i IR triplet lines at 777 nm in
different sets of spectra. Methods: The high-resolution and high
signal-to-noise solar center-to-limb spectra are analyzed with the
help of detailed synthetic line profiles based on 3D hydrodynamical
CO5BOLD model atmospheres and 3D non-LTE line formation calculations
with NLTE3D. The idea is to exploit the information contained in the
observations at different limb angles to simultaneously derive the
oxygen abundance, A(O), and the scaling factor SH that
describes the cross-sections for inelastic collisions with neutral
hydrogen relative to the classical Drawin formula. Using the same
codes and methods, we compare our 3D results with those obtained from
the semi-empirical Holweger-Müller model atmosphere as well as from
different one-dimensional (1D) reference models. Results: With
the CO5BOLD 3D solar model, the best fit of the center-to-limb variation
of the triplet lines is obtained when the collisions by neutral hydrogen
atoms are assumed to be efficient, i.e., when the scaling factor
SH is between 1.2 and 1.8, depending on the choice of the
observed spectrum and the triplet component used in the analysis. The
line profile fits achieved with standard 1D model atmospheres (with
fixed microturbulence, independent of disk position μ) are clearly
of inferior quality compared to the 3D case, and give the best match
to the observations when ignoring collisions with neutral hydrogen
(SH = 0). The results derived with the Holweger-Müller model
are intermediate between 3D and standard 1D. Conclusions: The
analysis of various observations of the triplet lines with different
methods yields oxygen abundance values (on a logarithmic scale where
A(H) = 12) that fall in the range 8.74 &lt;A(O) &lt; 8.78, and our
best estimate of the 3D non-LTE solar oxygen abundance is A(O) = 8.76
± 0.02. All 1D non-LTE models give much lower oxygen abundances,
by up to -0.15 dex. This is mainly a consequence of the assumption
of a μ-independent microturbulence. An independent determination of
the relevant collisional cross-sections is essential to substantially
improve the accuracy of the oxygen abundance derived from the O i IR
triplet. Appendices E and F are available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201526406/olm">http://www.aanda.org</A>

---------------------------------------------------------
Title: On the Evolution of Magnetic White Dwarfs
Authors: Tremblay, P. -E.; Fontaine, G.; Freytag, B.; Steiner, O.;
Ludwig, H. -G.; Steffen, M.; Wedemeyer, S.; Brassard, P.
2015ApJ...812...19T Altcode: 2015arXiv150905398T
We present the first radiation magnetohydrodynamic simulations of the
atmosphere of white dwarf stars. We demonstrate that convective energy
transfer is seriously impeded by magnetic fields when the plasma-β
parameter, the thermal-to-magnetic-pressure ratio, becomes smaller
than unity. The critical field strength that inhibits convection
in the photosphere of white dwarfs is in the range B = 1-50 kG,
which is much smaller than the typical 1-1000 MG field strengths
observed in magnetic white dwarfs, implying that these objects have
radiative atmospheres. We have employed evolutionary models to study the
cooling process of high-field magnetic white dwarfs, where convection
is entirely suppressed during the full evolution (B ≳ 10 MG). We
find that the inhibition of convection has no effect on cooling rates
until the effective temperature (Teff) reaches a value of
around 5500 K. In this regime, the standard convective sequences start
to deviate from the ones without convection due to the convective
coupling between the outer layers and the degenerate reservoir of
thermal energy. Since no magnetic white dwarfs are currently known
at the low temperatures where this coupling significantly changes the
evolution, the effects of magnetism on cooling rates are not expected
to be observed. This result contrasts with a recent suggestion
that magnetic white dwarfs with Teff ≲ 10,000 K cool
significantly slower than non-magnetic degenerates.

---------------------------------------------------------
Title: 3D Model Atmospheres for Extremely Low-mass White Dwarfs
Authors: Tremblay, P. -E.; Gianninas, A.; Kilic, M.; Ludwig, H. -G.;
Steffen, M.; Freytag, B.; Hermes, J. J.
2015ApJ...809..148T Altcode: 2015arXiv150701927T
We present an extended grid of mean three-dimensional (3D) spectra
for low-mass, pure-hydrogen atmosphere DA white dwarfs (WDs). We use
CO5BOLD radiation-hydrodynamics 3D simulations covering Teff
= 6000-11,500 K and log g = 5-6.5 (g in cm s-2) to derive
analytical functions to convert spectroscopically determined 1D
temperatures and surface gravities to 3D atmospheric parameters. Along
with the previously published 3D models, the 1D to 3D corrections are
now available for essentially all known convective DA WDs (i.e., log g
= 5-9). For low-mass WDs, the correction in temperature is relatively
small (a few percent at the most), but the surface gravities measured
from the 3D models are lower by as much as 0.35 dex. We revisit
the spectroscopic analysis of the extremely low-mass (ELM) WDs, and
demonstrate that the 3D models largely resolve the discrepancies seen
in the radius and mass measurements for relatively cool ELM WDs in
eclipsing double WD and WD + millisecond pulsar binary systems. We
also use the 3D corrections to revise the boundaries of the ZZ Ceti
instability strip, including the recently found ELM pulsators.

---------------------------------------------------------
Title: The photospheric solar oxygen project. III. Investigation of
the centre-to-limb variation of the 630 nm [O I]-Ni I blend
Authors: Caffau, E.; Ludwig, H. -G.; Steffen, M.; Livingston, W.;
Bonifacio, P.; Malherbe, J. -M.; Doerr, H. -P.; Schmidt, W.
2015A&A...579A..88C Altcode: 2015arXiv150600931C
Context. The solar photospheric abundance of oxygen is still a matter
of debate. For about ten years some determinations have favoured a
low oxygen abundance which is at variance with the value inferred by
helioseismology. Among the oxygen abundance indicators, the forbidden
line at 630 nm has often been considered the most reliable even
though it is blended with a Ni i line. In Papers I and II of this
series we reported a discrepancy in the oxygen abundance derived
from the 630 nm and the subordinate [O I] line at 636 nm in dwarf
stars, including the Sun. Aims: Here we analyse several,
in part new, solar observations of the centre-to-limb variation
of the spectral region including the blend at 630 nm in order to
separate the individual contributions of oxygen and nickel. 
Methods: We analyse intensity spectra observed at different limb
angles in comparison with line formation computations performed on a
CO5BOLD 3D hydrodynamical simulation of the solar atmosphere. 
Results: The oxygen abundances obtained from the forbidden line at
different limb angles are inconsistent if the commonly adopted nickel
abundance of 6.25 is assumed in our local thermodynamic equilibrium
computations. With a slightly lower nickel abundance, A(Ni) ≈ 6.1,
we obtain consistent fits indicating an oxygen abundance of A(O) = 8.73
± 0.05. At this value the discrepancy with the subordinate oxygen
line remains. Conclusions: The derived value of the oxygen
abundance supports the notion of a rather low oxygen abundance in the
solar photosphere. However, it is disconcerting that the forbidden
oxygen lines at 630 and 636 nm give noticeably different results,
and that the nickel abundance derived here from the 630 nm blend is
lower than expected from other nickel lines.

---------------------------------------------------------
Title: TOPoS . II. On the bimodality of carbon abundance in CEMP
stars Implications on the early chemical evolution of galaxies
Authors: Bonifacio, P.; Caffau, E.; Spite, M.; Limongi, M.; Chieffi,
A.; Klessen, R. S.; François, P.; Molaro, P.; Ludwig, H. -G.; Zaggia,
S.; Spite, F.; Plez, B.; Cayrel, R.; Christlieb, N.; Clark, P. C.;
Glover, S. C. O.; Hammer, F.; Koch, A.; Monaco, L.; Sbordone, L.;
Steffen, M.
2015A&A...579A..28B Altcode: 2015arXiv150405963B
Context. In the course of the Turn Off Primordial Stars (TOPoS) survey,
aimed at discovering the lowest metallicity stars, we have found several
carbon-enhanced metal-poor (CEMP) stars. These stars are very common
among the stars of extremely low metallicity and provide important
clues to the star formation processes. We here present our analysis
of six CEMP stars. Aims: We want to provide the most complete
chemical inventory for these six stars in order to constrain the
nucleosynthesis processes responsible for the abundance patterns. Methods: We analyse both X-Shooter and UVES spectra acquired at the
VLT. We used a traditional abundance analysis based on OSMARCS 1D local
thermodynamic equilibrium (LTE) model atmospheres and the turbospectrum
line formation code. Results: Calcium and carbon are the only
elements that can be measured in all six stars. The range is -5.0 ≤
[Ca/H] &lt;-2.1 and 7.12 ≤ A(C) ≤ 8.65. For star SDSS J1742+2531
we were able to detect three Fe i lines from which we deduced [Fe/H]
= -4.80, from four Ca ii lines we derived [Ca/H] = -4.56, and from
synthesis of the G-band we derived A(C) = 7.26. For SDSS J1035+0641 we
were not able to detect any iron lines, yet we could place a robust
(3σ) upper limit of [Fe/H] &lt; -5.0 and measure the Ca abundance,
with [Ca/H] = -5.0, and carbon, A(C) = 6.90, suggesting that this star
could be even more metal-poor than SDSS J1742+2531. This makes these
two stars the seventh and eighth stars known so far with [Fe/H] &lt;
-4.5, usually termed ultra-iron-poor (UIP) stars. No lithium is detected
in the spectrum of SDSS J1742+2531 or SDSS J1035+0641, which implies a
robust upper limit of A(Li) &lt; 1.8 for both stars. Conclusions:
Our measured carbon abundances confirm the bimodal distribution of
carbon in CEMP stars, identifying a high-carbon band and a low-carbon
band. We propose an interpretation of this bimodality according to which
the stars on the high-carbon band are the result of mass transfer from
an AGB companion, while the stars on the low-carbon band are genuine
fossil records of a gas cloud that has also been enriched by a faint
supernova (SN) providing carbon and the lighter elements. The abundance
pattern of the UIP stars shows a large star-to-star scatter in the
[X/Ca] ratios for all elements up to aluminium (up to 1 dex), but
this scatter drops for heavier elements and is at most of the order
of a factor of two. We propose that this can be explained if these
stars are formed from gas that has been chemically enriched by several
SNe, that produce the roughly constant [X/Ca] ratios for the heavier
elements, and in some cases the gas has also been polluted by the
ejecta of a faint SN that contributes the lighter elements in variable
amounts. The absence of lithium in four of the five known unevolved
UIP stars can be explained by a dominant role of fragmentation in the
formation of these stars. This would result either in a destruction
of lithium in the pre-main-sequence phase, through rotational mixing
or to a lack of late accretion from a reservoir of fresh gas. The
phenomenon should have varying degrees of efficiency. Based on
observations obtained at ESO Paranal Observatory, programme 091.D-0288,
091.D-0305, 189.D-0165.Appendix A is available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201425266/olm">http://www.aanda.org</A>Tables
4 is only available at the CDS via anonymous ftp to <A
href="http://cdsarc.u-strasbg.fr">http://cdsarc.u-strasbg.fr</A>
(ftp://130.79.128.5) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/579/A28">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/579/A28</A>

---------------------------------------------------------
Title: Calibration of the Mixing-Length Free Parameter for White
Dwarf Structures
Authors: Tremblay, P. -E.; Ludwig, H. -G.; Freytag, B.; Fontaine,
G.; Steffen, M.; Brassard, P.
2015ASPC..493...89T Altcode:
We present a comparison of our grid of 3D radiation-hydrodynamical
simulations for 70 pure-hydrogen DA white dwarfs, in the surface
gravity range 7.0 ≤log g≤ 9.0, with 1D envelope models based on the
mixing-length theory (MLT) for convection. We perform a calibration
of the mixing-length parameter for the lower part of the convection
zone. The 3D simulations are often restricted to the upper convective
layers, and in those cases, we rely on the asymptotic entropy value of
the adiabatic 3D upflows to calibrate 1D envelopes. Our results can be
applied to 1D structure calculations, and in particular for pulsation
and convective mixing studies. We demonstrate that while the 1D MLT
only provides a bottom boundary of the convection zone based on the
Schwarzschild criterion, the 3D stratifications are more complex. There
is a large overshoot region below the convective layers that is likely
critical for chemical diffusion applications.

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Title: The Born-again Planetary Nebulae Abell 30 and Abell 78
Authors: Todt, H.; Guerrero, M. A.; Fang, X.; Toala, J. A.;
Arthur, J. S.; Blair, W. P.; Chu, Y. -H.; Gruendl, R. A.; Hamann,
W. -R.; Marquez-Lugo, R. A.; Oskinova, L.; Ruiz, N.; Steffen, M.;
Schoenberner, D.
2015ASPC..493..141T Altcode:
The planetary nebulae Abell 30 and Abell 78 are born-again nebulae,
which are believed to have undergone a very late thermal pulse,
resulting in the ejection of hydrogen-poor material. Born-again PNe
are a rare phenomenon, only a few are known. It is really intriguing
that in all cases the expansion of the hydrogen-poor ejecta is highly
asymmetrical. Here we present new HST observations of the expansion
of the ejecta in Abell 30 and Abell 78, and compare them to ≍20 yrs
older HST images.

---------------------------------------------------------
Title: VizieR Online Data Catalog: Abundances of 3 CEMP stars
(Bonifacio+, 2015)
Authors: Bonifacio, P.; Caffau, E.; Spite, M.; Limongi, M.; Chieffi,
A.; Klessen, R. S.; Francois, P.; Molaro, P.; Ludwig, H. -G.; Zaggia,
S.; Spite, F.; Plez, B.; Cayrel, R.; Christlieb, N.; Clark, P. C.;
Glover, S. C. O.; Hammer, F.; Koch, A.; Monaco, L.; Sbordone, L.;
Steffen, M.
2015yCat..35790028B Altcode:
We analyse both X-Shooter and UVES spectra acquired at the VLT. We used
a traditional abundance analysis based on OSMARCS 1D Local Thermodynamic
Equilibrium (LTE) model atmospheres and the TURBOSPECTRUM line formation
code. (2 data files).

---------------------------------------------------------
Title: Three-dimensional hydrodynamical CO5BOLD model
atmospheres of red giant stars. IV. Oxygen diagnostics in extremely
metal-poor red giants with infrared OH lines
Authors: Dobrovolskas, V.; Kučinskas, A.; Bonifacio, P.; Caffau,
E.; Ludwig, H. -G.; Steffen, M.; Spite, M.
2015A&A...576A.128D Altcode: 2015arXiv150206587D
Context. Although oxygen is an important tracer of Galactic chemical
evolution, measurements of its abundance in the atmospheres of the
oldest Galactic stars are still scarce and rather imprecise. This
is mainly because only a few spectral lines are available for the
abundance diagnostics. At the lowest end of the metallicity scale,
oxygen can only be measured in giant stars and in most of cases such
measurements rely on a single forbidden [O i] 630 nm line that is very
weak and frequently blended with telluric lines. Although molecular
OH lines located in the ultraviolet and infrared could also be used
for the diagnostics, oxygen abundances obtained from the OH lines and
the [O i] 630 nm line are usually discrepant to a level of ~ 0.3-0.4
dex. Aims: We study the influence of convection on the formation
of the infrared (IR) OH lines and the forbidden [O i] 630 nm line in the
atmospheres of extremely metal-poor (EMP) red giant stars. Our ultimate
goal is to clarify whether a realistic treatment of convection with
state-of-the-art 3D hydrodynamical model atmospheres may help to bring
the oxygen abundances obtained using the two indicators into closer
agreement. Methods: We used high-resolution (R = 50 000) and high
signal-to-noise ratio (S/N ≈ 200-600) spectra of four EMP red giant
stars obtained with the VLT CRIRES spectrograph. For each EMP star,
4-14 IR OH vibrational-rotational lines located in the spectral range of
1514-1548 and 1595-1632 nm were used to determine oxygen abundances by
employing standard 1D local thermodynamic equilibrium (LTE) abundance
analysis methodology. We then corrected the 1D LTE abundances
obtained from each individual OH line for the 3D hydrodynamical
effects, which was done by applying 3D-1D LTE abundance corrections
that were determined using 3D hydrodynamical CO5BOLD and
1D hydrostatic LHD model atmospheres. Results: We find that
the influence of convection on the formation of [O i] 630 nm line in
the atmospheres of EMP giants studied here is minor, which leads to
very small 3D-1D abundance corrections (Δ3D-1D ≤ -0.01
dex). On the contrary, IR OH lines are strongly affected by convection
and thus the abundance corrections for these lines are significant,
Δ3D-1D ≈ -0.2···-0.3 dex. These abundance corrections
do indeed bring the 1D LTE oxygen abundances of EMP red giants obtained
using IR OH lines into better agreement with those determined from the
[O i] 630 nm line. Since in the EMP red giants IR OH lines are typically
at least a factor of two stronger than the [O i] line, OH lines may be
useful indicators of oxygen abundances in the EMP stars, provided that
the analysis is based on 3D hydrodynamical model atmospheres. Based
on observations obtained at the European Southern Observatory (ESO)
Very Large Telescope (VLT) at Paranal Observatory, Chile (observing
programme 089.D-0079).Appendices are available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201424885/olm">http://www.aanda.org</A>

---------------------------------------------------------
Title: The Chandra Planetary Nebula Survey (ChanPlaNS). III. X-Ray
Emission from the Central Stars of Planetary Nebulae
Authors: Montez, R., Jr.; Kastner, J. H.; Balick, B.; Behar, E.;
Blackman, E.; Bujarrabal, V.; Chu, Y. -H.; Corradi, R. L. M.; De Marco,
O.; Frank, A.; Freeman, M.; Frew, D. J.; Guerrero, M. A.; Jones, D.;
Lopez, J. A.; Miszalski, B.; Nordhaus, J.; Parker, Q. A.; Sahai, R.;
Sandin, C.; Schonberner, D.; Soker, N.; Sokoloski, J. L.; Steffen,
M.; Toalá, J. A.; Ueta, T.; Villaver, E.; Zijlstra, A.
2015ApJ...800....8M Altcode: 2014arXiv1412.2793M
We present X-ray spectral analysis of 20 point-like X-ray sources
detected in Chandra Planetary Nebula Survey observations of 59 planetary
nebulae (PNe) in the solar neighborhood. Most of these 20 detections
are associated with luminous central stars within relatively young,
compact nebulae. The vast majority of these point-like X-ray-emitting
sources at PN cores display relatively "hard" (&gt;=0.5 keV) X-ray
emission components that are unlikely to be due to photospheric emission
from the hot central stars (CSPN). Instead, we demonstrate that these
sources are well modeled by optically thin thermal plasmas. From the
plasma properties, we identify two classes of CSPN X-ray emission:
(1) high-temperature plasmas with X-ray luminosities, L X,
that appear uncorrelated with the CSPN bolometric luminosity, L
bol and (2) lower-temperature plasmas with L X/L
bol ~ 10-7. We suggest these two classes
correspond to the physical processes of magnetically active binary
companions and self-shocking stellar winds, respectively. In many cases
this conclusion is supported by corroborative multiwavelength evidence
for the wind and binary properties of the PN central stars. By thus
honing in on the origins of X-ray emission from PN central stars,
we enhance the ability of CSPN X-ray sources to constrain models of
PN shaping that invoke wind interactions and binarity.

---------------------------------------------------------
Title: Calibration of the Mixing-length Theory for Convective White
Dwarf Envelopes
Authors: Tremblay, P. -E.; Ludwig, H. -G.; Freytag, B.; Fontaine,
G.; Steffen, M.; Brassard, P.
2015ApJ...799..142T Altcode: 2014arXiv1412.1789T
A calibration of the mixing-length parameter in the local mixing-length
theory (MLT) is presented for the lower part of the convection zone in
pure-hydrogen-atmosphere white dwarfs. The parameterization is performed
from a comparison of three-dimensional (3D) CO5BOLD simulations with
a grid of one-dimensional (1D) envelopes with a varying mixing-length
parameter. In many instances, the 3D simulations are restricted to the
upper part of the convection zone. The hydrodynamical calculations
suggest, in those cases, that the entropy of the upflows does not
change significantly from the bottom of the convection zone to regions
immediately below the photosphere. We rely on this asymptotic entropy
value, characteristic of the deep and adiabatically stratified layers,
to calibrate 1D envelopes. The calibration encompasses the convective
hydrogen-line (DA) white dwarfs in the effective temperature range
6000 &lt;= T eff (K) &lt;=15, 000 and the surface gravity
range 7.0 &lt;= log g &lt;= 9.0. It is established that the local
MLT is unable to reproduce simultaneously the thermodynamical, flux,
and dynamical properties of the 3D simulations. We therefore propose
three different parameterizations for these quantities. The resulting
calibration can be applied to structure and envelope calculations,
in particular for pulsation, chemical diffusion, and convective mixing
studies. On the other hand, convection has no effect on the white dwarf
cooling rates until there is a convective coupling with the degenerate
core below T eff ~ 5000 K. In this regime, the 1D structures
are insensitive to the MLT parameterization and converge to the mean
3D results, hence they remain fully appropriate for age determinations.

---------------------------------------------------------
Title: Oxygen in the Early Galaxy: OH Lines as Tracers of Oxygen
Abundance in Extremely Metal-Poor Giant Stars
Authors: Kucinskas, A.; Dobrovolskas, V.; Bonifacio, P.; Caffau, E.;
Ludwig, H. -G.; Steffen, M.; Spite, M.
2015csss...18..327K Altcode: 2014arXiv1409.3153K
Oxygen is a powerful tracer element of Galactic chemical
evolution. Unfortunately, only a few oxygen lines are available in the
ultraviolet-infrared stellar spectra for the reliable determination of
its abundance. Moreover, oxygen abundances obtained using different
spectral lines often disagree significantly. In this contribution we
therefore investigate whether the inadequate treatment of convection in
1D hydrostatic model atmospheres used in the abundance determinations
may be responsible for this disagreement. For this purpose, we used VLT
CRIRES spectra of three EMP giants, as well as 3D hydrodynamical COBOLD
and 1D hydrostatic LHD model atmospheres, to investigate the role of
convection in the formation of infrared (IR) OH lines. Our results show
that the presence of convection leads to significantly stronger IR OH
lines. As a result, the difference in the oxygen abundance determined
from IR OH lines with 3D hydrodynamical and classical 1D hydrostatic
model atmospheres may reach -0.2 dots -0.3 dex. In case of the three
EMP giants studied here, we obtain a good agrement between the 3D LTE
oxygen abundances determined by us using vibrational-rotational IR
OH lines in the spectral range of 1514-1626 nm, and oxygen abundances
determined from forbidden [O I] 630 nm line in previous studies.

---------------------------------------------------------
Title: Properties of small-scale magnetism of stellar atmospheres
Authors: Steiner, Oskar; Salhab, René; Freytag, Bernd; Rajaguru,
Paul; Schaffenberger, Werner; Steffen, Matthias
2014PASJ...66S...5S Altcode: 2014PASJ..tmp...95S
The magnetic field outside of sunspots is concentrated in the
intergranular space, where it forms a delicate filigree of bright
ribbons and dots as seen on broad band images of the Sun. We expect this
small-scale magnetic field to exhibit a similar behavior in stellar
atmospheres. In order to find out more about it, we perform numerical
simulations of the surface layers of stellar atmospheres. Here, we
report on preliminary results from simulations in the range between
4000 K and 6500 K effective temperature with an initial vertical,
homogeneous magnetic field of 50 G strength. We find that the field
strength of the strongest magnetic flux concentrations increases with
decreasing effective temperature at the height level where the average
Rosseland optical depth is one. On the other hand, at the same level,
the field is less strong than the thermal equipartition value in the
coolest model but assumes superequipartition in the models hotter
than 5000 K. While the Wilson depression of the strongest field
concentrations is about one pressure scale height in the coolest
model, it is more than four times the pressure scale height in the
hottest one. We also find that the relative contribution of the bright
filigree to the bolometric, vertically directed radiative intensity is
most significant for the Teff = 5000 K model (0.6%-0.79%)
and least significant for the hottest and coolest models (0.1%-0.46%
and 0.14%-0.32%, respectively). This behavior suggests that the effect
of the small-scale magnetic field on the photometric variability is more
significant for K dwarf stars than for F-type and also M-type stars.

---------------------------------------------------------
Title: Detecting weak magnetic fields in the central stars of
planetary nebulae
Authors: Steffen, M.; Hubrig, S.; Todt, H.; Schöller, M.; Sandin,
C.; Hamann, W. -R.; Schönberner, D.
2014psce.conf..351S Altcode:
We have carried out low-resolution spectropolarimetric observations
with FORS 2, installed on the VLT, ESO, of a representative sample
of 12 bright central stars of Planetary Nebulae (PNe) with different
morphology. Two of the sample are hydrogen-deficient (Wolf-Rayet
type) stars. Our measurements rule out the existence of strong global
magnetic fields of the order of kG in any of the PN central stars of
our sample. Even so, our data may indicate the presence of weak mean
longitudinal magnetic fields of a few hundred Gauss in the central
stars of two elliptical nebulae, IC 418 and NGC 2392, and a very weak
magnetic field of about 100 G in the Wolf-Rayet type central star
Hen 2-113. However, the significance of these marginal detections
depends on the method adopted for estimating the uncertainties in the
magnetic-field measurements.

---------------------------------------------------------
Title: The Chandra Planetary Nebula Survey (ChanPlaNS). II. X-Ray
Emission from Compact Planetary Nebulae
Authors: Freeman, M.; Montez, R., Jr.; Kastner, J. H.; Balick, B.;
Frew, D. J.; Jones, D.; Miszalski, B.; Sahai, R.; Blackman, E.;
Chu, Y. -H.; De Marco, O.; Frank, A.; Guerrero, M. A.; Lopez, J. A.;
Zijlstra, A.; Bujarrabal, V.; Corradi, R. L. M.; Nordhaus, J.; Parker,
Q. A.; Sandin, C.; Schönberner, D.; Soker, N.; Sokoloski, J. L.;
Steffen, M.; Toalá, J. A.; Ueta, T.; Villaver, E.
2014ApJ...794...99F Altcode: 2014arXiv1407.4141F
We present results from the most recent set of observations obtained
as part of the Chandra X-ray observatory Planetary Nebula Survey
(ChanPlaNS), the first comprehensive X-ray survey of planetary nebulae
(PNe) in the solar neighborhood (i.e., within ~1.5 kpc of the Sun). The
survey is designed to place constraints on the frequency of appearance
and range of X-ray spectral characteristics of X-ray-emitting PN
central stars and the evolutionary timescales of wind-shock-heated
bubbles within PNe. ChanPlaNS began with a combined Cycle 12 and
archive Chandra survey of 35 PNe. ChanPlaNS continued via a Chandra
Cycle 14 Large Program which targeted all (24) remaining known
compact (R neb &lt;~ 0.4 pc), young PNe that lie within
~1.5 kpc. Results from these Cycle 14 observations include first-time
X-ray detections of hot bubbles within NGC 1501, 3918, 6153, and 6369,
and point sources in HbDs 1, NGC 6337, and Sp 1. The addition of the
Cycle 14 results brings the overall ChanPlaNS diffuse X-ray detection
rate to ~27% and the point source detection rate to ~36%. It has
become clearer that diffuse X-ray emission is associated with young
(lsim 5 × 103 yr), and likewise compact (R neb
&lt;~ 0.15 pc), PNe with closed structures and high central electron
densities (ne &gt;~ 1000 cm-3), and is rarely
associated with PNe that show H2 emission and/or pronounced
butterfly structures. Hb 5 is one such exception of a PN with a
butterfly structure that hosts diffuse X-ray emission. Additionally,
two of the five new diffuse X-ray detections (NGC 1501 and NGC 6369)
host [WR]-type central stars, supporting the hypothesis that PNe with
central stars of [WR]-type are likely to display diffuse X-ray emission.

---------------------------------------------------------
Title: The Gaia-ESO Survey: The analysis of high-resolution UVES
spectra of FGK-type stars
Authors: Smiljanic, R.; Korn, A. J.; Bergemann, M.; Frasca, A.;
Magrini, L.; Masseron, T.; Pancino, E.; Ruchti, G.; San Roman,
I.; Sbordone, L.; Sousa, S. G.; Tabernero, H.; Tautvaišienė,
G.; Valentini, M.; Weber, M.; Worley, C. C.; Adibekyan, V. Zh.;
Allende Prieto, C.; Barisevičius, G.; Biazzo, K.; Blanco-Cuaresma,
S.; Bonifacio, P.; Bragaglia, A.; Caffau, E.; Cantat-Gaudin, T.;
Chorniy, Y.; de Laverny, P.; Delgado-Mena, E.; Donati, P.; Duffau,
S.; Franciosini, E.; Friel, E.; Geisler, D.; González Hernández,
J. I.; Gruyters, P.; Guiglion, G.; Hansen, C. J.; Heiter, U.; Hill, V.;
Jacobson, H. R.; Jofre, P.; Jönsson, H.; Lanzafame, A. C.; Lardo, C.;
Ludwig, H. -G.; Maiorca, E.; Mikolaitis, Š.; Montes, D.; Morel, T.;
Mucciarelli, A.; Muñoz, C.; Nordlander, T.; Pasquini, L.; Puzeras,
E.; Recio-Blanco, A.; Ryde, N.; Sacco, G.; Santos, N. C.; Serenelli,
A. M.; Sordo, R.; Soubiran, C.; Spina, L.; Steffen, M.; Vallenari,
A.; Van Eck, S.; Villanova, S.; Gilmore, G.; Randich, S.; Asplund,
M.; Binney, J.; Drew, J.; Feltzing, S.; Ferguson, A.; Jeffries, R.;
Micela, G.; Negueruela, I.; Prusti, T.; Rix, H. -W.; Alfaro, E.;
Babusiaux, C.; Bensby, T.; Blomme, R.; Flaccomio, E.; François, P.;
Irwin, M.; Koposov, S.; Walton, N.; Bayo, A.; Carraro, G.; Costado,
M. T.; Damiani, F.; Edvardsson, B.; Hourihane, A.; Jackson, R.; Lewis,
J.; Lind, K.; Marconi, G.; Martayan, C.; Monaco, L.; Morbidelli, L.;
Prisinzano, L.; Zaggia, S.
2014A&A...570A.122S Altcode: 2014arXiv1409.0568S
Context. The ongoing Gaia-ESO Public Spectroscopic Survey is using
FLAMES at the VLT to obtain high-quality medium-resolution Giraffe
spectra for about 105 stars and high-resolution UVES spectra
for about 5000 stars. With UVES, the Survey has already observed
1447 FGK-type stars. Aims: These UVES spectra are analyzed
in parallel by several state-of-the-art methodologies. Our aim is
to present how these analyses were implemented, to discuss their
results, and to describe how a final recommended parameter scale is
defined. We also discuss the precision (method-to-method dispersion)
and accuracy (biases with respect to the reference values) of the
final parameters. These results are part of the Gaia-ESO second
internal release and will be part of its first public release of
advanced data products. Methods: The final parameter scale is
tied to the scale defined by the Gaia benchmark stars, a set of stars
with fundamental atmospheric parameters. In addition, a set of open
and globular clusters is used to evaluate the physical soundness
of the results. Each of the implemented methodologies is judged
against the benchmark stars to define weights in three different
regions of the parameter space. The final recommended results are
the weighted medians of those from the individual methods. 
Results: The recommended results successfully reproduce the atmospheric
parameters of the benchmark stars and the expected Teff-log
g relation of the calibrating clusters. Atmospheric parameters and
abundances have been determined for 1301 FGK-type stars observed with
UVES. The median of the method-to-method dispersion of the atmospheric
parameters is 55 K for Teff, 0.13 dex for log g and 0.07
dex for [Fe/H]. Systematic biases are estimated to be between 50-100
K for Teff, 0.10-0.25 dex for log g and 0.05-0.10 dex for
[Fe/H]. Abundances for 24 elements were derived: C, N, O, Na, Mg, Al,
Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Mo, Ba, Nd, and
Eu. The typical method-to-method dispersion of the abundances varies
between 0.10 and 0.20 dex. Conclusions: The Gaia-ESO sample of
high-resolution spectra of FGK-type stars will be among the largest of
its kind analyzed in a homogeneous way. The extensive list of elemental
abundances derived in these stars will enable significant advances in
the areas of stellar evolution and Milky Way formation and evolution. Based on observations made with the ESO/VLT, at Paranal Observatory,
under program 188.B-3002 (The Gaia-ESO Public Spectroscopic Survey, PIs
Gilmore and Randich). Appendices are available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201423937/olm">http://www.aanda.org</A>

---------------------------------------------------------
Title: Weak magnetic fields in central stars of planetary nebulae?
Authors: Steffen, M.; Hubrig, S.; Todt, H.; Schöller, M.; Hamann,
W. -R.; Sandin, C.; Schönberner, D.
2014A&A...570A..88S Altcode: 2014arXiv1408.3000S
Context. It is not yet clear whether magnetic fields play an essential
role in shaping planetary nebulae (PNe), or whether stellar rotation
alone and/or a close binary companion, stellar or substellar, can
account for the variety of the observed nebular morphologies. 
Aims: In a quest for empirical evidence verifying or disproving the
role of magnetic fields in shaping planetary nebulae, we follow up on
previous attempts to measure the magnetic field in a representative
sample of PN central stars. Methods: We obtained low-resolution
polarimetric spectra with FORS 2 installed on the Antu telescope of
the VLT for a sample of 12 bright central stars of PNe with different
morphologies, including two round nebulae, seven elliptical nebulae,
and three bipolar nebulae. Two targets are Wolf-Rayet type central
stars. Results: For the majority of the observed central stars,
we do not find any significant evidence for the existence of surface
magnetic fields. However, our measurements may indicate the presence of
weak mean longitudinal magnetic fields of the order of 100 Gauss in the
central star of the young elliptical planetary nebula IC 418 as well
as in the Wolf-Rayet type central star of the bipolar nebula Hen 2-113
and the weak emission line central star of the elliptical nebula Hen
2-131. A clear detection of a 250 G mean longitudinal field is achieved
for the A-type companion of the central star of NGC 1514. Some of the
central stars show a moderate night-to-night spectrum variability,
which may be the signature of a variable stellar wind and/or rotational
modulation due to magnetic features. Conclusions: Since our
analysis indicates only weak fields, if any, in a few targets of our
sample, we conclude that strong magnetic fields of the order of kG are
not widespread among PNe central stars. Nevertheless, simple estimates
based on a theoretical model of magnetized wind bubbles suggest that
even weak magnetic fields below the current detection limit of the
order of 100 G may well be sufficient to contribute to the shaping of
the surrounding nebulae throughout their evolution. Our current sample
is too small to draw conclusions about a correlation between nebular
morphology and the presence of stellar magnetic fields. Based on
observations obtained at the European Southern Observatory, Paranal,
Chile (ESO program No. 088.D-0425(A)).

---------------------------------------------------------
Title: VizieR Online Data Catalog: Abundances of 47 Tuc turn-off stars
(Dobrovolskas+, 2014)
Authors: Dobrovolskas, V.; Kucinskas, A.; Bonifacio, P.; Korotin,
S. A.; Steffen, M.; Sbordone, L.; Caffau, E.; Ludwig, H. -G.; Royer,
F.; Prakapavicius, D.
2014yCat..35650121D Altcode: 2014yCat..35659121D
Spectra of the TO stars in 47 Tuc investigated in this work were
obtained with the GIRAFFE spectrograph in August-September, 2008,
under the programme 081.D-0287(A) (PI: Shen). The same data set
was independently analysed by D'Orazi et al. (2010ApJ...713L...1D,
Cat. J/ApJ/713/L1). (1 data file).

---------------------------------------------------------
Title: Abundances of lithium, oxygen, and sodium in the turn-off
stars of Galactic globular cluster 47 Tucanae
Authors: Dobrovolskas, V.; Kučinskas, A.; Bonifacio, P.; Korotin,
S. A.; Steffen, M.; Sbordone, L.; Caffau, E.; Ludwig, H. -G.; Royer,
F.; Prakapavičius, D.
2014A&A...565A.121D Altcode: 2013arXiv1311.1072D
Context. The cluster 47 Tuc is among the most metal-rich Galactic
globular clusters and its metallicity is similar to that of metal-poor
disc stars and open clusters. Like other globular clusters, it
displays variations in the abundances of elements lighter than Si,
which is generally interpreted as evidence of the presence of multiple
stellar populations. Aims: We aim to determine abundances of
Li, O, and Na in a sample of of 110 turn-off (TO) stars, in order
to study the evolution of light elements in this cluster and to put
our results in perspective with observations of other globular and
open clusters, as well as with field stars. Methods: We use
medium resolution spectra obtained with the GIRAFFE spectrograph at
the ESO 8.2 m Kueyen VLT telescope and use state of the art 1D model
atmospheres and NLTE line transfer to determine the abundances. We
also employ CO5BOLD hydrodynamical simulations to assess
the impact of stellar granulation on the line formation and inferred
abundances. Results: Our results confirm the existence of
Na-O abundance anti-correlation and hint towards a possible Li-O
anti-correlation in the TO stars of 47 Tuc. At the same time,
we find no convincing evidence supporting the existence of Li-Na
correlation. The obtained 3D NLTE mean lithium abundance in a sample
of 94 TO stars where Li lines were detected reliably, ⟨A(Li)3D
NLTE⟩ = 1.78 ± 0.18 dex, appears to be significantly lower
than what is observed in other globular clusters. At the same time,
star-to-star spread in Li abundance is also larger than seen in other
clusters. The highest Li abundance observed in 47 Tuc is about 0.1
dex lower than the lowest Li abundance observed among the un-depleted
stars of the metal-poor open cluster NGC 2243. Conclusions: The
correlations/anti-correlations among light element abundances confirm
that chemical enrichment history of 47 Tuc was similar to that of other
globular clusters, despite the higher metallicity of 47 Tuc. The lithium
abundances in 47 Tuc, when put into context with observations in other
clusters and field stars, suggest that stars that are more metal-rich
than [Fe/H] ~ -1.0 experience significant lithium depletion during
their lifetime on the main sequence, while the more metal-poor stars
do not. Rather strikingly, our results suggest that initial lithium
abundance with which the star was created may only depend on its age
(the younger the star, the higher its Li content) and not on its
metallicity. Appendices are available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201322868/olm">http://www.aanda.org</A>

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Title: A super lithium-rich red-clump star in the open cluster
Trumpler 5
Authors: Monaco, L.; Boffin, H. M. J.; Bonifacio, P.; Villanova, S.;
Carraro, G.; Caffau, E.; Steffen, M.; Ahumada, J. A.; Beletsky, Y.;
Beccari, G.
2014A&A...564L...6M Altcode: 2014arXiv1403.6461M
Context. The existence of lithium-rich low-mass red giant stars still
represents a challenge for stellar evolution models. Stellar clusters
are privileged environments for this kind of investigation. 
Aims: To investigate the chemical abundance pattern of the old open
cluster Trumpler 5, we observed a sample of four red-clump stars with
high-resolution optical spectrographs. One of them (#3416) reveals
extremely strong lithium lines in its spectrum. Methods:
One-dimensional, local thermodynamic equilibrium analysis was
performed on the spectra of the observed stars. A 3D-NLTE analysis
was performed to derive the lithium abundance of star #3416. 
Results: Star #3416 is super Li-rich with A(Li) = 3.75 dex. The lack
of 6Li enrichment (6Li/7Li &lt;
2%), the low carbon isotopic ratio (12C/13C =
14 ± 3), and the lack of evidence for radial velocity variation
or enhanced rotational velocity (vsini = 2.8 km s-1)
all suggest that lithium production has occurred in this star
through the Cameron &amp; Fowler mechanism. Conclusions:
We identified a super Li-rich core helium-burning, red-clump star
in an open cluster. Internal production is the most likely cause
of the observed enrichment. Given the expected short duration of a
star's Li-rich phase, enrichment is likely to have occurred at the
red clump or in the immediately preceding phases, namely during the
He-flash at the tip of the red giant branch (RGB) or while ascending
the brightest portion of the RGB. Based on observations made
with ESO Telescopes at the La Silla Paranal Observatory under program
ID 088.D-0045(A).Appendix A is available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201323348/olm">http://www.aanda.org</A>

---------------------------------------------------------
Title: Detection of magnetic fields in central stars of planetary
nebulae
Authors: Todt, H.; Steffen, M.; Hubrig, S.; Schöller, M.; Hamann,
W. -R.; Sandin, C.; Schönberner, D.
2014apn6.confE.103T Altcode:
Magnetic fields may play an essential role in shaping planetary
nebulae. However, convincing evidence for the presence of magnetic
fields on their central stars is still missing. We obtained
low-resolution polarimetric spectra with FORS2 for twelve central
stars. Their nebulae cover different morphologies, comprising two round
nebulae, seven elliptical nebulae, and three bipolar nebulae. For
one central star, which has a bipolar nebula, we clearly detect a
weak magnetic field of the order of 100 Gauss. Interestingly, the
central star exhibits a hydrogen-free, Wolf-Rayet type spectrum. For
a few further stars in our sample we find marginal evidence for weak
magnetic fields. The detection of magnetic fields via polarimetry
strongly depends on the quality of the observations, and on the type of
the line spectrum. We perform careful estimates of the error margins,
using statistical tests and Monte-Carlo simulations, which confirm
the significance of the detection reported here.

---------------------------------------------------------
Title: 3D modeling of stellar atmospheres and the impact on the
understanding of the reliability of elemental abundances in stars
as tracers of galactic chemical evolution
Authors: Ludwig, H. -G.; Steffen, M.; Bonifacio, P.; Caffau, E.;
Kučinskas, A.; Freytag, B.
2014IAUS..298..343L Altcode:
We present a critical review of the construction of 3D model atmospheres
with emphasis on modeling challenges. We discuss the basic physical
processes which give rise to the effects which set 3D models apart
from 1D standard models. We consider elemental abundances derived
from molecular features, and the determination of the microturbulence
with 3D models. The examples serve as illustration of the limitations
inherent to 1D, however, also to 3D modeling. We find that 3D models
can provide constraints on the microturbulence parameter, and predict
substantial corrections for abundances derived from molecular species.

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Title: 3D Model Atmospheres of White Dwarfs
Authors: Tremblay, Pier-Emmanuel; Ludwig, H.; Steffen, M.; Freytag, B.
2014AAS...22331507T Altcode:
We present the first grid of 3D model atmospheres for
hydrogen-atmosphere (DA) white dwarfs. These CO5BOLD
radiation-hydrodynamics simulations, unlike the previous 1D
calculations, do not rely on the mixing-length theory for the
treatment of convection. The simulations have been employed to compute
model spectra and we compared our improved Balmer line profiles to
spectroscopic data from the Sloan Digital Sky Survey and the White
Dwarf Catalog. The 3D surface gravities are found to be as much as
0.3 dex lower than the values derived from 1D models. The white dwarfs
with a radiative and a convective atmosphere have derived mean masses
that are the same within 0.01 Msun with our new models, in much better
agreement with our understanding of stellar evolution.

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Title: The first generations of stars
Authors: Caffau, E.; Gallagher, A.; Bonifacio, P.; Cayrel, R.;
Christlieb, N.; Clark, P. C.; Francois, P.; Glover, S.; Klessen,
R. S.; Koch, A.; Ludwig, H. G.; Monaco, L.; Plez, B.; Sbordone, L.;
Spite, M.; Spite, F.; Steffen, M.; Zaggia, S.
2014nic..confE..53C Altcode: 2014PoS...204E..53C
No abstract at ADS

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Title: A hydrodynamical study of multiple-shell planetary
nebulae. III. Expansion properties and internal kinematics: Theory
versus observation
Authors: Schönberner, D.; Jacob, R.; Lehmann, H.; Hildebrandt, G.;
Steffen, M.; Zwanzig, A.; Sandin, C.; Corradi, R. L. M.
2014AN....335..378S Altcode: 2014arXiv1404.0391S
We present the result of a study on the expansion properties and
internal kinematics of round/elliptical planetary nebulae of the
Milky Way disk, the halo, and of the globular cluster M 15. The
purpose of this study is to considerably enlarge the small sample of
nebulae with precisely determined expansion properties (Schönberner
et al. \cite{SJSPCA.05}). To this aim, we selected a representative
sample of objects with different evolutionary stages and metallicities
and conducted high-resolution échelle spectroscopy. In most cases
we succeeded in detecting the weak signals from the outer nebular
shell which are attached to the main line emission from the bright
nebular rim. Next to the measurement of the motion of the rim gas by
decomposition of the main line components into Gaussians, we were
able to measure separately, for most objects for the first time,
the gas velocity immediately behind the leading shock of the shell,
i.e. the post-shock velocity. We more than doubled the number of objects
for which the velocities of both rim and shell are known and confirm
that the overall expansion of planetary nebulae is accelerating with
time. There are, however, differences between the expansion behaviour of
the shell and the rim: The post-shock velocity is starting at values as
low as around 20 km s-1 for the youngest nebulae, just above
the AGB wind velocity of ∼ 10-15 km s-1, and is reaching
values of about 40 km s-1 for the nebulae around hotter
central stars. Contrarily, the rim matter is at first decelerated
below the typical AGB-wind velocity and remains at about 5-10 km
s-1 for a while until finally a typical flow velocity of
up to 30 km s-1 is reached. This observed distinct velocity
evolution of both rim and shell is explained by radiation-hydrodynamics
simulations, at least qualitatively: It is due to the ever changing
stellar radiation field and wind-wind interaction together with the
varying density profile ahead of the leading shock during the progress
of evolution. The wind-wind interaction works on the rim dynamics while
the radiation field and upstream density gradient is responsible for the
shell dynamics. Because of these time-dependent boundary conditions,
a planetary nebula will never evolve into a simple self-similar
expansion. Also the metal-poor objects behave as theory predicts:
The post-shock velocities are higher and the rim flow velocities are
equal or even lower compared to disk objects at similar evolutionary
stage. The old nebula around low-luminosity central stars contained
in our sample expand still fast and are dominated by reionisation. We
detected, for the first time, in some objects an asymmetric expansion
behaviour: The relative expansions between rim and shell appear to
be different for the receding and approaching parts of the nebular
envelope. Based partly on observations obtained at the European
Southern Observatory, Paranal, Chile (ESO programme No. 077.D-0652).

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Title: TOPoS: chemical study of extremely metal-poor stars.
Authors: Caffau, E.; Sbordone, L.; Bonifacio, P.; Cayrel, R.;
Christlieb, N.; Clark, P.; François, P.; Glover, S.; Klessen, R.;
Koch, A.; Ludwig, H. -G.; Monaco, L.; Plez, B.; Spite, F.; Spite,
M.; Steffen, M.; Zaggia, S.
2014MmSAI..85..222C Altcode:
The extremely metal-poor (EMP) stars hold in their atmospheres
the fossil record of the chemical composition of the early phases
of the Galactic evolution. The chemical analysis of such objects
provides important constraints on these early phases. EMP stars
are very rare objects; to dig them out, large amounts of data have
to be processed. With an automatic procedure, we analysed objects
with colours of Turn-Off stars from the Sloan Digital Sky Survey to
select a sample of good candidate EMP stars. In the latest years,
we observed a sample of these candidates with X-Shooter and UVES,
and we have an ongoing ESO large programme to use these spectrographs
to observe EMP stars. I will report here the results on metallicity
and Strontium abundance. Based on observations obtained at ESO
Paranal Observatory, programme 189.D-0165(A)

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Title: 6Li/7Li isotopic ratio in the most metal-poor binary
CS22876-032
Authors: Gonzalez-Hernandez, J.; Caffau, E.; Ludwig, H. G.; Bonifacio,
P.; Steffen, M.; Monaco, L.; Cayrel, R.
2014nic..confE..23G Altcode: 2014PoS...204E..23G
No abstract at ADS

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Title: Isotope spectroscopy
Authors: Caffau, E.; Steffen, M.; Bonifacio, P.; Ludwig, H. -G.;
Monaco, L.; Lo Curto, G.; Kamp, I.
2014AN....335...59C Altcode: 2013arXiv1310.6058C
The measurement of isotopic ratios provides a privileged insight
both into nucleosynthesis and into the mechanisms operating in
stellar envelopes, such as gravitational settling. In this article,
we give a few examples of how isotopic ratios can be determined from
high-resolution, high-quality stellar spectra. We consider examples of
the lightest elements, H and He, for which the isotopic shifts are very
large and easily measurable, and examples of heavier elements for which
the determination of isotopic ratios is more difficult. The presence
of 6Li in the stellar atmospheres causes a subtle extra
depression in the red wing of the 7Li 670.7 nm doublet which
can only be detected in spectra of the highest quality. But even with
the best spectra, the derived 6Li abundance can only be as
good as the synthetic spectra used for their interpretation. It is now
known that 3D non-LTE modelling of the lithium spectral line profiles
is necessary to account properly for the intrinsic line asymmetry,
which is produced by convective flows in the atmospheres of cool stars,
and can mimic the presence of 6Li. We also discuss briefly
the case of the carbon isotopic ratio in metal-poor stars, and provide a
new determination of the nickel isotopic ratios in the solar atmosphere.

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Title: TOPoS. I. Survey design and analysis of the first sample
Authors: Caffau, E.; Bonifacio, P.; Sbordone, L.; François, P.;
Monaco, L.; Spite, M.; Plez, B.; Cayrel, R.; Christlieb, N.; Clark,
P.; Glover, S.; Klessen, R.; Koch, A.; Ludwig, H. -G.; Spite, F.;
Steffen, M.; Zaggia, S.
2013A&A...560A..71C Altcode: 2013arXiv1310.6963C
Context. The metal-weak tail of the metallicity distribution function
(MDF) of the Galactic Halo stars contains crucial information on the
formation mode of the first generation of stars. To determine this
observationally, it is necessary to observe large numbers of extremely
metal-poor stars. Aims: We present here the Turn-Off Primordial
Stars survey (TOPoS) that is conducted as an ESO Large Programme at the
VLT. This project has four main goals: (i) to understand the formation
of low-mass stars in a low-metallicity gas: determine the metal-weak
tail of the halo MDF below [M/H] = -3.5; in particular, we aim at
determining the critical metallicity, that is the lowest metallicity
sufficient for the formation of low-mass stars; (ii) to determine in
extremely metal-poor stars the relative abundances of the elements that
are the signature of the massive first stars; (iii) to determine the
trend of the lithium abundance at the time when the Galaxy formed; and
(iv) to derive the fraction of C-enhanced extremely metal-poor stars
with respect to normal extremely metal-poor stars. The large number of
stars observed in the SDSS provides a good sample of candidate stars
at extremely low metallicity. Methods: Candidates with turn-off
colours down to magnitude g = 20 were selected from the low-resolution
spectra of SDSS by means of an automated procedure. X-Shooter has the
potential of performing the necessary follow-up spectroscopy, providing
accurate metallicities and abundance ratios for several key elements
for these stars. Results: We present here the stellar parameters
of the first set of stars. The nineteen stars range in iron abundance
between -4.1 and -2.9 dex relative to the Sun. Two stars have a high
radial velocity and, according to our estimate of their kinematics,
appear to be marginally bound to the Galaxy and are possibly accreted
from another galaxy. Based on observations obtained at ESO Paranal
Observatory, GTO programme 189.D-0165(A).

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Title: Spectroscopic analysis of DA white dwarfs with 3D model
atmospheres
Authors: Tremblay, P. -E.; Ludwig, H. -G.; Steffen, M.; Freytag, B.
2013A&A...559A.104T Altcode: 2013arXiv1309.0886T
We present the first grid of mean three-dimensional (3D) spectra for
pure-hydrogen (DA) white dwarfs based on 3D model atmospheres. We use
CO5BOLD radiation-hydrodynamics 3D simulations instead of the
mixing-length theory for the treatment of convection. The simulations
cover the effective temperature range of 6000 &lt; Teff
(K) &lt; 15 000 and the surface gravity range of 7 &lt; log g &lt;
9 where the large majority of DAs with a convective atmosphere
are located. We rely on horizontally averaged 3D structures (over
constant Rosseland optical depth) to compute ⟨3D⟩ spectra. It
is demonstrated that our ⟨3D⟩ spectra can be smoothly connected
to their 1D counterparts at higher and lower Teff where
the 3D effects are small. Analytical functions are provided in order
to convert spectroscopically determined 1D effective temperatures
and surface gravities to 3D atmospheric parameters. We apply our
improved models to well studied spectroscopic data sets from the
Sloan Digital Sky Survey and the White Dwarf Catalog. We confirm
that the so-called high-log g problem is not present when employing
⟨3D⟩ spectra and that the issue was caused by inaccuracies in
the 1D mixing-length approach. The white dwarfs with a radiative
and a convective atmosphere have derived mean masses that are
the same within ~0.01 M⊙, in much better agreement
with our understanding of stellar evolution. Furthermore, the 3D
atmospheric parameters are in better agreement with independent
Teff and log g values from photometric and parallax
measurements. Appendices are only available in electronic form
at <A href="http://www.aanda.org">http://www.aanda.org</A>

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Title: Three-dimensional hydrodynamical CO5BOLD model
atmospheres of red giant stars. III. Line formation in the atmospheres
of giants located close to the base of the red giant branch
Authors: Dobrovolskas, V.; Kučinskas, A.; Steffen, M.; Ludwig,
H. -G.; Prakapavičius, D.; Klevas, J.; Caffau, E.; Bonifacio, P.
2013A&A...559A.102D Altcode: 2013arXiv1310.7791D
 Aims: We utilize state-of-the-art three-dimensional (3D)
hydrodynamical and classical 1D stellar model atmospheres to study
the influence of convection on the formation properties of various
atomic and molecular spectral lines in the atmospheres of four red
giant stars, located close to the base of the red giant branch, RGB
(Teff ≈ 5000 K, log g = 2.5), and characterized by four
different metallicities, [M/H] = 0.0, -1.0, -2.0, -3.0. Methods:
The role of convection in the spectral line formation is assessed with
the aid of abundance corrections, i.e., the differences in abundances
predicted for a given equivalent width of a particular spectral line
with the 3D and 1D model atmospheres. The 3D hydrodynamical and
classical 1D model atmospheres used in this study were calculated
with the CO5BOLD and 1D LHD codes, respectively. Identical
atmospheric parameters, chemical composition, equation of state, and
opacities were used with both codes, therefore allowing a strictly
differential analysis of the line formation properties in the 3D and 1D
models. Results: We find that for lines of certain neutral atoms,
such as Mg i, Ti i, Fe i, and Ni i, the abundance corrections strongly
depend both on the metallicity of a given model atmosphere and the line
excitation potential, χ. While abundance corrections for all lines of
both neutral and ionized elements tend to be small at solar metallicity
(≤±0.1 dex), for lines of neutral elements with low ionization
potential and low-to-intermediate χ they quickly increase with
decreasing metallicity, reaching in their extremes -0.6 to -0.8 dex. In
all such cases the large abundance corrections are due to horizontal
temperature fluctuations in the 3D hydrodynamical models. Lines of
neutral elements with higher ionization potentials (Eion
≳ 10 eV) generally behave very similarly to lines of ionized elements
characterized by low ionization potentials (Eion ≲ 6
eV). In the latter case, the abundance corrections are small (generally,
≤±0.1 dex) and are caused by approximately equal contributions
from the horizontal temperature fluctuations and differences between
the temperature profiles in the 3D and 1D model atmospheres. Abundance
corrections of molecular lines are very sensitive to the metallicity of
the underlying model atmosphere and may be larger (in absolute value)
than ~-0.5 dex at [M/H] = -3.0 (~-1.5 dex in the case of CO). At fixed
metallicity and excitation potential, the abundance corrections show
little variation within the wavelength range studied here, 400-1600
nm. We also find that an approximate treatment of scattering in the
3D model calculations (i.e., ignoring the scattering opacity in the
outer, optically thin, atmosphere) leads to abundance corrections
that are altered by less than ~0.1 dex, both for atomic and molecular
(CO) lines, with respect to the model where scattering is treated as
true absorption throughout the entire atmosphere, with the largest
differences for the resonance and low-excitation lines. Appendices
and Figs. 3, 5, 6, 8, 9, 11 are available in electronic form at <A
href="http://www.aanda.org">http://www.aanda.org</A>

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Title: The evolution of planetary nebulae. VIII. True expansion
rates and visibility times
Authors: Jacob, R.; Schönberner, D.; Steffen, M.
2013A&A...558A..78J Altcode: 2013arXiv1307.6189J
Context. The visibility time of planetary nebulae (PNe) in stellar
systems is an essential quantity for estimating the size of a PN
population in the context of general population studies. For instance,
it enters directly into the PN death rate determination. Aims:
The basic ingredient for determining visibility times is the typical
nebular expansion velocity, as a suited average over all PN sizes
of a PN population within a certain volume or stellar system. The
true expansion speed of the outer nebular edge of a PN is, however,
not accessible by spectroscopy - a difficulty that we surmount by
radiation-hydrodynamics modelling. Methods: We first discuss
the definition of the PN radius and possible differences between
the observable PN radius and its physical counterpart, the position
of the leading shock of the nebular shell. We also compare the Hα
surface-brightness evolution predicted by our radiation-hydrodynamics
models with the recent Hα surface-brightness radius calibration of Frew
(2008, Ph.D. Thesis, Macquarie University, Australia) and find excellent
agreement. We then carefully investigate the existing spectroscopic data
on nebular expansion velocities for a local PN sample with objects up
to a distance of 2 kpc with well-defined round/elliptical shapes. We
evaluate, by means of our radiation-hydrodynamics models, how these
observed expansion velocities must be corrected in order to get the
true expansion speed of the outer nebular edge. Results: We find
a mean true expansion velocity of 42 km s-1, i.e. nearly
twice as high as the commonly adopted value to date. Accordingly,
the time for a PN to expand to a radius of, say 0.9 pc, is only
21 000 ± 5000 years. This visibility time of a PN holds for all
central star masses since a nebula does not become extinct as the
central star fades. There is, however, a dependence on metallicity
in the sense that the visibility time becomes shorter for lower
nebular metal content. Conclusions: These statements on the
visibility time only hold for volume-limited samples. Extragalactic
samples that contain spatially unresolved nebulae are flux limited,
and in this case the visibility time directly depends on the limiting
magnitude of the survey. To reach a visibility time of 21 000 years,
the survey must reach about 7 mag below the bright cut-off of the
planetary nebula luminosity function. With the higher expansion
rate of PNe derived here we determined their local death-rate
density as (1.4 ± 0.5) × 10-12 PN pc-3
yr-1, using the local PN density advocated by Frew
(2008). Appendix A is available in electronic form at <A
href="http://www.aanda.org">http://www.aanda.org</A>Dedicated to the
memory of Volker Weidemann who died on March 14, 2012, at the age of
87 years.

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Title: Granulation properties of giants, dwarfs, and white dwarfs
from the CIFIST 3D model atmosphere grid
Authors: Tremblay, P. -E.; Ludwig, H. -G.; Freytag, B.; Steffen, M.;
Caffau, E.
2013A&A...557A...7T Altcode: 2013arXiv1307.2810T
Three-dimensional model atmospheres for giants, dwarfs, and white
dwarfs, computed with the CO5BOLD code and part of the CIFIST grid,
have been used for spectroscopic and asteroseismic studies. Unlike
existing plane-parallel 1D structures, these simulations predict
the spatially and temporally resolved emergent intensity so that
granulation can be analysed, which provides insights on how convective
energy transfer operates in stars. The wide range of atmospheric
parameters of the CIFIST 3D simulations (3600 &lt; Teff
(K) &lt; 13 000 and 1 &lt; log g &lt; 9) allows the comparison of
convective processes in significantly different environments. We
show that the relative intensity contrast is correlated with both
the Mach and Péclet numbers in the photosphere. The horizontal size
of granules varies between 3 and 10 times the local pressure scale
height, with a tight correlation between the factor and the Mach
number of the flow. Given that convective giants, dwarfs, and white
dwarfs cover the same range of Mach and Péclet numbers, we conclude
that photospheric convection operates in a very similar way in those
objects. Table 1 and Appendix A are available in electronic form
at <A href="http://www.aanda.org">http://www.aanda.org</A>

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Title: Evaluating local correlation tracking using CO5BOLD simulations
of solar granulation
Authors: Verma, M.; Steffen, M.; Denker, C.
2013A&A...555A.136V Altcode: 2013arXiv1305.6033V
Context. Flows on the solar surface are intimately linked to
solar activity, and local correlation tracking (LCT) is one of the
standard techniques for capturing the dynamics of these processes by
cross-correlating solar images. However, the link between contrast
variations in successive images to the underlying plasma motions has
to be quantitatively confirmed. Aims: Radiation hydrodynamics
simulations of solar granulation (e.g., CO5BOLD) provide
access to both the wavelength-integrated, emergent continuum intensity
and the three-dimensional velocity field at various heights in the
solar atmosphere. Thus, applying LCT to continuum images yields
horizontal proper motions, which are then compared to the velocity
field of the simulated (non-magnetic) granulation. In this study, we
evaluate the performance of an LCT algorithm previously developed for
bulk-processing Hinode G-band images, establish it as a quantitative
tool for measuring horizontal proper motions, and clearly work out
the limitations of LCT or similar techniques designed to track optical
flows. Methods: Horizontal flow maps and frequency distributions
of the flow speed were computed for a variety of LCT input parameters
including the spatial resolution, the width of the sampling window,
the time cadence of successive images, and the averaging time used to
determine persistent flow properties. Smoothed velocity fields from
the hydrodynamics simulation at three atmospheric layers (log τ = -1,
0, and +1) served as a point of reference for the LCT results. 
Results: LCT recovers many of the granulation properties, e.g., the
shape of the flow speed distributions, the relationship between mean
flow speed and averaging time, and also - with significant smoothing
of the simulated velocity field - morphological features of the
flow and divergence maps. However, the horizontal proper motions are
grossly underestimated by as much as a factor of three. The LCT flows
match best the flows deeper in the atmosphere at log τ = +1. 
Conclusions: Despite the limitations of optical flow techniques, they
are a valuable tool in describing horizontal proper motions on the Sun,
as long as the results are not taken at face value but with a proper
understanding of the input parameter space and the limitations inherent
to the algorithm. Movies are available in electronic form at <A
href="http://www.aanda.org">http://www.aanda.org</A>

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Title: The Intensity-Velocity Phase Spectra of Evanescent Oscillations
and Acoustic Sources
Authors: Severino, G.; Straus, T.; Oliviero, M.; Steffen, M.; Fleck, B.
2013SoPh..284..297S Altcode: 2012SoPh..tmp..295S
There are three major issues in modeling solar evanescent oscillations:
the variation of the intensity [I]-velocity [V] phase difference of
p-modes close to the base of photosphere; the existence of a plateau
of negative I-V phase differences below and between the ridges of the
low-frequency p-modes; the explanation of the I-V cross-spectra of
the evanescent oscillations. We present new interpretations for the
first two issues, based on modeling intensity fluctuations taking steep
temperature gradients, opacity, and non-adiabatic cooling into account.

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Title: The photospheric solar oxygen project. II. Non-concordance
of the oxygen abundance derived from two forbidden lines
Authors: Caffau, E.; Ludwig, H. -G.; Malherbe, J. -M.; Bonifacio,
P.; Steffen, M.; Monaco, L.
2013A&A...554A.126C Altcode: 2013arXiv1305.1763C
Context. In the Sun, the two forbidden [O i] lines at 630 and 636 nm
were previously found to provide discrepant oxygen abundances. Aims: We investigate whether this discrepancy is peculiar to the
Sun or whether it is also observed in other stars. Methods:
We make use of high-resolution, high signal-to-noise ratio spectra of
four dwarf to turn-off stars, five giant stars, and one sub-giant star
observed with THEMIS, HARPS, and UVES to investigate the coherence of
the two lines. Results: The two lines provide oxygen abundances
that are consistent, within observational errors, in all the giant
stars examined by us. On the other hand, for the two dwarf stars for
which a measurement was possible, for Procyon, and for the sub-giant
star Capella, the 636 nm line provides systematically higher oxygen
abundances, as already seen for the Sun. Conclusions: The
only two possible reasons for the discrepancy are a serious error
in the oscillator strength of the Ni i line blending the 630 nm line
or the presence of an unknown blend in the 636 nm line, which makes
the feature stronger. The CN lines blending the 636 nm line cannot
be responsible for the discrepancy. The Ca i autoionisation line, on
the red wing of which the 636 nm line is formed, is not well modelled
by our synthetic spectra. However, a better reproduction of this line
would result in even higher abundances from the 636 nm, thus increasing
the discrepancy. Based on observations collected at ESO Paranal
Observatory, Programme 182.D-5053(A).

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Title: Pure-hydrogen 3D model atmospheres of cool white dwarfs
Authors: Tremblay, P. -E.; Ludwig, H. -G.; Steffen, M.; Freytag, B.
2013A&A...552A..13T Altcode: 2013arXiv1302.2013T
A sequence of pure-hydrogen CO5BOLD 3D model atmospheres of DA white
dwarfs is presented for a surface gravity of log g = 8 and effective
temperatures from 6000 to 13 000 K. We show that convective properties,
such as flow velocities, characteristic granulation size and intensity
contrast of the granulation patterns, change significantly over this
range. We demonstrate that these 3D simulations are not sensitive to
numerical parameters unlike the 1D structures that considerably depend
on the mixing-length parameters. We conclude that 3D spectra can be used
directly in the spectroscopic analyses of DA white dwarfs. We confirm
the result of an earlier preliminary study that 3D model spectra provide
a much better characterization of the mass distribution of white dwarfs
and that shortcomings of the 1D mixing-length theory are responsible
for the spurious high-log g determinations of cool white dwarfs. In
particular, the 1D theory is unable to account for the cooling effect
of the convective overshoot in the upper atmospheres.

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Title: Detection of Diffuse X-Ray Emission from Planetary Nebulae
with Nebular O VI
Authors: Ruiz, N.; Chu, Y. -H.; Gruendl, R. A.; Guerrero, M. A.;
Jacob, R.; Schönberner, D.; Steffen, M.
2013ApJ...767...35R Altcode: 2013arXiv1302.3886R
The presence of O VI ions can be indicative of plasma temperatures of
a few ×105 K that are expected in heat conduction layers
between the hot shocked stellar wind gas at several 106
K and the cooler (104 K) nebular gas of planetary nebulae
(PNe). We have used FUSE observations of PNe to search for nebular O
VI emission or absorption as a diagnostic of the conduction layer to
ensure the presence of hot interior gas. Three PNe showing nebular O VI,
namely IC 418, NGC 2392, and NGC 6826, have been selected for Chandra
observations and diffuse X-ray emission is indeed detected in each of
these PNe. Among the three, NGC 2392 has peculiarly high diffuse X-ray
luminosity and plasma temperature compared with those expected from
its stellar wind's mechanical luminosity and terminal velocity. The
limited effects of heat conduction on the plasma temperature of a hot
bubble at the low terminal velocity of the stellar wind of NGC 2392 may
partially account for its high plasma temperature, but the high X-ray
luminosity needs to be powered by processes other than the observed
stellar wind, probably the presence of an unseen binary companion of the
central star of the PN (CSPN) of NGC 2392. We have compiled relevant
information on the X-ray, stellar, and nebular properties of PNe with
a bubble morphology and found that the expectations of bubble models
including heat conduction compare favorably with the present X-ray
observations of hot bubbles around H-rich CSPNe, but have notable
discrepancies for those around H-poor [WR] CSPNe. We note that PNe
with more massive central stars can produce hotter plasma and higher
X-ray surface brightness inside central hot bubbles.

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Title: Three-dimensional hydrodynamical CO5BOLD model
atmospheres of red giant stars. II. Spectral line formation in the
atmosphere of a giant located near the RGB tip
Authors: Kučinskas, A.; Steffen, M.; Ludwig, H. -G.; Dobrovolskas,
V.; Ivanauskas, A.; Klevas, J.; Prakapavičius, D.; Caffau, E.;
Bonifacio, P.
2013A&A...549A..14K Altcode: 2012arXiv1211.7313K
 Aims: We investigate the role of convection in the formation of
atomic and molecular lines in the atmosphere of a red giant star. For
this purpose we study the formation properties of spectral lines that
belong to a number of astrophysically important tracer elements,
including neutral and singly ionized atoms (Li I, N I, O I, Na I,
Mg I, Al I, Si I, Si II, S I, K I, Ca I, Ca II, Ti I, Ti II, Cr I,
Cr II, Mn I, Fe I, Fe II, Co I, Ni I, Zn I, Sr II, Ba II, and Eu II),
and molecules (CH, CO, C2, NH, CN, and OH). Methods:
We focus our investigation on a prototypical red giant located close to
the red giant branch (RGB) tip (Teff = 3660 K, log g = 1.0,
[M/H] = 0.0). We used two types of model atmospheres, 3D hydrodynamical
and classical 1D, calculated with the CO5BOLD and LHD
stellar atmosphere codes, respectively. Both codes share the same
atmospheric parameters, chemical composition, equation of state,
and opacities, which allowed us to make a strictly differential
comparison between the line formation properties predicted in 3D and
1D. The influence of convection on the spectral line formation was
assessed with the aid of 3D-1D abundance corrections, which measure
the difference between the abundances of chemical species derived
with the 3D hydrodynamical and 1D classical model atmospheres. Results: We find that convection plays a significant role in
the spectral line formation in this particular red giant. The
derived 3D-1D abundance corrections rarely exceed ± 0.1 dex when
lines of neutral atoms and molecules are considered, which is in
line with the previous findings for solar-metallicity red giants
located on the lower RGB. The situation is different with lines that
belong to ionized atoms, or to neutral atoms with high ionization
potential. In both cases, the corrections for high-excitation lines
(χ &gt; 8 eV) may amount to Δ3D-1D ~ -0.4 dex. The
3D-1D abundance corrections generally show a significant wavelength
dependence; in most cases they are smaller in the near-infrared, at
1600-2500 nm. Appendices are available in electronic form at <A
href="http://www.aanda.org">http://www.aanda.org</A>

---------------------------------------------------------
Title: First steps with CO5BOLD using HLLMHD and PP reconstruction .
Authors: Steiner, O.; Rajaguru, S. P.; Vigeesh, G.; Steffen, M.;
Schaffenberger, W.; Freytag, B.
2013MSAIS..24..100S Altcode:
We report on first experiences with real-life applications using
the MHD-module of CO5BOLD together with the piecewise parabolic
reconstruction scheme and present preliminary results of stellar
magnetic models with Teff = 4000 K to Teff =
5770 K.

---------------------------------------------------------
Title: Granulation in DA white dwarfs from CO5BOLD 3D model
atmospheres
Authors: Tremblay, P. -E.; Ludwig, H. -G.; Freytag, B.; Steffen, M.
2013MSAIS..24...61T Altcode:
Time-dependent 3D simulations of pure-hydrogen DA white dwarf
atmospheres have been computed in recent years. Synthetic Balmer lines
spectra drawn from these radiation-hydrodynamics (RHD) simulations
have been shown to predict surface gravities significantly lower than
the standard 1D models, in much better agreement with the expectation
that white dwarfs cool at constant mass. We have now computed a grid
of CO5BOLD pure-hydrogen 3D model atmospheres for surface gravities
from log g = 7 to log g = 8.5 and effective temperatures from 6000 to
13,000 K. Over this range, we observe a significant variation of the
intensity contrast of the surface granulation patterns, which indicates
the strength of the 3D effects. Furthermore, the size and appearance
of granules are also varying considerably. An explanation of these
behaviours can lead to a better understanding of the physical processes
responsible for the energy transfer in white dwarf atmospheres.

---------------------------------------------------------
Title: Micro- and macroturbulence predictions from CO5BOLD 3D stellar
atmospheres .
Authors: Steffen, M.; Caffau, E.; Ludwig, H. -G.
2013MSAIS..24...37S Altcode: 2013arXiv1306.4307S
We present an overview of the current status of our efforts to derive
the microturbulence and macroturbulence parameters (xi_mic and xi_mac)
from the CIFIST grid of CO5BOLD 3D model atmospheres as a function
of the basic stellar parameters T_{eff}, log g, and [M/H]. The latest
results for the Sun and Procyon show that the derived microturbulence
parameter depends significantly on the numerical resolution of the
underlying 3D simulation, confirming that `low-resolution' models tend
to underestimate the true value of xi_mic . Extending the investigation
to 12 further simulations with different T_{eff}, log g, and [M/H],
we obtain a first impression of the predicted trend of xi_mic over the
Hertzsprung-Russell diagram: in agreement with empirical evidence,
microturbulence increases towards higher effective temperature and
lower gravity. The metallicity dependence of xi_mic must be interpreted
with care, since it also reflects the deviation between the 1D and 3D
photospheric temperature stratifications that increases systematically
towards lower [M/H].

---------------------------------------------------------
Title: Spectral line asymmetries in the metal-poor red giant HD
122563: CO5BOLD predictions versus observations
Authors: Klevas, J.; Ludwig, A. Kučinskas H. -G.; Bonifacio, P.;
Steffen, M.
2013MSAIS..24...78K Altcode: 2013arXiv1303.6838K
We study the influence of convection on the asymmetries and Doppler
shifts of Fe I spectral lines in the metal-poor red giant HD 122563. To
this end, we compute theoretical Fe I line shifts and line bisectors
using 3D hydrodynamical model atmosphere of HD 122563 calculated with
the CO5BOLD code. We then make a detailed comparison of the
theoretical line shifts and bisectors with those derived from the high
quality HARPS spectrum of HD 122563 taken from the ESO Science Archive
Facility (R=115 000, average signal-to-noise ratio, S/N ≈ 310). In
general, we find a good agreement between the theoretically predicted
and observed Doppler shifts of Fe I line cores, with somewhat larger
discrepancies seen in the case of weaker (equivalent width W&lt;5
pm) and stronger lines (W&gt;11 pm). Both observed and theoretical
coreshifts cover a range between 0 and -1 km/s, with increasingly
stronger blueshifts for weaker lines and slight hints of a coreshift
dependence on wavelength. Theoretical bisectors reproduce the observed
ones reasonably well too, however, theoretical bisectors of the weak red
(lambda &gt; 600 nm) Fe I lines have blueshifts that are by up to ∼200
m/s larger than observed. The obtained results therefore suggest that
the current CO5BOLD models are capable of reproducing the
large-scale velocity fields in the atmosphere of HD 122563 sufficiently
well. Nevertheless, further efforts are needed in order to understand
the physical reasons behind the discrepancies in theoretical predictions
and observed properties of the weakest and strongest Fe I lines.

---------------------------------------------------------
Title: The influence of convection on the atmospheric structures
and observable properties of red giant stars.
Authors: Kučinskas, A.; Ludwig, H. -G.; Steffen, M.; Dobrovolskas,
V.; Klevas, J.; Prakapavičius, D.; Caffau, E.; Bonifacio, P.
2013MSAIS..24...68K Altcode: 2013arXiv1305.3441K
During the recent years significant progress has been made in the
modeling of red giant atmospheres with the aid of 3D hydrodynamical
model atmosphere codes. In this contribution we provide an overview of
selected results obtained in this context by utilizing 3D hydrodynamical
CO5BOLD stellar model atmospheres. Hydrodynamical simulations
show that convective motions lead to significant differences in the
atmospheric structures of red giants with respect to those predicted by
the classical 1D model atmospheres. Results of these simulations also
show that in certain cases 1D models fail to reproduce even the average
properties of the 3D hydrodynamical models, such as P-T profiles. Large
horizontal temperature fluctuations in the 3D model atmospheres, as well
as differences between the temperature profiles of the average xtmean
{3D} and 1D models, lead to large discrepancies in the strengths of
spectral lines predicted by the 3D and 1D model atmospheres. This is
especially important in models at lowest metallicities ([M/H]&lt;-2.0)
where the 3D-1D abundance differences may reach (or even exceed) -0.6
dex for lines of neutral atoms and molecules. We also discuss several
simplifications and numerical aspects involved in the present 3D
hydrodynamical modeling of red giant atmospheres, and briefly address
several issues where urgent progress may be needed.

---------------------------------------------------------
Title: Molecular bands in extremely metal-poor stars: Granulation
effects
Authors: Bonifacio, P.; Caffau, E.; Ludwig, H. -G.; Spite, M.; Plez,
B.; Steffen, M.; Spite, F.
2013MSAIS..24..138B Altcode: 2013arXiv1305.2065B
The bands of diatomic molecules are important abundance indicators,
especially in metal-poor stars, where they are still measurable in
metallicity regimes where the atomic lines of their constituting
metallic elements have become vanishingly small. In order to use
them for abundance determinations it is imperative to understand the
formation of these bands. In this contribution we report on our results
obtained using CO^5{BOLD} hydrodynamical simulations. Some effects
that are qualitatively different from what found in 1D computations
are highlighted. Due to the large number of lines that form the bands,
their spectrum synthesis is computationally challenging. We discuss
some of the computational strategies we employed to parallelise the
computation and possible future developments.

---------------------------------------------------------
Title: Opacities in CO5BOLD
Authors: Ludwig, H. -G.; Steffen, M.
2013MSAIS..24...53L Altcode:
We describe the present treatment of the frequency-dependence of
the radiative transfer in CO5BOLD. This mostly refers to
the way opacities are binned into groups. We discuss the basic ideas
behind the Opacity Binning Method, give some details of the practical
implementation in CO5BOLD, and point to some issues where
improvements are needed.

---------------------------------------------------------
Title: Oxygen spectral line synthesis: 3D non-LTE with
CO5BOLD hydrodynamical model atmospheres.
Authors: Prakapavičius, D.; Steffen, M.; Kučinskas, A.; Ludwig,
H. -G.; Freytag, B.; Caffau, E.; Cayrel, R.
2013MSAIS..24..111P Altcode: 2013arXiv1303.2016P
In this work we present first results of our current project aimed at
combining the 3D hydrodynamical stellar atmosphere approach with non-LTE
(NLTE) spectral line synthesis for a number of key chemical species. We
carried out a full 3D-NLTE spectrum synthesis of the oxygen IR 777 nm
triplet, using a modified and improved version of our NLTE3D package to
calculate departure coefficients for the atomic levels of oxygen in a
CO5BOLD 3D hydrodynamical solar model atmosphere. Spectral
line synthesis was subsequently performed with the Linfor3D code. In
agreement with previous studies, we find that the lines of the
oxygen triplet produce deeper cores under NLTE conditions, due to
the diminished line source function in the line forming region. This
means that the solar oxygen IR 777 nm lines should be stronger in NLTE,
leading to negative 3D NLTE-LTE abundance corrections. Qualitatively
this result would support previous claims for a relatively low solar
oxygen abundance. Finally, we outline several further steps that need
to be taken in order to improve the physical realism and numerical
accuracy of our current 3D-NLTE calculations.

---------------------------------------------------------
Title: 3D Model Atmospheres of DA White Dwarfs
Authors: Tremblay, P. -E.; Ludwig, H. -G.; Steffen, M.; Freytag, B.
2013ASPC..469..155T Altcode:
The spectroscopically determined surface gravities of cool
hydrogen-atmosphere DA white dwarfs are significantly higher than
the mean value of log g ∼ 8 found for hotter objects with radiative
atmospheres (Teff &gt; 13000 K). It was recently suggested
that a problem with the treatment of convective energy transport in
the 1D mixing-length theory was the explanation for this high-log
g problem. We have now computed a grid of pure-hydrogen 3D model
atmospheres with the CO5BOLD code for surface gravities
from log g = 7 to 8.5 and effective temperatures from 6000 to 13000
K. Over this range, the intensity contrast of the surface granulation
pattern, which describes the strength of the 3D effects, is varying
significantly. We confirm the result of an earlier investigation that
3D model spectra provide a much better characterization of the mass
distribution of cool white dwarfs.

---------------------------------------------------------
Title: Magnetic field detection in the bright A0-type supergiant
HD 92207
Authors: Hubrig, S.; Schöller, M.; Kholtygin, A. F.; González,
J. F.; Kharchenko, N. V.; Steffen, M.
2012A&A...546L...6H Altcode:
Context. Recent developments in observational techniques and theories
revealed the potential significance of magnetic fields for stellar
structure, evolution, and circumstellar environment. At present,
the distribution of magnetic field strengths in massive stars from
the zero-age main sequence to more evolved stages, which would shed
light on the origin of the magnetic field, has not been studied. 
Aims: We searched for the presence of a magnetic field in the visually
brightest early A-type supergiant HD 92207. Methods: Observations
were obtained using the low-resolution spectropolarimetric mode of
FORS 2 (FOcal Reducer low dispersion Spectrograph) mounted on the 8 m
Antu telescope of the VLT. For the mean longitudinal magnetic field
measurements, we applied a linear regression analysis in two ways:
using only the absorption hydrogen Balmer lines or using the entire
spectrum including all available absorption lines. Results: A
mean longitudinal magnetic field at a significance level of more than
3σ was detected in two out of three observations distributed over
about one year. It is one of the rare cases where a field of about
a few hundred Gauss is detected in an early A-type supergiant. All
line profiles in the spectra of HD 92207 undergo distinct variations
in radial velocities and intensities, probably caused by previously
detected non-radial pulsations. Based on observations obtained at
the European Southern Observatory (Prgs. 074.D-0008(B), 078.D-0330(A),
087.D-0049(A), and 088.D-0425(A)).

---------------------------------------------------------
Title: The role of heat conduction to the formation of [WC]-type
planetary nebulae
Authors: Sandin, Christer; Steffen, Matthias; Jacob, Ralf;
Schönberner, Detlef; Rühling, Ute; Hamann, Wolf-Rainer; Todt, Helge
2012IAUS..283..494S Altcode:
X-ray observations of young Planetary Nebulæ (PNe) have revealed
diffuse emission in extended regions around both H-rich and
H-deficient central stars. In order to also reproduce physical
properties of H-deficient objects, we have, at first, extended our
time-dependent radiation-hydrodynamic models with heat conduction
for such conditions. Here we present some of the important physical
concepts, which determine how and when a hot wind-blown bubble
forms. In this study we have had to consider the, largely unknown,
evolution of the CSPN, the slow (AGB) wind, the fast hot-CSPN wind,
and the chemical composition. The main conclusion of our work is that
heat conduction is needed to explain X-ray properties of wind-blown
bubbles also in H-deficient objects.

---------------------------------------------------------
Title: Planetary nebula abundance determinations: A view from 1D-RHD
simulations
Authors: Jacob, Ralf; Sandin, Christer; Schönberner, Detlef; Steffen,
Matthias
2012IAUS..283..398J Altcode:
In the last years (metallicity-dependent) radiation-hydrodynamics
simulations have become a powerful tool to understand the
formation and evolution of PNe in terms of simple morphologies and
kinematics. Contrary to photoionization models, with their ad-hoc
assumptions on structure and physics, the RHD models are self-consistent
with respect to their density distribution, velocity field, chemical
composition, and stellar evolution. We use our models as simple proxies
for real PNe and investigate the reliability of common abundance
determination methods, which are based on either plasma diagnostics
or static photoionization (PI) models.

---------------------------------------------------------
Title: Modeling the diffuse X-ray emission of planetary nebulae with
different chemical composition
Authors: Steffen, Matthias; Sandin, Christer; Jacob, Ralf;
Schönberner, Detlef
2012IAUS..283..215S Altcode: 2011arXiv1109.6355S
Based on time-dependent radiation-hydrodynamics simulations of the
evolution of Planetary Nebulae (PNe), we have carried out a systematic
parameter study to address the non-trivial question of how the diffuse
X-ray emission of PNe with closed central cavities is expected to depend
on the evolutionary state of the nebula, the mass of the central star,
and the metallicity of stellar wind and circumstellar matter. We have
also investigated how the model predictions depend on the treatment of
thermal conduction at the interface between the central `hot bubble'
and the `cool' inner nebula, and compare the results with recent X-ray
observations. Our study includes models whose properties resemble
the extreme case of PNe with Wolf-Rayet type central stars. Indeed,
such models are found to produce the highest X-ray luminosities.

---------------------------------------------------------
Title: The Chandra X-Ray Survey of Planetary Nebulae (ChanPlaNS):
Probing Binarity, Magnetic Fields, and Wind Collisions
Authors: Kastner, J. H.; Montez, R., Jr.; Balick, B.; Frew, D. J.;
Miszalski, B.; Sahai, R.; Blackman, E.; Chu, Y. -H.; De Marco, O.;
Frank, A.; Guerrero, M. A.; Lopez, J. A.; Rapson, V.; Zijlstra, A.;
Behar, E.; Bujarrabal, V.; Corradi, R. L. M.; Nordhaus, J.; Parker,
Q. A.; Sandin, C.; Schönberner, D.; Soker, N.; Sokoloski, J. L.;
Steffen, M.; Ueta, T.; Villaver, E.
2012AJ....144...58K Altcode: 2012arXiv1204.6055K
We present an overview of the initial results from the Chandra Planetary
Nebula Survey (ChanPlaNS), the first systematic (volume-limited)
Chandra X-Ray Observatory survey of planetary nebulae (PNe) in the
solar neighborhood. The first phase of ChanPlaNS targeted 21 mostly
high-excitation PNe within ~1.5 kpc of Earth, yielding four detections
of diffuse X-ray emission and nine detections of X-ray-luminous point
sources at the central stars (CSPNe) of these objects. Combining these
results with those obtained from Chandra archival data for all (14)
other PNe within ~1.5 kpc that have been observed to date, we find an
overall X-ray detection rate of ~70% for the 35 sample objects. Roughly
50% of the PNe observed by Chandra harbor X-ray-luminous CSPNe, while
soft, diffuse X-ray emission tracing shocks—in most cases, "hot
bubbles"—formed by energetic wind collisions is detected in ~30%; five
objects display both diffuse and point-like emission components. The
presence (or absence) of X-ray sources appears correlated with PN
density structure, in that molecule-poor, elliptical nebulae are
more likely to display X-ray emission (either point-like or diffuse)
than molecule-rich, bipolar, or Ring-like nebulae. All but one of the
point-like CSPNe X-ray sources display X-ray spectra that are harder
than expected from hot (~100 kK) central stars emitting as simple
blackbodies; the lone apparent exception is the central star of the
Dumbbell nebula, NGC 6853. These hard X-ray excesses may suggest a high
frequency of binary companions to CSPNe. Other potential explanations
include self-shocking winds or PN mass fallback. Most PNe detected as
diffuse X-ray sources are elliptical nebulae that display a nested
shell/halo structure and bright ansae; the diffuse X-ray emission
regions are confined within inner, sharp-rimmed shells. All sample
PNe that display diffuse X-ray emission have inner shell dynamical
ages &lt;~ 5 × 103 yr, placing firm constraints on the
timescale for strong shocks due to wind interactions in PNe. The
high-energy emission arising in such wind shocks may contribute to the
high excitation states of certain archetypical "hot bubble" nebulae
(e.g., NGC 2392, 3242, 6826, and 7009).

---------------------------------------------------------
Title: Ablation and Wind Mass-Loading in the Born-Again Planetary
Nebula A 30
Authors: Guerrero, Martín A.; Chu, You-Hua; Hamann, Wolf-Rainer;
Oskinova, Lidia; Schönberner, Detlef; Todt, Helge; Steffen, Matthias;
Ruiz, Nieves; Gruendl, Robert A.; Blair, William P.
2012IAUS..283..378G Altcode:
We present XMM-Newton and Chandra observations of the born-again
planetary nebula A 30. These X-ray observations reveal a bright
unresolved source at the position of the central star whose X-ray
luminosity exceeds by far the model expectations for photospheric
emission and for shocks within the stellar wind. We suggest that
a “born-again hot bubble” may be responsible for this X-ray
emission. Diffuse X-ray emission associated with the petal-like
features and one of the H-poor knots seen in the optical is also
found. The weakened emission of carbon lines in the spectrum of the
diffuse emission can be interpreted as the dilution of stellar wind
by mass-loading or as the detection of material ejected during a very
late thermal pulse.

---------------------------------------------------------
Title: X-ray spectra of wind-driven bubbles with chemical gradients
Authors: Jacob, Ralf; Sandin, Christer; Schönberner, Detlef; Steffen,
Matthias
2012IAUS..283..396J Altcode:
As a tool helping to interpret diffuse X-ray emission of PNe, and as a
supplement to our RHD simulations, we have started to construct a grid
of theoretical X-ray spectra of wind-blown bubbles with temperature and
density profiles according to thermal conduction theory. We investigate
how the X-ray spectra depend on chemical composition (e.g. H-rich
vs. H-deficient) and how temperature and abundance determinations
reflect gradients of temperature and chemical composition within
the bubbles. These synthetic models shall allow to quickly perform
detailed parameter studies without the need for dedicated hydrodynamical
simulations. We report on ideas and goals.

---------------------------------------------------------
Title: On the internal kinematics of PNe
Authors: Jacob, Ralf; Schönberner, Detlef; Lehmann, Holger; Zwanzig,
Alena; Sandin, Christer; Steffen, Matthias
2012IAUS..283..400J Altcode:
Our sample of round/elliptical double-shell PNe around central stars
(CSs) with H-rich surface chemistry covers all evolutionary phases
across the HRD. By means of high-resolution and high-S/N spectra we
determine bulk matter velocities of the inner wind-driven rims and
the maximum (= post-shock) gas velocities of the surrounding thermally
expanding shells. Studying the details of the internal kinematics allows
a look at processes of PN formation and at PN expansion history helping,
for instance, to determine expansion distances.

---------------------------------------------------------
Title: Rebirth of X-Ray Emission from the Born-again Planetary
Nebula A30
Authors: Guerrero, M. A.; Ruiz, N.; Hamann, W. -R.; Chu, Y. -H.;
Todt, H.; Schönberner, D.; Oskinova, L.; Gruendl, R. A.; Steffen,
M.; Blair, W. P.; Toalá, J. A.
2012ApJ...755..129G Altcode: 2012arXiv1202.4463G
The planetary nebula A30 is believed to have undergone a very late
thermal pulse resulting in the ejection of knots of hydrogen-poor
material. Using multi-epoch Hubble Space Telescope images, we have
detected the angular expansion of these knots and derived an age of
850+280 - 150 yr. To investigate the spectral
and spatial properties of the soft X-ray emission detected by ROSAT,
we have obtained Chandra and XMM-Newton deep observations of A30. The
X-ray emission from A30 can be separated into two components: a point
source at the central star and diffuse emission associated with the
hydrogen-poor knots and the cloverleaf structure inside the nebular
shell. To help us assess the role of the current stellar wind in
powering this X-ray emission, we have determined the stellar parameters
and wind properties of the central star of A30 using a non-LTE model fit
to its optical and UV spectra. The spatial distribution and spectral
properties of the diffuse X-ray emission are highly suggestive that
it is generated by the post-born-again and present fast stellar winds
interacting with the hydrogen-poor ejecta of the born-again event. This
emission can be attributed to shock-heated plasma, as the hydrogen-poor
knots are ablated by the stellar winds, under which circumstances
the efficient mass loading of the present fast stellar wind raises
its density and damps its velocity to produce the observed diffuse
soft X-rays. Charge transfer reactions between the ions of the stellar
winds and material of the born-again ejecta have also been considered
as a possible mechanism for the production of diffuse X-ray emission,
and upper limits on the expected X-ray production by this mechanism
have been derived. The origin of the X-ray emission from the central
star of A30 is puzzling: shocks in the present fast stellar wind and
photospheric emission can be ruled out, while the development of a new,
compact hot bubble confining the fast stellar wind seems implausible.

---------------------------------------------------------
Title: A primordial star in the heart of the Lion
Authors: Caffau, E.; Bonifacio, P.; François, P.; Spite, M.; Spite,
F.; Zaggia, S.; Ludwig, H. -G.; Steffen, M.; Mashonkina, L.; Monaco,
L.; Sbordone, L.; Molaro, P.; Cayrel, R.; Plez, B.; Hill, V.; Hammer,
F.; Randich, S.
2012A&A...542A..51C Altcode: 2012arXiv1203.2607C
Context. The discovery and chemical analysis of extremely metal-poor
stars permit a better understanding of the star formation of the first
generation of stars and of the Universe emerging from the Big Bang. Aims: We report the study of a primordial star situated in the centre
of the constellation Leo (SDSS J102915+172927). Methods: The
star, selected from the low-resolution spectrum of the Sloan Digital
Sky Survey, was observed at intermediate (with X-Shooter at VLT) and
at high spectral resolution (with UVES at VLT). The stellar parameters
were derived from the photometry. The standard spectroscopic analysis
based on 1D ATLAS models was completed by applying 3D and non-LTE
corrections. Results: An iron abundance of [Fe/H ] = -4.89 makes
SDSS J102915+172927 one of the lowest [Fe/H] stars known. However,
the absence of measurable C and N enhancements indicates that it has
the lowest metallicity, Z ≤ 7.40 × 10-7 (metal-mass
fraction), ever detected. No oxygen measurement was possible. 
Conclusions: The discovery of SDSS J102915+172927 highlights that
low-mass star formation occurred at metallicities lower than previously
assumed. Even lower metallicity stars may yet be discovered, with a
chemical composition closer to the composition of the primordial gas
and of the first supernovae. Based on observations obtained at ESO
Paranal Observatory, GTO programme 086.D-0094 and programme 286.D-5045.

---------------------------------------------------------
Title: Excitation of Slow-Modes in Network Magnetic Elements
Authors: Kato, Y.; Steiner, O.; Steffen, M.; Suematsu, Y.
2012ASPC..455..237K Altcode:
From radiation magnetohydrodynamic (RMHD) simulations of the solar
atmosphere we have found a new mechanism for the excitation of
longitudinal slow modes within magnetic flux concentrations. It is
found that the convective downdrafts in the immediate surroundings of
magnetic elements are responsible for the excitation of slow modes. The
coupling between the external downdraft and the plasma motion internal
to the flux concentration is mediated by the inertial forces of the
downdraft that act on the magnetic flux concentration. These forces
pump the internal atmosphere in the downward direction, which entails
a fast downflow in the photospheric and chromospheric layers of the
magnetic element. Subsequent to the transient pumping phase, the
atmosphere rebounds, causing a slow mode traveling along the magnetic
flux concentration in the upward direction and developing into a shock
wave in chromospheric heights, possibly capable of producing some kind
of dynamic fibril. This event occurs recurrently. We compare the power
spectra of the temperature and velocity of the flux-sheet atmosphere
to the corresponding spectra of the unmagnetized atmosphere.

---------------------------------------------------------
Title: Using Hubble Space Telescope Images to Test Theoretical Models
of Planetary Nebulae
Authors: Braxton, Kelsey M.; Balick, B.; Jacob, R.; Steffen, M.;
Schonberner, D.
2012AAS...22043110B Altcode:
Classical ionization models that predict the ionization structure
of planetary nebulae generally assume constant gas density (or
a central void and constant density) in the shell. More recently,
Steffen, Jacob, Schoenberner (2005, A&amp;A, 441, 573) have computed
fully consistent one-dimensional radiation hydrodynamic ionization
models (RHI models) with realistic stellar winds and evolving central
stars. Their numerical results offer concrete predictions concerning
the projected three-dimensional shapes of simple round and elliptical
PNe. A first glance at their models shows them to be a superior way of
modeling ionization structures in realistic PNe. We present high-spatial
resolution and carefully flux calibrated images of PNe selected in
Halpha, [OIII], and [NII] as well as the ratios [OIII]/Halpha and
[NII]/Halpha in order to test the predictions of the RHI models. These
were obtained uniformly and at the same epoch using the WFPC2 camera on
the Hubble Space Telescope. Some of the targets are ionization bounded
and others are density bounded. In the future these images will be
compared to the models in detail. At this point we note many of the
common features--some of them unexpected--that the models must explain.

---------------------------------------------------------
Title: LTE Model Atmospheres: MARCS, ATLAS and CO5BOLD
Authors: Bonifacio, P.; Caffau, E.; Ludwig, H. -G.; Steffen, M.
2012IAUS..282..213B Altcode:
In this talk, we review the basic assumptions and physics covered
by classical 1D LTE model atmospheres. We will focus on ATLAS and
MARCS models of F-G-K stars and describe what resources are available
through the web, both in terms of codes and model-atmosphere grids. We
describe the advances made in hydrodynamical simulations of convective
stellar atmospheres with the CO5BOLD code and what grids
and resources are available, with a prospect of what will be available
in the near future.

---------------------------------------------------------
Title: The Gaia-ESO Public Spectroscopic Survey
Authors: Gilmore, G.; Randich, S.; Asplund, M.; Binney, J.; Bonifacio,
P.; Drew, J.; Feltzing, S.; Ferguson, A.; Jeffries, R.; Micela, G.;
Negueruela, I.; Prusti, T.; Rix, H. -W.; Vallenari, A.; Alfaro, E.;
Allende-Prieto, C.; Babusiaux, C.; Bensby, T.; Blomme, R.; Bragaglia,
A.; Flaccomio, E.; François, P.; Irwin, M.; Koposov, S.; Korn, A.;
Lanzafame, A.; Pancino, E.; Paunzen, E.; Recio-Blanco, A.; Sacco,
G.; Smiljanic, R.; Van Eck, S.; Walton, N.; Aden, D.; Aerts, C.;
Affer, L.; Alcala, J. -M.; Altavilla, G.; Alves, J.; Antoja, T.;
Arenou, F.; Argiroffi, C.; Asensio Ramos, A.; Bailer-Jones, C.;
Balaguer-Nunez, L.; Bayo, A.; Barbuy, B.; Barisevicius, G.; Barrado
y Navascues, D.; Battistini, C.; Bellas Velidis, I.; Bellazzini, M.;
Belokurov, V.; Bergemann, M.; Bertelli, G.; Biazzo, K.; Bienayme, O.;
Bland-Hawthorn, J.; Boeche, C.; Bonito, S.; Boudreault, S.; Bouvier,
J.; Brandao, I.; Brown, A.; de Bruijne, J.; Burleigh, M.; Caballero,
J.; Caffau, E.; Calura, F.; Capuzzo-Dolcetta, R.; Caramazza, M.;
Carraro, G.; Casagrande, L.; Casewell, S.; Chapman, S.; Chiappini,
C.; Chorniy, Y.; Christlieb, N.; Cignoni, M.; Cocozza, G.; Colless,
M.; Collet, R.; Collins, M.; Correnti, M.; Covino, E.; Crnojevic,
D.; Cropper, M.; Cunha, M.; Damiani, F.; David, M.; Delgado, A.;
Duffau, S.; Edvardsson, B.; Eldridge, J.; Enke, H.; Eriksson, K.;
Evans, N. W.; Eyer, L.; Famaey, B.; Fellhauer, M.; Ferreras, I.;
Figueras, F.; Fiorentino, G.; Flynn, C.; Folha, D.; Franciosini,
E.; Frasca, A.; Freeman, K.; Fremat, Y.; Friel, E.; Gaensicke, B.;
Gameiro, J.; Garzon, F.; Geier, S.; Geisler, D.; Gerhard, O.; Gibson,
B.; Gomboc, A.; Gomez, A.; Gonzalez-Fernandez, C.; Gonzalez Hernandez,
J.; Gosset, E.; Grebel, E.; Greimel, R.; Groenewegen, M.; Grundahl,
F.; Guarcello, M.; Gustafsson, B.; Hadrava, P.; Hatzidimitriou, D.;
Hambly, N.; Hammersley, P.; Hansen, C.; Haywood, M.; Heber, U.; Heiter,
U.; Held, E.; Helmi, A.; Hensler, G.; Herrero, A.; Hill, V.; Hodgkin,
S.; Huelamo, N.; Huxor, A.; Ibata, R.; Jackson, R.; de Jong, R.;
Jonker, P.; Jordan, S.; Jordi, C.; Jorissen, A.; Katz, D.; Kawata,
D.; Keller, S.; Kharchenko, N.; Klement, R.; Klutsch, A.; Knude,
J.; Koch, A.; Kochukhov, O.; Kontizas, M.; Koubsky, P.; Lallement,
R.; de Laverny, P.; van Leeuwen, F.; Lemasle, B.; Lewis, G.; Lind,
K.; Lindstrom, H. P. E.; Lobel, A.; Lopez Santiago, J.; Lucas, P.;
Ludwig, H.; Lueftinger, T.; Magrini, L.; Maiz Apellaniz, J.; Maldonado,
J.; Marconi, G.; Marino, A.; Martayan, C.; Martinez-Valpuesta, I.;
Matijevic, G.; McMahon, R.; Messina, S.; Meyer, M.; Miglio, A.;
Mikolaitis, S.; Minchev, I.; Minniti, D.; Moitinho, A.; Momany, Y.;
Monaco, L.; Montalto, M.; Monteiro, M. J.; Monier, R.; Montes, D.;
Mora, A.; Moraux, E.; Morel, T.; Mowlavi, N.; Mucciarelli, A.; Munari,
U.; Napiwotzki, R.; Nardetto, N.; Naylor, T.; Naze, Y.; Nelemans, G.;
Okamoto, S.; Ortolani, S.; Pace, G.; Palla, F.; Palous, J.; Parker, R.;
Penarrubia, J.; Pillitteri, I.; Piotto, G.; Posbic, H.; Prisinzano,
L.; Puzeras, E.; Quirrenbach, A.; Ragaini, S.; Read, J.; Read, M.;
Reyle, C.; De Ridder, J.; Robichon, N.; Robin, A.; Roeser, S.; Romano,
D.; Royer, F.; Ruchti, G.; Ruzicka, A.; Ryan, S.; Ryde, N.; Santos,
N.; Sanz Forcada, J.; Sarro Baro, L. M.; Sbordone, L.; Schilbach, E.;
Schmeja, S.; Schnurr, O.; Schoenrich, R.; Scholz, R. -D.; Seabroke, G.;
Sharma, S.; De Silva, G.; Smith, M.; Solano, E.; Sordo, R.; Soubiran,
C.; Sousa, S.; Spagna, A.; Steffen, M.; Steinmetz, M.; Stelzer, B.;
Stempels, E.; Tabernero, H.; Tautvaisiene, G.; Thevenin, F.; Torra,
J.; Tosi, M.; Tolstoy, E.; Turon, C.; Walker, M.; Wambsganss, J.;
Worley, C.; Venn, K.; Vink, J.; Wyse, R.; Zaggia, S.; Zeilinger, W.;
Zoccali, M.; Zorec, J.; Zucker, D.; Zwitter, T.; Gaia-ESO Survey Team
2012Msngr.147...25G Altcode:
The Gaia-ESO Public Spectroscopic Survey has begun and will obtain high
quality spectroscopy of some 100000 Milky Way stars, in the field and
in open clusters, down to magnitude 19, systematically covering all the
major components of the Milky Way. This survey will provide the first
homogeneous overview of the distributions of kinematics and chemical
element abundances in the Galaxy. The motivation, organisation and
implementation of the Gaia-ESO Survey are described, emphasising the
complementarity with the ESA Gaia mission. Spectra from the very first
observing run of the survey are presented.

---------------------------------------------------------
Title: Simulations of the solar near-surface layers with the CO5BOLD,
MURaM, and Stagger codes
Authors: Beeck, B.; Collet, R.; Steffen, M.; Asplund, M.; Cameron,
R. H.; Freytag, B.; Hayek, W.; Ludwig, H. -G.; Schüssler, M.
2012A&A...539A.121B Altcode: 2012arXiv1201.1103B
Context. Radiative hydrodynamic simulations of solar and stellar surface
convection have become an important tool for exploring the structure and
gas dynamics in the envelopes and atmospheres of late-type stars and for
improving our understanding of the formation of stellar spectra. Aims: We quantitatively compare results from three-dimensional,
radiative hydrodynamic simulations of convection near the solar surface
generated with three numerical codes (CO5BOLD, MURaM,
and Stagger) and different simulation setups in order to investigate
the level of similarity and to cross-validate the simulations. Methods: For all three simulations, we considered the average
stratifications of various quantities (temperature, pressure, flow
velocity, etc.) on surfaces of constant geometrical or optical depth,
as well as their temporal and spatial fluctuations. We also compared
observables, such as the spatially resolved patterns of the emerging
intensity and of the vertical velocity at the solar optical surface
as well as the center-to-limb variation of the continuum intensity
at various wavelengths. Results: The depth profiles of the
thermodynamical quantities and of the convective velocities as well as
their spatial fluctuations agree quite well. Slight deviations can be
understood in terms of differences in box size, spatial resolution
and in the treatment of non-gray radiative transfer between the
simulations. Conclusions: The results give confidence in the
reliability of the results from comprehensive radiative hydrodynamic
simulations.

---------------------------------------------------------
Title: Simulations of stellar convection with CO5BOLD
Authors: Freytag, B.; Steffen, M.; Ludwig, H. -G.; Wedemeyer-Böhm,
S.; Schaffenberger, W.; Steiner, O.
2012JCoPh.231..919F Altcode: 2011arXiv1110.6844F
High-resolution images of the solar surface show a granulation
pattern of hot rising and cooler downward-sinking material - the
top of the deep-reaching solar convection zone. Convection plays a
role for the thermal structure of the solar interior and the dynamo
acting there, for the stratification of the photosphere, where most
of the visible light is emitted, as well as for the energy budget of
the spectacular processes in the chromosphere and corona. Convective
stellar atmospheres can be modeled by numerically solving the coupled
equations of (magneto)hydrodynamics and non-local radiation transport
in the presence of a gravity field. The CO5BOLD code described in this
article is designed for so-called "realistic" simulations that take
into account the detailed microphysics under the conditions in solar
or stellar surface layers (equation-of-state and optical properties of
the matter). These simulations indeed deserve the label "realistic"
because they reproduce the various observables very well - with only
minor differences between different implementations. The agreement
with observations has improved over time and the simulations are now
well-established and have been performed for a number of stars. Still,
severe challenges are encountered when it comes to extending these
simulations to include ideally the entire star or substellar object:
the strong stratification leads to completely different conditions in
the interior, the photosphere, and the corona. Simulations have to cover
spatial scales from the sub-granular level to the stellar diameter and
time scales from photospheric wave travel times to stellar rotation
or dynamo cycle periods. Various non-equilibrium processes have to be
taken into account. Last but not least, realistic simulations are based
on detailed microphysics and depend on the quality of the input data,
which can be the actual accuracy limiter. This article provides an
overview of the physical problem and the numerical solution and the
capabilities of CO5BOLD, illustrated with a number of applications.

---------------------------------------------------------
Title: 6Li detection in metal-poor stars: can 3D model
atmospheres solve the second lithium problem?
Authors: Steffen, M.; Cayrel, R.; Caffau, E.; Bonifacio, P.; Ludwig,
H. -G.; Spite, M.
2012MSAIS..22..152S Altcode: 2012arXiv1206.2239S
The presence of 6Li in the atmospheres of metal-poor
halo stars is usually inferred from the detection of a subtle extra
depression in the red wing of the 7Li doublet line at
670.8 nm. However, as pointed out recently by \cite{Cayrel2007},
the intrinsic line asymmetry caused by convective flows in the
photospheres of cool stars is almost indistinguishable from the
asymmetry produced by a weak 6Li blend on a (presumed)
symmetric 7Li profile. Previous determinations of the
6Li/ 7Li isotopic ratio based on 1D model
atmospheres, ignoring the convection-induced line asymmetry, must
therefore be considered as upper limits. By comparing synthetic
1D LTE and 3D non-LTE line profiles of the iLi 670.8 nm
feature, we quantify the differential effect of the convective line
asymmetry on the derived 6Li abundance as a function of
effective temperature, gravity, and metallicity. As expected, we
find that the asymmetry effect systematically reduces the resulting
6Li/7Li ratios. Depending on the stellar
parameters, the 3D-1D offset in 6Li/7Li ranges
between -0.005 and -0.020. When this purely theoretical correction is
taken into account for the \cite{A2006} sample of stars, the number of
significant 6Li detections decreases from 9 to 5 (2sigma
criterion), or from 5 to 2 (3sigma criterion). We also present
preliminary results of a re-analysis of high-resolution, high S/N
spectra of individual metal-poor turn-off stars, to see whether the
second Lithium problem actually disappears when accounting properly for
convection and non-LTE line formation in 3D stellar atmospheres. Out
of 8 stars, HD 84937 seems to be the only significant (2sigma )
detection of 6Li. In view of our results, the existence of
a 6Li plateau appears questionable.

---------------------------------------------------------
Title: 3D Model Atmospheres of Red Giant Stars
Authors: Ludwig, Hans-Günter; Steffen, Matthias
2012ASSP...26..125L Altcode: 2011arXiv1108.4554L; 2012rgps.book..125L
We provide a brief overview of the modelling of the atmospheres of
red giant stars with the 3D radiation-hydrodynamics code CO5BOLD. We
emphasize aspects where 3D modelling provides additional insight
beyond standard hydrostatic 1D models, and comment on present modelling
challenges.

---------------------------------------------------------
Title: Radiation-Hydrodynamics Simulations of Cool Stellar and
Substellar Atmospheres
Authors: Freytag, B.; Allard, F.; Ludwig, H. -G.; Homeier, D.;
Steffen, M.
2011ASPC..448..855F Altcode: 2011csss...16..855F
In the atmospheres of brown dwarfs, not only molecules but much
larger and heavier "dust" particles can form. The latter should sink
under the influence of gravity into deeper layers and vanish from the
atmosphere, clearing it from condensable material. However, observed
spectra can only be reproduced by models assuming the presence of dust
and its resulting greenhouse effect in the visible layers. Apparently,
hydrodynamical mixing can counteract the gravitational settling. We
present new 2D and 3D radiation-hydrodynamics simulations with CO5BOLD
of the upper part of the convection zone and the atmosphere of cool
stars and brown dwarfs in a range of temperatures and gravities
that enable the formation of dust clouds in the visible layers. We
find that the differences between 2D and 3D models are remarkably
small. Lowering the gravity has a somewhat similar effect on the surface
intensity contrast as increasing the effective temperature. The biggest
uncertainties of the simulations come from approximations made in the
description of the dust chemistry. Global circulation and rotation
likely play an important role.

---------------------------------------------------------
Title: Radiation Hydrodynamics Simulations of Dust Clouds in the
Atmospheres of Substellar Objects
Authors: Freytag, B.; Allard, F.; Homeier, D.; Ludwig, H.; Steffen, M.
2011ASPC..450..125F Altcode:
The temperature structure and the motions in the atmospheres of cool
stars are affected by the underlying convection zone. The radiation
hydrodynamics code CO5BOLD has been developed to simulate (small patches
of the) convective surface layers of these stars. Updated opacity tables
based on PHOENIX data and a description for the formation, destruction,
advective transport, and settling of dust have made the code fit to
handle the conditions in brown dwarf atmospheres. Currently, objects
from 8500 K down to about 900 K have been simulated. Recently, incident
radiation has been included, allowing simulations with conditions
found on hot planets. In non-irradiated brown dwarf models we encounter
mixing by gravity waves and in the cooler models convection within the
clouds. The qualitative effects of incident radiation are surprisingly
small, as long as the effective temperature of the object stays well
below the dust condensation temperature. Beyond that point, there are
no layers where dust could form, anymore.

---------------------------------------------------------
Title: LTE model atmopsheres MARCS, ATLAS and CO5BOLD
Authors: Bonifacio, Piercarlo; Caffau, Elisabetta; Ludwig,
Hans-Guenter; Steffen, Matthias
2011arXiv1109.0717B Altcode:
In this talk we review the basic assumptions and physics covered by
classical 1D LTE model atmospheres. We will focus on ATLAS and MARCS
models of F-G-K stars and describe what resources are available through
the web, both in terms of codes and model-atmosphere grids. We describe
the advances made in hydrodynamical simulations of convective stellar
atmospheres with the CO5BOLD code and what grids and resources are
available, with a prospect of what will be available in the near future.

---------------------------------------------------------
Title: The Galactic evolution of phosphorus
Authors: Caffau, E.; Bonifacio, P.; Faraggiana, R.; Steffen, M.
2011A&A...532A..98C Altcode: 2011arXiv1107.2657C
Context. As a galaxy evolves, its chemical composition changes and
the abundance ratios of different elements are powerful probes of
the underlying evolutionary processes. Phosphorous is an element
whose evolution has remained quite elusive until now, because it is
difficult to detect in cool stars. The infrared weak P i lines of
the multiplet 1, at 1050-1082 nm, are the most reliable indicators
of the presence of phosphorus. The availability of CRIRES at VLT has
permitted access to this wavelength range in stellar spectra. 
Aims: We attempt to measure the phosphorus abundance of twenty cool
stars in the Galactic disk. Methods: The spectra are analysed
with one-dimensional model-atmospheres computed in local thermodynamic
equilibrium (LTE). The line formation computations are performed
assuming LTE. Results: The ratio of phosphorus to iron behaves
similarly to sulphur, increasing towards lower metallicity stars. Its
ratio with respect to sulphur is roughly constant and slightly larger
than solar, [P/S] = 0.10 ± 0.10. Conclusions: We succeed in
taking an important step towards the understanding of the chemical
evolution of phosphorus in the Galaxy. However, the observed rise in
the P/Fe abundance ratio is steeper than predicted by Galactic chemical
evolution model developed by Kobayashi and collaborators. Phosphorus
appears to evolve differently from the light odd-Z elements sodium
and aluminium. The constant value of [P/S] with metallicity implies
that P production is insensitive to the neutron excess, thus processes
other than neutron captures operate. We suggest that proton captures on
30Si and α captures on 27Al are possibilities
to investigate. We see no clear distinction between our results for
stars with planets and stars without any detected planet. Based
on observations obtained with the CRIRES spectrograph at ESO-VLT Antu
8.2 m telescope at Paranal, Programme 386.D-0130, P.I. E. Caffau.

---------------------------------------------------------
Title: Excitation of magneto-acoustic waves in network magnetic
elements
Authors: Kato, Yoshiaki; Steiner, Oskar; Steffen, Matthias; Suematsu,
Yoshinori
2011IAUS..273..442K Altcode:
From radiation magnetohydrodynamic (RMHD) simulations we track the
temporal evolution of a vertical magnetic flux sheet embedded in a
two-dimensional non-stationary atmosphere that reaches all the way
from the upper convection zone to the low chromosphere. Examining its
temporal behavior near the interface between the convection zone and
the photosphere, we describe the excitation of propagating longitudinal
waves within the magnetic element as a result of convective motion in
its surroundings.

---------------------------------------------------------
Title: Solution to the problem of the surface gravity distribution
of cool DA white dwarfs from improved 3D model atmospheres
Authors: Tremblay, P. -E.; Ludwig, H. -G.; Steffen, M.; Bergeron,
P.; Freytag, B.
2011A&A...531L..19T Altcode: 2011arXiv1106.6007T
The surface gravities of cool (Teff &lt; 13 000 K)
hydrogen-atmosphere DA white dwarfs, determined from spectroscopic
analyses, are found to be significantly higher than the canonical value
of log g ~ 8 expected for these stars. It was recently concluded that
a problem with the treatment of convective energy transport within
the framework of the mixing-length theory was the most plausible
explanation for this high-log g problem. We pursue the investigation
of this discrepancy by computing model spectra of cool convective
white dwarfs from a small sequence (11 300 K &lt; Teff
&lt; 12 800 K) of 3D hydrodynamical model atmospheres, which feature
a sophisticated treatment of convection and radiative transfer. Our
approach is to proceed with a differential analysis between 3D and
standard 1D models. We find that the 3D spectra predict significantly
lower surface gravities, with corrections of the right amplitude
as a function of effective temperature to obtain values of log g ~
8 on average. We conclude that the surface gravity distribution of
cool convective DA white dwarfs is much closer to that of hotter
radiative objects when using, for the treatment of the convection,
3D models instead of the mixing-length framework. Figure 2 is
available in electronic form at http://www.aanda.org

---------------------------------------------------------
Title: Excitation of Slow Modes in Network Magnetic Elements Through
Magnetic Pumping
Authors: Kato, Yoshiaki; Steiner, Oskar; Steffen, Matthias; Suematsu,
Yoshinori
2011ApJ...730L..24K Altcode: 2011arXiv1102.5164K
From radiation magnetohydrodynamic simulations of the solar atmosphere,
we find a new mechanism for the excitation of longitudinal slow modes
within magnetic flux concentrations. We find that the convective
downdrafts in the immediate surroundings of magnetic elements are
responsible for the excitation of slow modes. The coupling between
the external downdraft and the plasma motion internal to the flux
concentration is mediated by the inertial forces of the downdraft that
act on the magnetic flux concentration. These forces, in conjunction
with the downward movement, pump the internal atmosphere in the
downward direction, which entails a fast downdraft in the photospheric
and chromospheric layers of the magnetic element. Subsequent to the
transient pumping phase, the atmosphere rebounds, causing a slow
mode traveling along the magnetic flux concentration in the upward
direction. It develops into a shock wave in chromospheric heights,
possibly capable of producing some kind of dynamic fibril. We propose
an observational detection of this process.

---------------------------------------------------------
Title: The solar photospheric abundance of zirconium
Authors: Caffau, E.; Faraggiana, R.; Ludwig, H. -G.; Bonifacio, P.;
Steffen, M.
2011AN....332..128C Altcode: 2010arXiv1012.1038C
Zirconium (Zr), together with strontium and yttrium, is an important
element in the understanding of the Galactic nucleosynthesis. In
fact, the triad Sr-Y-Zr constitutes the first peak of s-process
elements. Despite its general relevance not many studies of the solar
abundance of Zr were conducted. We derive the zirconium abundance in
the solar photosphere with the same CO5BOLD hydrodynamical
model of the solar atmosphere that we previously used to investigate
the abundances of C-N-O. We review the zirconium lines available in
the observed solar spectra and select a sample of lines to determine
the zirconium abundance, considering lines of neutral and singly
ionised zirconium. We apply different line profile fitting strategies
for a reliable analysis of Zr lines that are blended by lines of other
elements. The abundance obtained from lines of neutral zirconium is very
uncertain because these lines are commonly blended and weak in the solar
spectrum. However, we believe that some lines of ionised zirconium are
reliable abundance indicators. Restricting the set to Zr II lines,
from the CO5BOLD 3D model atmosphere we derive A(Zr)
{=2.62± 0.06}, where the quoted error is the RMS line-to-line scatter.

---------------------------------------------------------
Title: Solar Chemical Abundances Determined with a CO5BOLD 3D Model
Atmosphere
Authors: Caffau, E.; Ludwig, H. -G.; Steffen, M.; Freytag, B.;
Bonifacio, P.
2011SoPh..268..255C Altcode: 2010SoPh..tmp...66C; 2010arXiv1003.1190C
In the last decade, the photospheric solar metallicity as determined
from spectroscopy experienced a remarkable downward revision. Part
of this effect can be attributed to an improvement of atomic data and
the inclusion of NLTE computations, but also the use of hydrodynamical
model atmospheres seemed to play a role. This "decrease" with time of
the metallicity of the solar photosphere increased the disagreement
with the results from helioseismology. With a CO5BOLD 3D
model of the solar atmosphere, the CIFIST team at the Paris Observatory
re-determined the photospheric solar abundances of several elements,
among them C, N, and O. The spectroscopic abundances are obtained by
fitting the equivalent width and/or the profile of observed spectral
lines with synthetic spectra computed from the 3D model atmosphere. We
conclude that the effects of granular fluctuations depend on the
characteristics of the individual lines, but are found to be relevant
only in a few particular cases. 3D effects are not responsible for
the systematic lowering of the solar abundances in recent years. The
solar metallicity resulting from this analysis is Z=0.0153, Z/X=0.0209.

---------------------------------------------------------
Title: Watching planetary nebulae grow with HST
Authors: Balick, B.; Huehnerhoff, J.; Steffen, M.; Schoenberner, D.;
Hajian, A.; Behr, B. B.
2011apn5.confP..43B Altcode: 2011apn5.procA..43B
HST/WFPC2/PC images of PNe spanning over a decade allow secular changes
in their sharp-edged structures to monitored and compared to the
predictions of hydrodynamical interacting winds models. We selected a
sample of 18 simple, bright, and relatively nearby targets for second-
or third-epoch observations in 2008 using the F502N ([OIII]) and F658N
([NII]) filters. All data sets were identically calibrated and compared
using difference images. About 75% showed signs of expansion (by up to
1%) and changes in surface brightness (by up to ±5%). In most cases
the expansions were largely uniform. Exceptions tend to be found in
ionization fronts at the perimeter of the ionized gas, though NGC6543
shows signs of nonuniform expansion in its interior as well. Results
for NGC 2392, 3132, 3242, 3918, and 6543 were presented.

---------------------------------------------------------
Title: On the chemical composition of the metal-poor planetary
nebula PNG135.9+55.9
Authors: Sandin, C.; Jacob, R.; Schoenberner, D.; Steffen, M.; Roth,
M. M.
2011apn5.confP..51S Altcode: 2011apn5.procA..51S
The metal-poor planetary nebula PN G135.9+55.9 has a particularly-low
oxygen abundance, that is a matter of an ongoing discussion. We
report on our recent results of both new accurate observations by
means of integral field spectroscopy with PMAS, and on the outcome of
new radiation hydrodynamics models. Our goal with these studies was
to calculate new abundance estimates. We find that expansion cooling,
and deviations from thermal equilibrium, become increasingly important
to the physical structure at metalicities that are as low as in this
object. The resulting low electron temperatures cause substantial
deviations in the estimated abundances compared to an approach using
standard hydrostatic photo-ionization models.

---------------------------------------------------------
Title: The role of thermal conduction in WR-type planetary nebulae
Authors: Sandin, C.; Steffen, M.; Schoenberner, D.; Ruehling, U.;
Hamann, W. R.
2011apn5.confP..53S Altcode: 2011apn5.procA..53S
Thermal conduction plays an important role to the interpretation of
X-ray spectra of planetary nebulae. Models including this effect so
far only used a theoretical formulation that assumes a pure hydrogen
composition. To permit modeling of objects with other compositions,
such as Wolf-Rayet stars, we have now extended the thermal-conduction
description in our models. We will present the outcome of our study
in terms of how these changes affect the new models and the predicted
X-ray emission spectra.

---------------------------------------------------------
Title: What does the Sun suggest about global oscillation amplitudes
in solar-like stars?
Authors: Severino, G.; Straus, Th.; Oliviero, M.; Steffen, M.
2010AN....331..896S Altcode:
Motivated by the results of CoRoT and based on the solar experience,
we develop a new stellar scaling law for the intensity-velocity
amplitude ratio (gain) of resonant oscillations, which is a quantity
independent of the excitation model. The comparison of our approach
with observations gives new interesting results for the Sun as a
star. Moreover, for a sample of three solar-like stars observed
by CoRoT, we found that our scaling law provides an explanation of
the low observed luminosity amplitudes which is an alternative to
non-adiabatic effects.

---------------------------------------------------------
Title: The metal-poor end of the Spite plateau. I. Stellar parameters,
metallicities, and lithium abundances
Authors: Sbordone, L.; Bonifacio, P.; Caffau, E.; Ludwig, H. -G.;
Behara, N. T.; González Hernández, J. I.; Steffen, M.; Cayrel, R.;
Freytag, B.; van't Veer, C.; Molaro, P.; Plez, B.; Sivarani, T.; Spite,
M.; Spite, F.; Beers, T. C.; Christlieb, N.; François, P.; Hill, V.
2010A&A...522A..26S Altcode: 2010arXiv1003.4510S
Context. The primordial nature of the Spite plateau is at odds with
the WMAP satellite measurements, implying a primordial Li production
at least three times higher than observed. It has also been suggested
that A(Li) might exhibit a positive correlation with metallicity below
[Fe/H] ~ -2.5. Previous samples studied comprised few stars below
[Fe/H] = -3. Aims: We present VLT-UVES Li abundances of 28
halo dwarf stars between [Fe/H] = -2.5 and -3.5, ten of which have
[Fe/H] &lt;-3. Methods: We determined stellar parameters and
abundances using four different Teff scales. The direct
infrared flux method was applied to infrared photometry. Hα wings were
fitted with two synthetic grids computed by means of 1D LTE atmosphere
models, assuming two different self-broadening theories. A grid of Hα
profiles was finally computed by means of 3D hydrodynamical atmosphere
models. The Li i doublet at 670.8 nm has been used to measure A(Li)
by means of 3D hydrodynamical NLTE spectral syntheses. An analytical
fit of A(Li)3D, NLTE as a function of equivalent width,
Teff, log g, and [Fe/H] has been derived and is made
available. Results: We confirm previous claims that A(Li)
does not exhibit a plateau below [Fe/H] = -3. We detect a strong
positive correlation with [Fe/H] that is insensitive to the choice of
Teff estimator. From a linear fit, we infer a steep slope
of about 0.30 dex in A(Li) per dex in [Fe/H], which has a significance
of 2-3σ. The slopes derived using the four Teff estimators
are consistent to within 1σ. A significant slope is also detected
in the A(Li)-Teff plane, driven mainly by the coolest
stars in the sample (Teff &lt; 6250), which appear to be
Li-poor. However, when we remove these stars the slope detected in
the A(Li)-[Fe/H] plane is not altered significantly. When the full
sample is considered, the scatter in A(Li) increases by a factor
of 2 towards lower metallicities, while the plateau appears very
thin above [Fe/H] = -2.8. At this metallicity, the plateau lies at
&lt;A(Li)3D, NLTE&gt; = 2.199±0.086. Conclusions:
The meltdown of the Spite plateau below [Fe/H] ~ -3 is established,
but its cause is unclear. If the primordial A(Li) were that derived
from standard BBN, it appears difficult to envision a single depletion
phenomenon producing a thin, metallicity independent plateau above
[Fe/H] = -2.8, and a highly scattered, metallicity dependent
distribution below. That no star below [Fe/H] = -3 lies above the
plateau suggests that they formed at plateau level and experienced
subsequent depletion. Based on observations made with the ESO Very
Large Telescope at Paranal Observatory, Chile (Programmes 076.A-0463
and 077.D-0299).Full Table 3 is available in electronic form at the
CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/522/A26">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/522/A26</A>IDL
code (appendix) is only available in electronic form at <A
href="http://www.aanda.org">http://www.aanda.org</A>

---------------------------------------------------------
Title: Diffuse X-rays from PNe with WR-type central stars
Authors: Rühling, U.; Sandin, C.; Steffen, M.; Schönberner, D.;
Hamann, W. -R.; Todt, H.
2010AIPC.1273..213R Altcode:
With the help of detailed nebula modeling and X-ray observations, we
want to shed light on the enigmatic origin of Wolf-Rayet type central
stars of planetary nebulae. This method allows us to assign observed
[WC] stars to one of the proposed evolutionary scenarios, attributing
the loss of hydrogen to a “late”, “very late”, or an “AGB final”
thermal pulse (LTP, VLTP and AFTP, respectively). We conclude that
BD+30° 3639 evolved through an AFTP.

---------------------------------------------------------
Title: The evolution of planetary nebulae. VII. Modelling planetary
nebulae of distant stellar systems
Authors: Schönberner, D.; Jacob, R.; Sandin, C.; Steffen, M.
2010A&A...523A..86S Altcode: 2010arXiv1009.0999S
 Aims: By means of hydrodynamical models we do the first
investigations of how the properties of planetary nebulae are affected
by their metal content and what can be learned from spatially unresolved
spectrograms of planetary nebulae in distant stellar systems. 
Methods: We computed a new series of 1D radiation-hydrodynamics
planetary nebulae model sequences with central stars of 0.595
M⊙ surrounded by initial envelope structures that differ
only by their metal content. At selected phases along the evolutionary
path, the hydrodynamic terms were switched off, allowing the models
to relax for fixed radial structure and radiation field into their
equilibrium state with respect to energy and ionisation. The analyses
of the line spectra emitted from both the dynamical and static models
enabled us to systematically study the influence of hydrodynamics
as a function of metallicity and evolution. We also recomputed
selected sequences already used in previous publications, but now
with different metal abundances. These sequences were used to study
the expansion properties of planetary nebulae close to the bright
cut-off of the planetary nebula luminosity function. Results:
Our simulations show that the metal content strongly influences the
expansion of planetary nebulae: the lower the metal content, the weaker
the pressure of the stellar wind bubble, but the faster the expansion of
the outer shell because of the higher electron temperature. This is in
variance with the predictions of the interacting-stellar-winds model
(or its variants) according to which only the central-star wind is
thought to be responsible for driving the expansion of a planetary
nebula. Metal-poor objects around slowly evolving central stars
become very dilute and are prone to depart from thermal equilibrium
because then adiabatic expansion contributes to gas cooling. We find
indications that photoheating and line cooling are not fully balanced
in the evolved planetary nebulae of the Galactic halo. Expansion rates
based on widths of volume-integrated line profiles computed from our
radiation-hydrodynamics models compare very well with observations
of distant stellar system. Objects close to the bright cut-off of the
planetary nebula luminosity function consist of rather massive central
stars (&gt;0.6 M⊙) with optically thick (or nearly thick)
nebular shells. The half-width-half-maximum velocity during this bright
phase is virtually independent of metallicity, as observed, but somewhat
depends on the final AGB-wind parameters. Conclusions: The
observed expansion properties of planetary nebulae in distant stellar
systems with different metallicities are explained very well by our 1D
radiation-hydrodynamics models. This result demonstrates convincingly
that the formation and acceleration of a planetary nebula occurs mainly
because of ionisation and heating of the circumstellar matter by the
stellar radiation field, and that the pressure exerted by the shocked
stellar wind is less important. Determinations of nebular abundances
by means of photoionisation modelling may become problematic for those
cases where expansion cooling must be considered.

---------------------------------------------------------
Title: CO5BOLD: COnservative COde for the COmputation of COmpressible
COnvection in a BOx of L Dimensions with l=2,3
Authors: Freytag, Bernd; Steffen, Matthias; Wedemeyer-Böhm, Sven;
Ludwig, Hans-Günter; Leenaarts, Jorrit; Schaffenberger, Werner;
Allard, France; Chiavassa, Andrea; Höfner, Susanne; Kamp, Inga;
Steiner, Oskar
2010ascl.soft11014F Altcode:
CO5BOLD - nickname COBOLD - is the short form of "COnservative
COde for the COmputation of COmpressible COnvection in a BOx of L
Dimensions with l=2,3". It is used to model solar and stellar
surface convection. For solar-type stars only a small fraction of the
stellar surface layers are included in the computational domain. In
the case of red supergiants the computational box contains the entire
star. Recently, the model range has been extended to sub-stellar objects
(brown dwarfs). CO5BOLD solves the coupled non-linear equations
of compressible hydrodynamics in an external gravity field together
with non-local frequency-dependent radiation transport. Operator
splitting is applied to solve the equations of hydrodynamics (including
gravity), the radiative energy transfer (with a long-characteristics
or a short-characteristics ray scheme), and possibly additional 3D
(turbulent) diffusion in individual sub steps. The 3D hydrodynamics
step is further simplified with directional splitting (usually). The 1D
sub steps are performed with a Roe solver, accounting for an external
gravity field and an arbitrary equation of state from a table. The radiation transport is computed with either one of three
modules: MSrad module: It uses long characteristics. The lateral
boundaries have to be periodic. Top and bottom can be closed or open
("solar module"). LHDrad module: It uses long characteristics
and is restricted to an equidistant grid and open boundaries at all
surfaces (old "supergiant module"). SHORTrad module: It uses
short characteristics and is restricted to an equidistant grid and
open boundaries at all surfaces (new "supergiant module"). The
code was supplemented with an (optional) MHD version [Schaffenberger
et al. (2005)] that can treat magnetic fields. There are also modules
for the formation and advection of dust available. The current version
now contains the treatment of chemical reaction networks, mostly used
for the formation of molecules [Wedemeyer-Böhm et al. (2005)], and
hydrogen ionization [Leenaarts &amp; Wedemeyer-Böhm (2005)], too. CO5BOLD is written in Fortran90. The parallelization is done with
OpenMP directives.

---------------------------------------------------------
Title: VizieR Online Data Catalog: Fe Abundances in metal-poor stars
(Sbordone+ 2010)
Authors: Sbordone, L.; Bonifacio, P.; Caffau, E.; Ludwig, H. -G.;
Behara, N. T.; Gonzalez Hernandez, J. I.; Steffen, M.; Cayrel, R.;
Freytag, B.; van't Veer, C.; Molaro, P.; Plez, B.; Sivarani, T.; Spite,
M.; Spite, F.; Beers, T. C.; Christlieb, N.; Francois, P.; Hill, V.
2010yCat..35220026S Altcode: 2010yCat..35229026S
Line-by-line abundances for FeI and FeII lines used to estimate
metallicity and gravity for the program stars. The first column lists
the star name, then the ion (FeI or FeII) The the wavelength in nm,
the loggf, the measured EW (pm) and the derived abundance assuming the
four stellar parameter sets used in the article, respectively 3D, BA,
ALI and IRFM. (3 data files).

---------------------------------------------------------
Title: The solar photospheric abundance of carbon. Analysis of atomic
carbon lines with the CO5BOLD solar model
Authors: Caffau, E.; Ludwig, H. -G.; Bonifacio, P.; Faraggiana, R.;
Steffen, M.; Freytag, B.; Kamp, I.; Ayres, T. R.
2010A&A...514A..92C Altcode: 2010arXiv1002.2628C
Context. The analysis of the solar spectra using hydrodynamical
simulations, with a specific selection of lines, atomic data, and method
for computing deviations from local thermodynamical equilibrium, has
led to a downward revision of the solar metallicity, Z. We are using
the latest simulations computed with the CO5BOLD code to reassess
the solar chemical composition. Our previous analyses of the key
elements, oxygen and nitrogen, have not confirmed any extreme downward
revision of Z, as derived in other works based on hydrodynamical
models. Aims: We determine the solar photospheric carbon
abundance with a radiation-hydrodynamical CO5BOLD model and compute
the departures from local thermodynamical equilibrium by using the
Kiel code. Methods: We measured equivalent widths of atomic C
I lines on high-resolution, high signal-to-noise ratio solar atlases
of disc-centre intensity and integrated disc flux. These equivalent
widths were analysed with our latest solar 3D hydrodynamical simulation
computed with CO5BOLD. Deviations from local thermodynamic equilibrium
we computed in 1D with the Kiel code, using the average temperature
structure of the hydrodynamical simulation as a background model. 
Results: Our recommended value for the solar carbon abundance relies
on 98 independent measurements of observed lines and is A(C)=8.50
± 0.06. The quoted error is the sum of statistical and systematic
errors. Combined with our recent results for the solar oxygen and
nitrogen abundances, this implies a solar metallicity of Z = 0.0154
and Z/X = 0.0211. Conclusions: Our analysis implies a solar
carbon abundance that is about 0.1 dex higher than what was found in
previous analyses based on different 3D hydrodynamical computations. The
difference is partly driven by our equivalent width measurements
(we measure, on average, larger equivalent widths than the other work
based on a 3D model), in part because of the different properties of
the hydrodynamical simulations and the spectrum synthesis code. The
solar metallicity we obtain from the CO5BOLD analyses is in slightly
better agreement with the constraints of helioseismology than the
previous 3D abundance results.

---------------------------------------------------------
Title: A 3D-NLTE study of the 670 nm solar lithium feature
Authors: Caffau, Elisabetta; Ludwig, Hans-Günter; Steffen, Matthias;
Bonifacio, Piercarlo
2010IAUS..268..329C Altcode:
We derive the 3D-NLTE lithium abundance in the solar photosphere from
the Lii line at 670 nm as measured in several solar atlases. The Li
abundance is obtained from line profile fitting with 1D/3D-LTE/3D-NLTE
synthetic spectra, considering several possibilities for the
atomic parameters of the lines blending the Li feature. The 670 nm
spectral region shows considerable differences in the two available
disc-centre solar atlases, while the two integrated disc spectra are
very similar. We obtain A(Li)3D-NLTE = 1.03. The 1D-LTE
abundance is 0.07 dex smaller. The line-lists giving the best fit
for the Sun may fail for other stars, while some line-lists fail to
reproduce the solar profile satisfactorily. We need a better knowledge
of the atomic parameters of the lines blending the Li feature in order
to be able to reproduce both the solar spectrum and the spectra of
other stars. An improved line-list is also required to derive reliable
estimates of the isotopic Li ratio in solar-metallicity stars.

---------------------------------------------------------
Title: The role of convection, overshoot, and gravity waves for the
transport of dust in M dwarf and brown dwarf atmospheres
Authors: Freytag, B.; Allard, F.; Ludwig, H. -G.; Homeier, D.;
Steffen, M.
2010A&A...513A..19F Altcode: 2010arXiv1002.3437F
Context. Observationally, spectra of brown dwarfs indicate the presence
of dust in their atmospheres while theoretically it is not clear what
prevents the dust from settling and disappearing from the regions of
spectrum formation. Consequently, standard models have to rely on ad
hoc assumptions about the mechanism that keeps dust grains aloft in
the atmosphere. Aims: We apply hydrodynamical simulations to
develop an improved physical understanding of the mixing properties of
macroscopic flows in M dwarf and brown dwarf atmospheres, in particular
of the influence of the underlying convection zone. Methods: We
performed two-dimensional radiation hydrodynamics simulations including
a description of dust grain formation and transport with the CO5BOLD
code. The simulations cover the very top of the convection zone and
the photosphere including the dust layers for a sequence of effective
temperatures between 900 K and 2800 K, all with log g = 5 assuming solar
chemical composition. Results: Convective overshoot occurs in the
form of exponentially declining velocities with small scale heights, so
that it affects only the region immediately above the almost adiabatic
convective layers. From there on, mixing is provided by gravity waves
that are strong enough to maintain thin dust clouds in the hotter
models. With decreasing effective temperature, the amplitudes of the
waves become smaller but the clouds become thicker and develop internal
convective flows that are more efficient in transporting and mixing
material than gravity waves. The presence of clouds often leads to a
highly structured appearance of the stellar surface on short temporal
and small spatial scales (presently inaccessible to observations). Conclusions: We identify convectively excited gravity waves as an
essential mixing process in M dwarf and brown dwarf atmospheres. Under
conditions of strong cloud formation, dust convection is the dominant
self-sustaining mixing component.

---------------------------------------------------------
Title: Convection and 6Li in the atmospheres of metal-poor
halo stars
Authors: Steffen, Matthias; Cayrel, R.; Bonifacio, P.; Ludwig, H. -G.;
Caffau, E.
2010IAUS..268..215S Altcode: 2010arXiv1001.3274S
Based on 3D hydrodynamical model atmospheres computed with the
CO5BOLD code and 3D non-LTE (NLTE) line formation
calculations, we study the effect of the convection-induced line
asymmetry on the derived 6Li abundance for a range in
effective temperature, gravity, and metallicity covering the stars
of the Asplund et al. (2006) sample. When the asymmetry effect
is taken into account for this sample of stars, the resulting
6Li/7Li ratios are reduced by about 1.5% on
average with respect to the isotopic ratios determined by Asplund et
al. (2006). This purely theoretical correction diminishes the number
of significant 6Li detections from 9 to 4 (2σ criterion),
or from 5 to 2 (3σ criterion). In view of this result the existence
of a 6Li plateau appears questionable. A careful reanalysis
of individual objects by fitting the observed lithium 6707 Å doublet
both with 3D NLTE and 1D LTE synthetic line profiles confirms that the
inferred 6Li abundance is systematically lower when using
3D NLTE instead of 1D LTE line fitting. Nevertheless, halo stars with
unquestionable 6Li detection do exist even if analyzed in
3D-NLTE, the most prominent example being HD 84937.

---------------------------------------------------------
Title: The metal-poor end of the Spite plateau: gravity sensitivity
of the Hα wings fitting.
Authors: Sbordone, L.; Bonifacio, P.; Caffau, E.; Ludwig, H. -G.;
Behara, N.; Gonzalez-Hernandez, J. I.; Steffen, M.; Cayrel, R.;
Freytag, B.; Van't Veer, C.; Molaro, P.; Plez, B.; Sivarani, T.; Spite,
M.; Spite, F.; Beers, T. C.; Christlieb, N.; François, P.; Hill, V.
2010IAUS..268..355S Altcode:
We recently presented (Sbordone et al., 2009a) the largest sample to
date of lithium abundances in extremely metal-poor (EMP) Halo dwarf and
Turn-Off (TO) stars. One of the most crucial aspects in estimating Li
abundances is the Teff determination, since the Li I 670.8
nm doublet is highly temperature sensitive. In this short contribution
we concentrate on the Teff determination based on Hα wings
fitting, and on its sensitivity to the chosen stellar gravity.

---------------------------------------------------------
Title: Main-sequence and sub-giant stars in the globular cluster
NGC 6397: The complex evolution of the lithium abundance
Authors: González Hernández, J. I.; Bonifacio, P.; Caffau, E.;
Steffen, M.; Ludwig, H. -G.; Behara, N.; Sbordone, L.; Cayrel, R.;
Zaggia, S.
2010IAUS..268..257G Altcode: 2009arXiv0912.4105G
Thanks to the high multiplex and efficiency of Giraffe at the VLT
we have been able for the first time to observe the Li I doublet in
the Main Sequence stars of a globular cluster. At the same time we
observed Li in a sample of Sub-Giant stars of the same B-V colour. Our final sample is composed of 84 SG stars and 79 MS stars. In
spite of the fact that SG and MS span the same temperature range we
find that the equivalent widths of the Li I doublet in SG stars are
systematically larger than those in MS stars, suggesting a higher Li
content among SG stars. This is confirmed by our quantitative analysis
carried out making use of 1D hydrostatic plane-parallel models and
3D hydrodynamical simulations of the stellar atmospheres. We
derived the effective temperatures of stars in our the sample from Hα
fitting. Theoretical profiles were computed using 3D hydrodynamical
simulations and 1D ATLAS models. Therefore, we are able to determined
1D and 3D-based effective temperatures. We then infer Li abundances
taking into account non-local thermodynamical equilibrium effects when
using both 1D and 3D models. We find that SG stars have a mean
Li abundance higher by 0.1 dex than MS stars. This result is obtained
using both 1D and 3D models. We also detect a positive slope of Li
abundance with effective temperature, the higher the temperature the
higher the Li abundance, both for SG and MS stars, although the slope
is slightly steeper for MS stars. These results provide an unambiguous
evidence that the Li abundance changes with evolutionary status. The physical mechanisms responsible for this behaviour are not yet
clear, and none of the existing models seems to describe accurately
these observations. Based on these conclusions, we believe that the
cosmological lithium problem still remains an open question.

---------------------------------------------------------
Title: The evolution of planetary nebulae. VI. On the chemical
composition of the metal-poor PN G135.9+55.9
Authors: Sandin, C.; Jacob, R.; Schönberner, D.; Steffen, M.; Roth,
M. M.
2010A&A...512A..18S Altcode: 2009arXiv0912.5430S
The actual value of the oxygen abundance of the metal-poor planetary
nebula <ASTROBJ>PN G135.9+55.9</ASTROBJ> has frequently been debated
in the literature. We wanted to clarify the situation by making an
improved abundance determination based on a study that includes both
new accurate observations and new models. We made observations using
the method of integral field spectroscopy with the PMAS instrument, and
also used ultraviolet observations that were measured with HST-STIS. In
our interpretation of the reduced and calibrated spectrum we used
for the first time, recent radiation hydrodynamic models, which were
calculated with several setups of scaled values of mean Galactic disk
planetary nebula metallicities. For evolved planetary nebulae, such
as <ASTROBJ>PN G135.9+55.9</ASTROBJ>, it turns out that departures
from thermal equilibrium can be significant, leading to much lower
electron temperatures, hence weaker emission in collisionally excited
lines. Based on our time-dependent hydrodynamic models and the observed
emission line [O iii] λ5007, we found a very low oxygen content of
about 1/80 of the mean Galactic disk value. This result is consistent
with emission line measurements in the ultraviolet wavelength range. The
C/O and Ne/O ratios are unusually high and similar to those of another
halo object, BoBn-1. Based in part on observations collected at
the Centro Astronómico Hispano Alemán (CAHA), operated jointly by the
Max-Planck-Institut für Astronomie and the Instituto de Astrofísica
de Andalucia (CSIC).

---------------------------------------------------------
Title: 6Li in metal-poor halo stars: real or spurious?
Authors: Steffen, M.; Cayrel, R.; Bonifacio, P.; Ludwig, H. -G.;
Caffau, E.
2010IAUS..265...23S Altcode: 2009arXiv0910.5917S
The presence of convective motions in the atmospheres of metal-poor
halo stars leads to systematic asymmetries of the emergent spectral
line profiles. Since such line asymmetries are very small, they can be
safely ignored for standard spectroscopic abundance analysis. However,
when it comes to the determination of the 6Li/7Li
isotopic ratio, q(Li)=n(6Li)/n(7Li), the
intrinsic asymmetry of the 7Li line must be taken into
account, because its signature is essentially indistinguishable from
the presence of a weak 6Li blend in the red wing of the
7Li line. In this contribution we quantity the error of the
inferred 6Li/7Li isotopic ratio that arises if
the convective line asymmetry is ignored in the fitting of the λ6707
Å lithium blend. Our conclusion is that 6Li/7Li
ratios derived by Asplund et al. (2006), using symmetric line profiles,
must be reduced by typically Δq(Li) ≈ 0.015. This diminishes the
number of certain 6Li detections from 9 to 4 stars or less,
casting some doubt on the existence of a 6Li plateau.

---------------------------------------------------------
Title: Solar abundances and 3D model atmospheres
Authors: Ludwig, Hans-Günter; Caffau, Elisabetta; Steffen, Matthias;
Bonifacio, Piercarlo; Freytag, Bernd; Cayrel, Roger
2010IAUS..265..201L Altcode: 2009arXiv0911.4248L
We present solar photospheric abundances for 12 elements from optical
and near-infrared spectroscopy. The abundance analysis was conducted
employing 3D hydrodynamical (CO5BOLD) as well as standard
1D hydrostatic model atmospheres. We compare our results to others
with emphasis on discrepancies and still lingering problems, in
particular exemplified by the pivotal abundance of oxygen. We argue
that the thermal structure of the lower solar photosphere is very
well represented by our 3D model. We obtain an excellent match of
the observed center-to-limb variation of the line-blanketed continuum
intensity, also at wavelengths shortward of the Balmer jump.

---------------------------------------------------------
Title: The metal-poor end of the Spite plateau
Authors: Sbordone, L.; Bonifacio, P.; Caffau, E.; Ludwig, H. -G.;
Behara, N.; Gonzalez-Hernandez, J. I.; Steffen, M.; Cayrel, R.;
Freytag, B.; Van't Veer, C.; Molaro, P.; Plez, B.; Sivarani, T.; Spite,
M.; Spite, F.; Beers, T. C.; Christlieb, N.; François, P.; Hill, V.
2010IAUS..265...75S Altcode:
We present the largest sample available to date of lithium abundances in
extremely metal poor (EMP) Halo dwarfs. Four Teff estimators
are used, including IRFM and Hα wings fitting against 3D hydrodynamical
synthetic profiles. Lithium abundances are computed by means of 1D and
3D-hydrodynamical NLTE computations. Below [Fe/H]~-3, a strong positive
correlation of A(Li) with [Fe/H] appears, not influenced by the choice
of the Teff estimator. A linear fit finds a slope of about 0.30 dex in
A(Li) per dex in [Fe/H], significant to 2-3 σ, and consistent within
1 σ among all the Teff estimators. The scatter in A(Li)
increases significantly below [Fe/H]~-3. Above, the plateau lies at
&lt;A(Li)3D, NLTE&gt; = 2.199 ± 0.086. If the primordial
A(Li) is the one derived from standard Big Bang Nucleosynthesis
(BBN), it appears difficult to envision a single depletion phenomenon
producing a thin, metallicity independent plateau above [Fe/H] = -2.8,
and a highly scattered, metallicity dependent distribution below.

---------------------------------------------------------
Title: Accuracy of spectroscopy-based radioactive dating of stars
Authors: Ludwig, H. -G.; Caffau, E.; Steffen, M.; Bonifacio, P.;
Sbordone, L.
2010A&A...509A..84L Altcode: 2009arXiv0911.4251L
Context. Combined spectroscopic abundance analyses of stable and
radioactive elements can be applied for deriving stellar ages. The
achievable precision depends on factors related to spectroscopy,
nucleosynthesis, and chemical evolution. Aims: We quantify the
uncertainties arising from the spectroscopic analysis, and compare these
to the other error sources. Methods: We derive formulae for the
age uncertainties arising from the spectroscopic abundance analysis,
and apply them to spectroscopic and nucleosynthetic data compiled
from the literature for the Sun and metal-poor stars. Results:
We obtained ready-to-use analytic formulae of the age uncertainty for
the cases of stable+unstable and unstable+unstable chronometer pairs,
and discuss the optimal relation between to-be-measured age and mean
lifetime of a radioactive species. Application to the literature
data indicates that, for a single star, the achievable spectroscopic
accuracy is limited to about ±20% for the foreseeable future. At
present, theoretical uncertainties in nucleosynthesis and chemical
evolution models form the precision bottleneck. For stellar clusters,
isochrone fitting provides a higher accuracy than radioactive dating,
but radioactive dating becomes competitive when applied to many cluster
members simultaneously, reducing the statistical errors by a factor
√{N}. Conclusions: Spectroscopy-based radioactive stellar
dating would benefit from improvements in the theoretical understanding
of nucleosynthesis and chemical evolution. Its application to clusters
can provide strong constraints for nucleosynthetic models.

---------------------------------------------------------
Title: Lithium abundances of main-sequence and subgiant stars in
the globular cluster NGC 6397
Authors: González Hernández, J. I.; Bonifacio, P.; Caffau, E.;
Steffen, M.; Ludwig, H. -G.; Behara, N.; Sbordone, L.; Cayrel, R.;
Zaggia, S.
2010IAUS..266..407G Altcode: 2009arXiv0910.2305G
We present FLAMES/GIRAFFE spectroscopy obtained with the Very Large
Telescope (VLT). Using these observations, we have been able (for the
first time) to observe the Lii doublet in the main-sequence (MS) stars
of a globular cluster. We also observed Li in a sample of subgiant (SG)
stars of the same B - V colour. Our final sample is composed of 84 SG
and 79 MS stars. In spite of the fact that SG and MS stars span the same
temperature range, we find that the equivalent widths of the Lii doublet
in SG stars are systematically greater than in MS stars, suggesting a
higher Li content among SG stars. This is confirmed by our quantitative
analysis, which makes use of both 1D and 3D model atmospheres. We find
that SG stars show, on average, a higher Li abundance, by 0.1 dex, than
MS stars. We also detect a positive slope of Li abundance with effective
temperature: the higher the temperature the higher the Li abundance,
both for SG and MS stars, although the slope is slightly steeper for MS
stars. These results provide unambiguous evidence that the Li abundance
changes with evolutionary state. The physical mechanisms that contribute
to this are not yet clear, since none of the proposed models seem to
describe accurately the observations. Whether such a mechanism can
explain the cosmological lithium problem is still an open question.

---------------------------------------------------------
Title: On the Role of Acoustic-Gravity Waves in the Energetics of
the Solar Atmosphere
Authors: Straus, T.; Fleck, B.; Jefferies, S. M.; McIntosh, S. W.;
Severino, G.; Steffen, M.; Tarbell, T. D.
2009ASPC..415...95S Altcode: 2010arXiv1003.3773S
In a recent paper (Straus et al. 2008) we determined the energy
flux of internal gravity waves in the lower solar atmosphere using
a combination of 3D numerical simulations and observations obtained
with the IBIS instrument operated at the Dunn Solar Telescope and
the Michelson Doppler Imager (MDI) on SOHO. In this paper we extend
these studies using coordinated observations from SOT/NFI and SOT/SP
on Hinode and MDI. The new measurements confirm that gravity waves
are the dominant phenomenon in the quiet middle/upper photosphere and
that they transport more mechanical energy than the high-frequency
(&gt; 5 mHz) acoustic waves, even though we find an acoustic flux 3-5
times larger than the upper limit estimate of Fossum &amp; Carlsson
(2006). It therefore appears justified to reconsider the significance of
(non-M)HD waves for the energy balance of the solar chromosphere.

---------------------------------------------------------
Title: Hydrodynamical simulations of convection-related stellar
micro-variability. II. The enigmatic granulation background of the
CoRoT target HD 49933
Authors: Ludwig, H. -G.; Samadi, R.; Steffen, M.; Appourchaux, T.;
Baudin, F.; Belkacem, K.; Boumier, P.; Goupil, M. -J.; Michel, E.
2009A&A...506..167L Altcode: 2009arXiv0905.2695L
Context: Local-box hydrodynamical model atmospheres provide
statistical information about a star's emergent radiation field
which allows one to predict the level of its granulation-related
micro-variability. Space-based photometry is now sufficiently
accurate to test model predictions. Aims: We aim to model the
photometric granulation background of HD 49933 as well as the Sun,
and compare the predictions to the measurements obtained by the
&lt;sf&gt;CoRoT&lt;/sf&gt; and &lt;sf&gt;SOHO&lt;/sf&gt; satellite
missions. Methods: We construct hydrodynamical model atmospheres
representing HD 49933 and the Sun, and use a previously developed
scaling technique to obtain the observable disk-integrated brightness
fluctuations. We further performed exploratory magneto-hydrodynamical
simulations to gauge the impact of small scale magnetic fields
on the synthetic light-curves. Results: We find that the
granulation-related brightness fluctuations depend on metallicity. We
obtain a satisfactory correspondence between prediction and observation
for the Sun, validating our approach. For HD 49933, we arrive at
a significant over-estimation by a factor of two to three in total
power. Locally generated magnetic fields are unlikely to be responsible,
otherwise existing fields would need to be rather strong to sufficiently
suppress the granulation signal. Presently suggested updates on the
fundamental stellar parameters do not improve the correspondence;
however, an ad-hoc increase of the HD 49933 surface gravity by about
0.2 dex would eliminate most of the discrepancy. Conclusions:
We diagnose a puzzling discrepancy between the predicted and observed
granulation background in HD 49933, with only rather ad-hoc ideas for
remedies at hand. The &lt;sf&gt;CoRoT&lt;/sf&gt; space mission,
launched on December 27th 2006, has been developed and is operated by
CNES, with the contribution of Austria, Belgium, Brazil, ESA, Germany
and Spain. CIFIST Marie Curie Excellence Team.

---------------------------------------------------------
Title: Lithium in the globular cluster NGC 6397. Evidence for
dependence on evolutionary status
Authors: González Hernández, J. I.; Bonifacio, P.; Caffau, E.;
Steffen, M.; Ludwig, H. -G.; Behara, N. T.; Sbordone, L.; Cayrel,
R.; Zaggia, S.
2009A&A...505L..13G Altcode: 2009arXiv0909.0983G
Context: Most globular clusters are believed to host a single
stellar population. They can thus be considered a good place to
study the Spite plateau and to search for possible evolutionary
modifications of the Li content. Aims: We want to determine the
Li content of subgiant (SG) and main sequence (MS) stars of the old,
metal-poor globular cluster NGC 6397. This work was aimed not only
at studying possible Li abundance variations but also to investigate
the cosmological Li discrepancy. Methods: Here, we present
FLAMES/GIRAFFE observations of a sample of 84 SG and 79 MS stars in
NGC 6397 selected in a narrow range of B-V colour and, therefore,
effective temperatures. We determine both effective temperatures and
Li abundances using three-dimensional hydrodynamical model atmospheres
for all the MS and SG stars of the sample. Results: We find
a significant difference in the Li abundance between SG stars and
MS stars, the SG stars having an abundance higher by almost 0.1
dex on average. We also find a decrease in the lithium abundance
with decreasing effective temperature, both in MS and SG stars,
albeit with a significantly different slope for the two classes of
stars. This suggests that the lithium abundance in these stars is,
indeed, altered by some process, which is temperature-dependent. Conclusions: The lithium abundance pattern observed in NGC 6397 is
different from what is found among field stars, casting some doubt on
the use of globular cluster stars as representative of Population II
with respect to the lithium abundance. None of the available theories
of Li depletion appears to satisfactorily describe our observations. Based on observations obtained with FLAMES/GIRAFFE at VLT Kueyen
8.2 m telescope in programme 079.D-0399(A). Table and Figs. 3-10 are
only available in electronic form at http://www.aanda.org Table 2
is available in electronic form at http://www.aanda.org and at the
CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via
http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/505/L13

---------------------------------------------------------
Title: The Solar Photospheric Nitrogen Abundance: Determination with
3D and 1D Model Atmospheres
Authors: Maiorca, E.; Caffau, E.; Bonifacio, P.; Busso, M.; Faraggiana,
R.; Steffen, M.; Ludwig, H. -G.; Kamp, I.
2009PASA...26..345M Altcode: 2009arXiv0912.0375M
We present a new determination of the solar nitrogen abundance
making use of 3D hydrodynamical modelling of the solar photosphere,
which is more physically motivated than traditional static 1D
models. We selected suitable atomic spectral lines, relying on
equivalent width measurements already existing in the literature. For
atmospheric modelling we used the co 5 bold 3D radiation
hydrodynamics code. We investigated the influence of both deviations
from local thermodynamic equilibrium (non-LTE effects) and photospheric
inhomogeneities (granulation effects) on the resulting abundance. We
also compared several atlases of solar flux and centre-disc intensity
presently available. As a result of our analysis, the photospheric
solar nitrogen abundance is A(N) = 7.86 +/- 0.12.

---------------------------------------------------------
Title: VizieR Online Data Catalog: Lithium in NGC 6397 (Gonzalez
Hernandez+, 2009)
Authors: Gonzalez Hernandez, J. I.; Bonifacio, P.; Caffau, E.; Steffen,
M.; Ludwig, H. -G.; Behara, N. T.; Sbordone, L.; Cayrel, R.; Zaggia, S.
2009yCat..35059013G Altcode:
Photometric data of the dwarf and subgiant stars of the globular
cluster NGC 6397. We also provide the signal-to-noise of the spectra,
the 3D and 1D Halpha-based effective temperatures, 3D Li abundances,
and the equivalent widths and errors: dEWa: Error of the equivalent
width measurements estimated from a fitting routine that uses as
free parameters the velocity shift, the continuum location, and the
equivalent width of the Li line. dEWb: Error of the equivalent width
associated to the signal-to-noise ratio and the wavelength dispersion
of the spectra, derived using Cayrel's formula (Cayrel, 1988, IAU
Symp. 132: The Impact of Very High S/N Spectroscopy on Stellar Physics,
132, 345). (1 data file).

---------------------------------------------------------
Title: Impact of granulation effects on the use of Balmer lines as
temperature indicators
Authors: Ludwig, H. -G.; Behara, N. T.; Steffen, M.; Bonifacio, P.
2009A&A...502L...1L Altcode: 2009arXiv0906.4697L
Context: Balmer lines serve as important indicators of stellar
effective temperatures in late-type stellar spectra. One of their
modelling uncertainties is the influence of convective flows on
their shape. Aims: We aim to characterize the influence of
convection on the wings of Balmer lines. Methods: We perform
a differential comparison of synthetic Balmer line profiles obtained
from 3D hydrodynamical model atmospheres and 1D hydrostatic standard
ones. The model parameters are appropriate for F, G, K dwarf and
subgiant stars of metallicity ranging from solar to 10-3
solar. Results: The shape of the Balmer lines predicted by 3D
models can never be exactly reproduced by a 1D model, irrespective of
its effective temperature. We introduce the concept of a 3D temperature
correction, as the effective temperature difference between a 3D model
and a 1D model which provides the closest match to the 3D profile. The
temperature correction is different for the different members of
the Balmer series and depends on the adopted mixing-length parameter
αMLT in the 1D model. Among the investigated models, the
3D correction ranges from -300 K to +300 K. Horizontal temperature
fluctuations tend to reduce the 3D correction. Conclusions:
Accurate effective temperatures cannot be derived from the wings of
Balmer lines, unless the effects of convection are properly accounted
for. The 3D models offer a physically well justified way of doing
so. The use of 1D models treating convection with the mixing-length
theory do not appear to be suitable for this purpose. In particular,
there are indications that it is not possible to determine a single
value of αMLT which will optimally reproduce the Balmer
lines for any choice of atmospheric parameters. The investigation
of a more extended grid and direct comparison with observed Balmer
profiles will be carried out in the near future. Appendices is
only available in electronic form at http://www.aanda.org

---------------------------------------------------------
Title: The solar photospheric nitrogen abundance. Analysis of atomic
transitions with 3D and 1D model atmospheres
Authors: Caffau, E.; Maiorca, E.; Bonifacio, P.; Faraggiana, R.;
Steffen, M.; Ludwig, H. -G.; Kamp, I.; Busso, M.
2009A&A...498..877C Altcode: 2009arXiv0903.3406C
Context: In recent years, the solar chemical abundances have been
studied in considerable detail because of discrepant values of
solar metallicity inferred from different indicators, i.e., on the
one hand, the “sub-solar” photospheric abundances resulting
from spectroscopic chemical composition analyses with the aid of
3D hydrodynamical models of the solar atmosphere, and, on the other
hand, the high metallicity inferred by helioseismology. Aims:
After investigating the solar oxygen abundance using a CO^5BOLD 3D
hydrodynamical solar model in previous work, we undertake a similar
approach studying the solar abundance of nitrogen, since this element
accounts for a significant fraction of the overall solar metallicity,
Z. Methods: We used a selection of atomic spectral lines to
determine the solar nitrogen abundance, relying mainly on equivalent
width measurements in the literature. We investigate the influence on
the abundance analysis, of both deviations from local thermodynamic
equilibrium (“NLTE effects”) and photospheric inhomogeneities
(“granulation effects”). Results: We recommend use of a solar
nitrogen abundance of A(N) = 7.86 ± 0.12, whose error bar reflects
the line-to-line scatter. Conclusions: The solar metallicity
implied by the CO^5BOLD-based nitrogen and oxygen abundances is in the
range 0.0145≤ Z ≤ 0.0167. This result is a step towards reconciling
photospheric abundances with helioseismic constraints on Z. Our most
suitable estimates are Z=0.0156 and Z/X=0.0213.

---------------------------------------------------------
Title: Near-surface stellar magneto-convection: simulations for the
Sun and a metal-poor solar analog
Authors: Steffen, Matthias; Ludwig, H. -G.; Steiner, O.
2009IAUS..259..233S Altcode: 2009arXiv0902.2753S
We present 2D local box simulations of near-surface radiative
magneto-convection with prescribed magnetic flux, carried out with the
MHD version of the CO5BOLD code for the Sun and a solar-like
star with a metal-poor chemical composition (metal abundances reduced by
a factor 100, [M/H] = -2). The resulting magneto-hydrodynamical models
can be used to study the influence of the metallicity on the properties
of magnetized stellar atmospheres. A preliminary analysis indicates
that the horizontal magnetic field component tends to be significantly
stronger in the optically thin layers of metal-poor stellar atmospheres.

---------------------------------------------------------
Title: Convective mixing and dust clouds in the atmospheres of
brown dwarfs
Authors: Freytag, Bernd; Allard, France; Ludwig, Hans-Günter; Homeier,
Derek; Steffen, Matthias; Sharp, Christopher
2009AIPC.1094..489F Altcode: 2009csss...15..489F
Observed spectra of brown dwarfs demonstrate that their atmospheres
are influenced by dust. To investigate the mechanism that controls
the formation and gravitational settling of dust grains as well as the
mixing of fresh condensable material into the atmosphere, we performed
2D radiation-hydrodynamics simulations with CO5BOLD. The models comprise
the upper part of the convection zone and the atmosphere with the
dust cloud layers. We find that direct convective overshoot does not
play a major role. Instead, the mixing in the clouds is controlled by
gravity waves.

---------------------------------------------------------
Title: Effective temperatures of cool metal-poor stars derived from
the analysis of 3D Balmer lines
Authors: Behara, N. T.; Ludwig, H. -G.; Steffen, M.; Bonifacio, P.
2009AIPC.1094..784B Altcode: 2009csss...15..784B
Balmer lines are recognized as accurate indicators of the effective
temperature of late-type stars. The influence of convection on
the shape of Balmer line profiles has been investigated using LTE
3D hydrodynamical model atmospheres and `classical' LTE 1D stellar
atmospheres, where convection is modeled within the simplistic picture
of mixing-length theory. Models and line profiles computed with the
CO5BOLD and Linfor3D codes have been used to determine
the effective temperatures of the Sun and three well known metal-poor
stars HD84937, HD74000, and HD140283. Our 3D fit provides the best fit
thus far for the solar Hα temperature using the Barklem theory. The
resulting (3D-1D) Teff biases related to the different treatment of
convection in the 1D and 3D models are presented.

---------------------------------------------------------
Title: Micro- and macroturbulence derived from 3D hydrodynamical
stellar atmospheres .
Authors: Steffen, M.; Ludwig, H. -G.; Caffau, E.
2009MmSAI..80..731S Altcode: 2009arXiv0909.2831S
The theoretical prediction of micro- and macroturbulence (xi_mic
and xi_mac ) as a function of stellar parameters can be useful for
spectroscopic work based on 1D model atmospheres in cases where an
empirical determination of xi_mic is impossible due to a lack of
suitable lines and/or macroturbulence and rotational line broadening
are difficult to separate. In an effort to exploit the CIFIST 3D model
atmosphere grid for deriving the theoretical dependence of xi_mic and
xi_mac on effective temperature, gravity, and metallicity, we discuss
different methods to derive xi_mic from the numerical simulations,
and report first results for the Sun and Procyon. In both cases the
preliminary analysis indicates that the microturbulence found in the
simulations is significantly lower than in the real stellar atmospheres.

---------------------------------------------------------
Title: The CIFIST 3D model atmosphere grid.
Authors: Ludwig, H. -G.; Caffau, E.; Steffen, M.; Freytag, B.;
Bonifacio, P.; Kučinskas, A.
2009MmSAI..80..711L Altcode: 2009arXiv0908.4496L
Grids of stellar atmosphere models and associated synthetic spectra
are numerical products which have a large impact in astronomy due to
their ubiquitous application in the interpretation of radiation from
individual stars and stellar populations. 3D model atmospheres are
now on the verge of becoming generally available for a wide range
of stellar atmospheric parameters. We report on efforts to develop
a grid of 3D model atmospheres for late-type stars within the CIFIST
Team at Paris Observatory. The substantial demands in computational
and human labor for the model production and post-processing render
this apparently mundane task a challenging logistic exercise. At
the moment the CIFIST grid comprises 77 3D model atmospheres with
emphasis on dwarfs of solar and sub-solar metallicities. While the
model production is still ongoing, first applications are already
worked upon by the CIFIST Team and collaborators.

---------------------------------------------------------
Title: Solar abundances and granulation effects
Authors: Caffau, E.; Ludwig, H. -G.; Steffen, M.
2009MmSAI..80..643C Altcode: 2009arXiv0910.4733C
The solar abundances have undergone a major downward revision
in the last decade, reputedly as a result of employing 3D
hydrodynamical simulations to model the inhomogeneous structure of
the solar photosphere. The very low oxygen abundance advocated by
\citet{asplund04}, A(O)=8.66, together with the downward revision of
the carbon and nitrogen abundances, has created serious problems for
solar models to explain the helioseismic measurements. In an
effort to contribute to the dispute we have re-derived photospheric
abundances of several elements independently of previous analysis. We
applied a state-of-the art 3D (CO5BOLD) hydrodynamical simulation
of the solar granulation as well as different 1D model atmospheres
for the line by line spectroscopic abundance determinations. The
analysis is based on both standard disc-centre and disc-integrated
spectral atlases; for oxygen we acquired in addition spectra at
different heliocentric angles. The derived abundances are the result
of equivalent width and/or line profile fitting of the available atomic
lines. We discuss the different granulation effects on solar abundances
and compare our results with previous investigations. According to
our investigations hydrodynamical models are important in the solar
abundance determination, but are not responsible for the recent downward
revision in the literature of the solar metallicity.

---------------------------------------------------------
Title: NLTE Abundances of Sodium, Magnesium and Barium in the Globular
Clusters M10 and M71
Authors: Mishenina, T. V.; Kučinskas, A.; Andrievsky, S. M.; Korotin,
S. A.; Dobrovolskas, V.; Ivanauskas, A.; Caffau, E.; Ludwig, H. -G.;
Steffen, M.; Sperauskas, J.; Klochkova, V. G.; Panchuk, V. E.
2009BaltA..18..193M Altcode: 2009OAst...18..193M
We derive NLTE abundances of Na, Mg and Ba in four late-type giants
belonging to globular clusters M10 and M71. The obtained relative
[Na/Fe] ratios, which were measured only in M10, are positive, with
the average value [Na/Fe] = +0.3. The ratios [Mg/Fe] in both clusters
are supersolar, +0.15 to +0.28, while [Ba/Fe] scatter between --0.14
and +0.09. Differences between the NLTE abundances derived in this
work and those obtained in LTE by Mishenina et al. (2003) are small,
typically within ±0.1 dex. We also perform numerical simulations with
the CO5BOLD 3D hydrodynamical stellar atmosphere code to
investigate the influence of convection on the formation of spectral
lines used in our NLTE study. For this purpose we use a model of
late-type giant with T eff = 4020 K, log g = 1.0, [M/H] =
--1.0 and find that for Na, Mg and Ba the 3D--1D abundance corrections
are below ∼ 0.02 dex. However, their size strongly depends on the
value of microturbulent velocity used with the 1D model.

---------------------------------------------------------
Title: Simulations of dust clouds in the atmospheres of substellar
objects. Theory toddles after observations
Authors: Freytag, B.; Allard, F.; Ludwig, H. -G.; Homeier, D.;
Steffen, M.
2009MmSAI..80..670F Altcode:
The atmospheres of brown dwarfs allow the formation of dust grains
and their rain-out into deeper, invisible layers. However, observed
spectra of L dwarfs can only be reproduced when static 1D models
account for dust formation and its resulting greenhouse effect in the
visible layers. Time-dependent hydrodynamical processes can mix up the
material giving rise to complex unsteady weather phenomena on these
objects. We performed radiation hydrodynamics simulations in two and
three dimensions of the atmospheres of brown dwarfs with CO5BOLD,
including a treatment of dust particles. We find that exponential
overshoot (close to the gas convection zone), gravity waves (weak
omni-present mixing), and convection within dust layers (in the thick
clouds in cooler models) contribute to the atmospheric mixing, which
is far from being a stationary process. The presence of dust in the
atmospheres is accompanied by large temporal and spatial intensity
fluctuations.

---------------------------------------------------------
Title: 3D hydrodynamical simulations of stellar photospheres with
the CO5BOLD code. Photometric colors of a late-type giant
Authors: Kučinskas, A.; Ludwig, H. -G.; Caffau, E.; Steffen, M.
2009MmSAI..80..723K Altcode: 2009arXiv0910.3412K
We present synthetic broad-band photometric colors of a late-type
giant located close to the RGB tip (T_eff≈3640 K, log g=1.0 and
[M/H]=0.0). Johnson-Cousins-Glass BVRIJHK colors were obtained from
the spectral energy distributions calculated using 3D hydrodynamical
and 1D classical stellar atmosphere models. The differences between
photometric magnitudes and colors predicted by the two types of models
are significant, especially at optical wavelengths where they may
reach, e.g., Delta V≈0.16, Delta R≈0.13 and Delta (V-I)≈0.14,
Delta (V-K)≈0.20. Differences in the near-infrared are smaller but
still non-negligible (e.g., Delta K≈ 0.04). Such discrepancies may
lead to noticeably different photometric parameters when these are
inferred from photometry (e.g., effective temperature will change by
Delta T_eff≈60 K due to difference of Delta (V-K)≈0.20).

---------------------------------------------------------
Title: Solar Photosphere and Chromosphere
Authors: Steffen, Matthias
2009LanB...4B...28S Altcode: 2009LanB...4B.4114S
No abstract at ADS

---------------------------------------------------------
Title: Monitoring mass motions of Betelgeuse's photosphere using
robotic telescopes .
Authors: Weber, M.; Carroll, T.; Granzer, T.; Steffen, M.; Strassmeier,
K. G.
2009MmSAI..80..743W Altcode:
We started monitoring Betelgeuse using STELLA/SES, the STELLA
échelle spectrograph fed by a robotic 1.2 m telescope on Tenerife,
and the automatic photometric telescope (APT) T7 in Arizona in fall
2008. In this first observing season, we have collected 67 high
resolution spectra from 390 to 900 nm at a resolution of 50,000 and
a S/N between 100 and 300, and a comparable number of photometric
observations in the Halpha filter. In this presentation, we report on
the initial findings based on this first data set: Radial velocities,
effective temperature (along with surface gravity and metallicity) are
automatically computed by the STELLA/SES data reduction &amp; analysis
pipeline. We compare these global measurements and the photometric
brightness with velocities and temperature indicators derived from
individual spectral lines, to bring these values in line with recently
published observations. Furthermore we compute synthetic line profiles
from state-of-the-art 3D stellar convection models, and compare the
line-profiles, their shapes and positions to our observations. The
final aim of the observing program is to find out if the spectral line
variations can be explained using these non-magnetic convection models.

---------------------------------------------------------
Title: Models of surface convection and dust clouds in brown dwarfs
Authors: Freytag, B.; Allard, F.; Ludwig, H. -G.; Homeier, D.;
Steffen, M.
2008PhST..133a4005F Altcode:
The influence of dust grains on the atmospheres of brown dwarfs is
visible in observed spectra. To investigate what prevents the dust
grains from falling down, or how fresh condensable material is mixed
up in the atmosphere to allow new grains to form, we performed 2D
radiation-hydrodynamics simulations with CO5BOLD of the upper part
of the convection zone and the atmosphere containing the dust cloud
layers. We find that unlike in models of Cepheids, the convective
overshoot does not play a major role. Instead, the mixing in the dust
clouds is controlled by gravity waves.

---------------------------------------------------------
Title: he models comprise the upper part of the convection zone
and the atmosphere with %the dust cloud layers. We find that direct
convective overshoot does not play a major role. Instead, the mixing
in the clouds is controlled by gravity waves.
Authors: Freytag, B.; Allard, F.; Ludwig, H. -G.; Homeier, D.; Steffen,
M.; Sharp, C.
2008sf2a.conf..469F Altcode:
To investigate the mechanism that controls the formation and
gravitational settling of dust grains as well as the mixing of fresh
condensable material into the atmosphere of brown dwarfs, we performed
2D radiation-hydrodynamics simulations with CO5BOLD.

---------------------------------------------------------
Title: The evolution of planetary nebulae. V. The diffuse X-ray
emission
Authors: Steffen, M.; Schönberner, D.; Warmuth, A.
2008A&A...489..173S Altcode: 2008arXiv0807.3290S
Context: Observations with space-borne X-ray telescopes revealed the
existence of soft, diffuse X-ray emission from the inner regions of
planetary nebulae. Although the existing images support the idea that
this emission arises from the hot shocked central-star wind which
fills the inner cavity of a planetary nebula, existing models have
difficulties to explain the observations consistently. Aims: We
investigate how the inclusion of thermal conduction changes the physical
parameters of the hot shocked wind gas and the amount of X-ray emission
predicted by time-dependent hydrodynamical models of planetary nebulae
with central stars of normal, hydrogen-rich surface composition. Methods: We upgraded our 1D hydrodynamics code NEBEL by to account
for energy transfer due to heat conduction, which is of importance at
the interface separating the hot shocked wind gas (“hot bubble”)
from the much cooler nebular material. With this new version of
NEBEL we recomputed a selection of our already existing hydrodynamical
sequences and obtained synthetic X-ray spectra for representative models
along the evolutionary tracks by means of the freely available CHIANTI
package. Results: Heat conduction leads to lower temperatures and
higher densities within a bubble and brings the physical properties
of the X-ray emitting domain into close agreement with the values
derived from observations. The amount of X-rays emitted during the
course of evolution depends on the energy dumped into the bubble by
the fast stellar wind, on the efficiency of “evaporating” cool
nebular gas via heat conduction, and on the bubble's expansion rate. We
find from our models that the X-ray luminosity of a planetary nebula
increases during its evolution across the HR diagram until stellar
luminosity and wind power decline. Depending on the central-star mass
and the evolutionary phase, our models predict X-ray [ 0.45-2.5
keV] luminosities between 10-8 and 10-4
of the stellar bolometric luminosities, in good agreement with the
observations. Less than 1% of the wind power is radiated away in this
X-ray band. Although temperature, density, and also the mass of the
hot bubble is significantly altered by heat conduction, the dynamics
of the whole system remains practically the same. Conclusions:
Heat conduction allows the construction of nebular models which predict
the correct amount of X-ray emission and at the same time are fully
consistent with the observed mass-loss rate and wind speed. Thermal
conduction must be considered as a viable physical process for
explaining the diffuse X-ray emission from planetary nebulae with
closed inner cavities. Magnetic fields must then be absent or extremely
weak. Dedicated to the memory of M. Perinotto, a dear friend and
esteemed colleague who died unexpectedly and much too early on
August 15, 2007.

---------------------------------------------------------
Title: On the Role of Acoustic-gravity Waves in the Energetics of
the Solar Atmosphere
Authors: Straus, T.; Fleck, B.; Jefferies, S. M.; Cauzzi, G.; McIntosh,
S. W.; Reardon, K.; Severino, G.; Steffen, M.; Suter, M.; Tarbell,
T. D.
2008ESPM...12.2.11S Altcode:
We revisit the dynamics and energetics of the solar atmosphere, using a
combination of high-quality observations and 3D numerical simulations
of the overshoot region of compressible convection into the stable
photosphere. We discuss the contribution of acoustic-gravity waves
to the energy balance of the photosphere and low chromosphere. We
demonstrate the presence of propagating internal gravity waves at
low frequencies (&lt; 5mHz). Surprisingly, these waves are found
to be the dominant phenomenon in the quiet middle/upper photosphere
and to transport a significant amount of mechanical energy into the
atmosphere outweighing the contribution of high-frequency (&gt; 5mHz)
acoustic waves by more than an order of magnitude. We compare the
properties of high-frequency waves in the simulations with results
of recent high cadence, high resolution Doppler velocity measurements
obtained with SOT/SP and SOT/NFI on Hinode. Our results seem to be in
conflict with the simple picture of upward propagating sound waves. We
discuss the implications of our findings on the energy flux estimate
at high-frequencies.

---------------------------------------------------------
Title: The Solar Photospheric Oxygen Abundance and the Role of 3D
Model Atmospheres
Authors: Caffau, E.; Steffen, M.; Ludwig, H. -G.
2008ESPM...12..3.7C Altcode:
The solar oxygen abundance has undergone a major downward revision in
the last decade, reputedly as a result of employing 3D hydrodynamical
simulations to model the inhomogeneous structure of the solar
photosphere. The very low oxygen abundance advocated by Asplund
et al. 2004, A(O)=8.66, together with the downward revision of the
abundances of other key elements, has created serious problems for solar
models to explain the helioseismic measurements. In an effort to
contribute to the dispute of whether the Sun has "solar" or "sub-solar"
abundances, we have re-derived its photospheric abundance of oxygen,
nitrogen, and other elements, independently of previous analyses. We applied a state-of-the art 3D (CO5BOLD) hydrodynamical simulation
of the solar granulation as well as different 1D model atmospheres for
the line by line spectroscopic abundance determinations. The analysis
is based on both standard disk-center and full-disk spectral atlases;
for oxygen we acquired in addition spectra at different heliocentric
angles. The derived abundances are the result of equivalent width
and/or line profile fitting of the available atomic lines. Our
recommended oxygen abundance is A(O)=8.76+- 0.07, 0.1 dex higher
than the value of Asplund et al. (2004). Our current estimate of the
overall solar metallicity is 0.014&lt; Z&lt;0.016. Questions we
discuss include: (i) Is the general downward revision of the solar
abundances a 3D effect? (ii) How large are the abundance corrections
due to horizontal inhomogeneities? (iii) What is the main reason for
the differences between the abundances obtained in our study and those
derived by Apslund and coworkers? (iv) How large are the uncertainties
in the observed solar spectra? (v) What is the reason why the two
forbidden oxygen lines, [OI] lambda 630 nm and [OI] lambda 636.3 nm,
give significantly different answers for the solar oxygen abundance?

---------------------------------------------------------
Title: The photospheric solar oxygen project. I. Abundance analysis
of atomic lines and influence of atmospheric models
Authors: Caffau, E.; Ludwig, H. -G.; Steffen, M.; Ayres, T. R.;
Bonifacio, P.; Cayrel, R.; Freytag, B.; Plez, B.
2008A&A...488.1031C Altcode: 2008arXiv0805.4398C
Context: The solar oxygen abundance has undergone a major downward
revision in the past decade, the most noticeable one being the
update including 3D hydrodynamical simulations to model the solar
photosphere. Up to now, such an analysis has only been carried out
by one group using one radiation-hydrodynamics code. Aims:
We investigate the photospheric oxygen abundance considering lines
from atomic transitions. We also consider the relationship between
the solar model used and the resulting solar oxygen abundance, to
understand whether the downward abundance revision is specifically
related to 3D hydrodynamical effects. Methods: We performed
a new determination of the solar photospheric oxygen abundance by
analysing different high-resolution high signal-to-noise ratio atlases
of the solar flux and disc-centre intensity, making use of the latest
generation of CO5BOLD 3D solar model atmospheres. Results: We
find 8.73 ≤ log (N_O/N_H) +12 ≤ 8.79. The lower and upper values
represent extreme assumptions on the role of collisional excitation
and ionisation by neutral hydrogen for the NLTE level populations
of neutral oxygen. The error of our analysis is ± (0.04± 0.03)
dex, the last being related to NLTE corrections, the first error
to any other effect. The 3D “granulation effects” do not play a
decisive role in lowering the oxygen abundance. Conclusions:
Our recommended value is log (N_O/N_H) = 8.76 ± 0.07, considering our
present ignorance of the role of collisions with hydrogen atoms on the
NLTE level populations of oxygen. The reasons for lower O abundances in
the past are identified as (1) the lower equivalent widths adopted and
(2) the choice of neglecting collisions with hydrogen atoms in the
statistical equilibrium calculations for oxygen. This paper is
dedicated to the memory of Hartmut Holweger.

---------------------------------------------------------
Title: Velocity and Intensity Power and Cross Spectra in Numerical
Simulations of Solar Convection
Authors: Severino, G.; Straus, T.; Steffen, M.
2008SoPh..251..549S Altcode: 2008SoPh..tmp...54S
Fitting observed power and cross spectra of medium-degree p modes
in velocity (V) and intensity (I) has been widely used for getting
information about the p-mode excitation process and, in particular,
for trying to determine the type and location of the acoustic
sources. Numerical simulations of solar convection allow one to
"observe" velocity and temperature (T, used as proxy for I) fluctuations
in different reference frames. Sampling the oscillations on planes of
constant optical depth (τ-frame) closely corresponds to the observer's
point of view, whereas sampling the oscillations at constant geometrical
height (z-frame) is more appropriate for comparison with predictions
from theoretical models based on Eulerian hydrodynamics. The results of
the analysis in the two frames show significant differences. Considering
the effects introduced on oscillations by the steep temperature gradient
of the photosphere and by the temperature- and pressure-dependent
continuum opacity, we develop a new model for fitting the simulated V
and T power and cross spectra both in the τ- and z-frames and discuss
its merits and limitations.

---------------------------------------------------------
Title: Radiation hydrodynamics simulations of stellar surface
convection
Authors: Freytag, Bernd; Steffen, Matthias; Ludwig, Hans-Guenter;
Wedemeyer-Boehm, Sven
2008asd..soft...36F Altcode:
The code is used to perform radiation hydrodynamics simulations of
the convective surface layers and the photosphere of cool stars.

---------------------------------------------------------
Title: Spatially resolved spectroscopy of planetary nebulae and
their halos. I. Five galactic disk objects
Authors: Sandin, C.; Schönberner, D.; Roth, M. M.; Steffen, M.;
Böhm, P.; Monreal-Ibero, A.
2008A&A...486..545S Altcode: 2008arXiv0802.3813S
Strong mass loss off stars at the tip of the asymptotic giant
branch (AGB) profoundly affects properties of these stars and their
surroundings, including the subsequent planetary nebula (PN) stage. With
this study we wanted to determine physical properties of mass loss by
studying weakly emitting halos, focusing on objects in the galactic
disk. Halos surround the, up to several thousand times, brighter
central regions of PNe. Young halos, specifically, still contain
information of the preceeding final mass loss stage on the AGB. In
the observations we used the method of integral field spectroscopy
with the PMAS instrument. This is the first committed study of halos
of PNe that uses this technique. We improved our data analysis by a
number of steps. In a study of the influence of scattered light we found
that a moderate fraction of intensities in the inner halo originate in
adjacent regions. As we combine line intensities of distant wavelengths,
and because radial intensity gradients are steep, we corrected for
effects of differential atmospheric refraction. In order to increase
the signal-to-noise of weak emission lines we introduced a dedicated
method to bin spectra of individual spatial elements. We also developed
a general technique to part the temperature-sensitive oxygen line [O
III] λ4363 from the adjacent telluric mercury line Hg λ4358 - without
using separate sky exposures. By these steps we avoided introducing
errors of several thousand Kelvin to our temperature measurements in the
halo. For IC 3568 we detected a halo. For M 2-2 we found a halo radius
that is 2.5 times larger than reported earlier. We derived radially
densely sampled temperature and density structures for four nebulae,
which all extend from the central regions and out into the halo. NGC
7662, IC 3568, and NGC 6826 show steep radially increasing temperatures
and a hot halo, indicating that the gas in the halo is not in thermal
equilibrium. M 2-2 shows a larger temperature in the central region
and an otherwise constant value. From the density structures we made
estimates of core and halo masses and - for the first time reliable -
mass loss rates at the tip of the AGB. All four objects show inwards
radially increasing mass loss rate structures, which represent a rise
by a factor of about 4-7, during the final mass loss phase, that covers
a time period of approximately 104 years. Within a factor
of two, the average of the maximum mass loss rates, which are distance
dependent, is dot{M}max≃10-4 {M}⊙
yr-1. Based on observations collected at the Centro
Astronómico Hispano Alemán (CAHA), operated jointly by the Max-Planck
Institut für Astronomie and the Instituto de Astrofísica de Andalucia
(CSIC). Figures 24-29 are only available in electronic form at
http://www.aanda.org

---------------------------------------------------------
Title: The Energy Flux of Internal Gravity Waves in the Lower Solar
Atmosphere
Authors: Straus, Thomas; Fleck, Bernhard; Jefferies, Stuart M.;
Cauzzi, Gianna; McIntosh, Scott W.; Reardon, Kevin; Severino, Giuseppe;
Steffen, Matthias
2008ApJ...681L.125S Altcode:
Stably stratified fluids, such as stellar and planetary atmospheres,
can support and propagate gravity waves. On Earth these waves,
which can transport energy and momentum over large distances and can
trigger convection, contribute to the formation of our weather and
global climate. Gravity waves also play a pivotal role in planetary
sciences and modern stellar physics. They have also been proposed
as an agent for the heating of stellar atmospheres and coronae, the
exact mechanism behind which is one of the outstanding puzzles in solar
and stellar physics. Using a combination of high-quality observations
and 3D numerical simulations we have the first unambiguous detection
of propagating gravity waves in the Sun's (and hence a stellar)
atmosphere. Moreover, we are able to determine the height dependence of
their energy flux and find that at the base of the Sun's chromosphere it
is around 5 kW m-2. This amount of energy is comparable to
the radiative losses of the entire chromosphere and points to internal
gravity waves as a key mediator of energy into the solar atmosphere.

---------------------------------------------------------
Title: Toward Better Simulations of Planetary Nebulae Luminosity
Functions
Authors: Méndez, R. H.; Teodorescu, A. M.; Schönberner, D.; Jacob,
R.; Steffen, M.
2008ApJ...681..325M Altcode: 2008arXiv0804.0826M
We describe a procedure for the numerical simulation of the planetary
nebulae luminosity function (PNLF), improving on previous work. Earlier
PNLF simulations were based on an imitation of the observed distribution
of the intensities of [O III] λ5007 relative to Hβ, generated
predominantly using random numbers. We are now able to replace this
by a distribution derived from the predictions of hydrodynamical
PN models (Schönberner et al. 2007), which are made to evolve as
the central star moves across the HR diagram, using proper initial
and boundary conditions. In this way we move one step closer to a
physically consistent procedure for the generation of a PNLF. As an
example of these new simulations, we have been able to reproduce the
observed PNLF in the Small Magellanic Cloud.

---------------------------------------------------------
Title: The solar photospheric abundance of hafnium and
thorium. Results from CO5BOLD 3D hydrodynamic model
atmospheres
Authors: Caffau, E.; Sbordone, L.; Ludwig, H. -G.; Bonifacio, P.;
Steffen, M.; Behara, N. T.
2008A&A...483..591C Altcode: 2008arXiv0803.3585C
Context: The stable element hafnium (Hf) and the radioactive element
thorium (Th) were recently suggested as a suitable pair for radioactive
dating of stars. The applicability of this elemental pair needs to
be established for stellar spectroscopy. Aims: We aim at a
spectroscopic determination of the abundance of Hf and Th in the
solar photosphere based on a CO5BOLD 3D hydrodynamical
model atmosphere. We put this into a wider context by investigating 3D
abundance corrections for a set of G- and F-type dwarfs. Methods:
High-resolution, high signal-to-noise solar spectra were compared to
line synthesis calculations performed on a solar CO5BOLD
model. For the other atmospheres, we compared synthetic spectra
of CO5BOLD 3D and associated 1D models. Results:
For Hf we find a photospheric abundance A(Hf) = 0.87 ± 0.04, in good
agreement with a previous analysis, based on 1D model atmospheres. The
weak Th II 401.9 nm line constitutes the only Th abundance indicator
available in the solar spectrum. It lies in the red wing of a Ni-Fe
blend exhibiting a non-negligible convective asymmetry. Accounting for
the asymmetry-related additional absorption, we obtain A(Th) = 0.08 ±
0.03, consistent with the meteoritic abundance, and about 0.1 dex lower
than obtained in previous photospheric abundance determinations. Conclusions: Only for the second time, to our knowledge, has a
non-negligible effect of convective line asymmetries on an abundance
derivation been highlighted. Three-dimensional hydrodynamical
simulations should be employed to measure Th abundances in dwarfs
if similar blending is present, as in the solar case. In contrast,
3D effects on Hf abundances are small in G- to mid F-type dwarfs and
sub-giants, and 1D model atmospheres can be conveniently used.

---------------------------------------------------------
Title: Hydrodynamical Model Atmospheres of Metal-Poor Stars
Authors: Ludwig, Hans-Günter; González Hernández, Jonay I.; Behara,
Natalie; Caffau, Elisabetta; Steffen, Matthias
2008AIPC..990..268L Altcode:
Standard one-dimensional (1D) model atmospheres rely on the assumption
of radiative equilibrium in the non-convective part of the stellar
photosphere. However, gas-dynamical effects can lead to dramatic
deviations from radiative equilibrium conditions, especially in
metal-poor stellar atmospheres. These can be taken into account in
3D stellar atmosphere models representing the detailed interplay of
hydrodynamics and radiation. During the last two years efforts have been
invested to compute such 3D models for metal-poor atmospheres with the
CO5 BOLD code within the CIFIST (Cosmological Impact of the
FIrst STars) Team, an European Union funded research group dedicated
to the study of metal-poor stars. Based on the available models we will
give an account of the radiation-hydrodynamical processes at work, and
discuss consequences for the temperature scale and abundance analysis
of metal-poor stars.

---------------------------------------------------------
Title: Hydrodynamical Model Atmospheres and 3D Spectral Synthesis
Authors: Ludwig, Hans-Günter; Steffen, Matthias
2008psa..conf..133L Altcode: 2007arXiv0704.1176L
In this paper we discuss three issues in the context of
three-dimensional (3D) hydrodynamical model atmospheres for late-type
stars, related to spectral line shifts, radiative transfer in metal-poor
3D models, and the solar oxygen abundance. To establish the context we
start by giving a brief overview about the model construction, taking
the radiation-hydrodynamics code CO5BOLD (Conservative COde
for the Computation of COmpressible COnvection in a BOx of L Dimensions
with L=2,3; [3]) and the related spectral synthesis package Linfor3D
as examples.

---------------------------------------------------------
Title: Overview of the Li problem in metal-poor stars and new results
on 6Li
Authors: Cayrel, R.; Steffen, M.; Bonifacio, P.; Ludwig, H. -G.;
Caffau, E.
2008nuco.confE...2C Altcode: 2008arXiv0810.4290C; 2008PoS....53E...2C
Two problems are discussed here. The first one is the 0.4 dex
discrepancy between the 7Li abundance derived from the spectra
of metal-poor halo stars on the one hand, and from Big Bang
nucleosynthesis, based on the cosmological parameters constrained
by the WMAP measurements, on the other hand. Lithium, indeed,
can be depleted in the convection zone of unevolved stars. The
understanding of the hydrodynamics of the crucial zone near the
bottom of the convective envelope in dwarfs or turn-off stars of solar
metallicity has recently made enormous progress with the inclusion of
internal gravity waves. However, similar work for metal-poor stars is
still lacking. Therefore it is not yet clear whether the depletion
occurring in the metal-poor stars themselves is adequate to produce
a 7Li plateau. The second problem pertains to the large amount of
6Li recently found in metal-poor halo stars. The convection-related
asymmetry of the 7Li line could mimic the signal attributed so far
to the weak blend of 6Li in the red wing of the 7Li line. Theoretical
computations show that the signal generated by the asymmetry of 7Li is
2.0, 2.1, and 3.7 per cent for [Fe/H]= -3.0, -2.0, -1.0, respectively
(Teff =6250 K and log g=4.0 [cgs]). In addition we re-investigate
the statistical properties of the 6Li plateau and show that previous
analyses were biased. Our conclusion is that the 6Li plateau can be
reinterpreted in terms of intrinsic line asymmetry, without the need
to invoke a contribution of 6Li. (abridged)

---------------------------------------------------------
Title: Rotating `star-in-a-box' experiments
Authors: Steffen, M.; Freytag, B.
2007AN....328.1054S Altcode:
Using the radiation hydrodynamics code CO5BOLD in its
`star-in-a-box' setup, we have performed exploratory simulations
of global convection in a rotating reference frame. The goal is to
study the interaction of convection and rotation by direct numerical
simulation. For these first experiments, we chose an idealized
configuration (a scaled-down, fast rotating Sun) whose properties
resemble those of red supergiants in some respect. We describe the setup
and time evolution of these models, and discuss the particular problems
we have encountered. Finally, we derive the resulting differential
rotation pattern and meridional flow field by temporal and azimuthal
averaging of the simulation data. We find anti-solar differential
rotation for all cases studied so far. Movies are available via
http://www.aip.de/AN/movies

---------------------------------------------------------
Title: On the Mass-Loss History at the Tip of the AGB
Authors: Schönberner, D.; Steffen, M.
2007ASPC..378..343S Altcode:
We demonstrate by means of radiation-hydrodynamics simulations how the
spectral energy distribution (SED) of post-AGB objects depends on the
mass-loss properties during evolution off the tip of the AGB.

---------------------------------------------------------
Title: A hydrodynamical study of multiple-shell planetary
nebulae. II. Measuring the post-shock velocities in the shells
Authors: Corradi, R. L. M.; Steffen, M.; Schönberner, D.; Jacob, R.
2007A&A...474..529C Altcode:
Context: Planetary Nebulae (PNe) display complex radial surface
brightness profiles whose understanding is crucial for a correct
interpretation of their formation and evolution. In particular, the
intermediate shells commonly observed around the bright rims of PNe
contain important information for the discussion of the mass loss rate
at the end of the AGB, for the determination of the post-AGB age of
the central stars and the distance via the expansion parallax method,
and for the analysis of line profiles for unresolved, faraway objects
like extragalactic PNe. Aims: This paper describes the dynamical
properties of PNe shells and presents a new practical method to properly
determine their expansion velocity. Methods: The analysis is
guided by realistic radiative-hydrodynamical simulations which allow
to test and verify different methods for the kinematical study of
PNe shells using high-resolution long-slit spectra. Results: We
show that the use of the derivative of the line profile allows us to
determine accurately the post-shock gas velocity in the shells. This
method is generally superior to other techniques previously used. In
addition, if applied to long-slit data, it allows to determine the
velocities of the shells even when they are very close to those of the
PNe rims. Its application to 10 real PNe confirms the model predictions
and previous conclusions about the substantial mass loss increase
during the latest AGB evolution. Conclusions: The method and
discussion presented in this paper are meant to be used as a guide
for a correct determination of some basic kinematic and evolutionary
properties of Galactic and extragalactic PNe, including their expansion
parallaxes. Based on observations obtained at the 3.5 m NTT and
CAT telescopes of the European Southern Observatory in Chile, and at
the 2.6 m NOT telescope operated by NOTSA in the Spanish Observatorio
del Roque de Los Muchachos of the Instituto de Astrofísica de Canarias.

---------------------------------------------------------
Title: The solar photospheric abundance of phosphorus: results from
CO^5BOLD 3D model atmospheres
Authors: Caffau, E.; Steffen, M.; Sbordone, L.; Ludwig, H. -G.;
Bonifacio, P.
2007A&A...473L...9C Altcode: 2007arXiv0708.1607C
Aims:We determine the solar abundance of phosphorus using CO^5BOLD
3D hydrodynamic model atmospheres. Methods: High-resolution,
high signal-to-noise solar spectra of the P i lines of Multiplet 1 at
1051-1068 nm are compared to line-formation computations performed
on a CO^5BOLD solar model atmosphere. Results: We find A(P)
= 5.46 ± 0.04, in good agreement with previous analyses based on
1D model atmospheres, due to the P i lines of Mult. 1 not being
affected much by 3D effects. We cannot confirm an earlier claim by
other authors of a downward revision of the solar P abundance by 0.1
dex when employing a 3D model atmosphere. Concerning other stars,
we find modest (&lt;0.1 dex) 3D abundance corrections for P among
four F-dwarf model atmospheres of different metallicities, and these
corrections are largest at lowest metallicity. Conclusions:
We conclude that 3D abundance corrections are generally rather small
for the P i lines studied in this work. They are marginally relevant
for metal-poor stars, but may be neglected in the Sun. Tables
2-4 are only available in electronic form at http://www.aanda.org

---------------------------------------------------------
Title: Line shift, line asymmetry, and the ^6Li/^7Li isotopic ratio
determination
Authors: Cayrel, R.; Steffen, M.; Chand, H.; Bonifacio, P.; Spite,
M.; Spite, F.; Petitjean, P.; Ludwig, H. -G.; Caffau, E.
2007A&A...473L..37C Altcode: 2007arXiv0708.3819C
Context: Line asymmetries are generated by convective Doppler shifts in
stellar atmospheres, especially in metal-poor stars, where convective
motions penetrate to higher atmospheric levels. Such asymmetries are
usually neglected in abundance analyses. The determination of the
^6Li/^7Li isotopic ratio is prone to suffering from such asymmetries,
as the contribution of ^6Li is a slight blending reinforcement of the
red wing of each component of the corresponding ^7Li line, with respect
to its blue wing. Aims: The present paper studies the halo star
HD 74000 and estimates the impact of convection-related asymmetries
on the Li isotopic ratio determination. Methods: Two methods
are used to meet this aim. The first, which is purely empirical,
consists in deriving a template profile from another element that can
be assumed to originate in the same stellar atmospheric layers as Li
I, producing absorption lines of approximately the same equivalent
width as individual components of the ^7Li I resonance line. The
second method consists in conducting the abundance analysis based on
NLTE line formation in a 3D hydrodynamical model atmosphere, taking
into account the effects of photospheric convection. Results:
The results of the first method show that the convective asymmetry
generates an excess absorption in the red wing of the ^7Li absorption
feature that mimics the presence of ^6Li at a level comparable to
the hitherto published values. This opens the possibility that only
an upper limit on ^6Li/^7Li has thus far been derived. The second
method confirms these findings. Conclusions: From this work,
it appears that a systematic reappraisal of former determinations of
^6Li abundances in halo stars is warranted. Based on observations
carried out at the European Southern Observatory (ESO), under prog. ID
75.D-0600. Tables 1-3, and additional references are only available
in electronic form at http://www.aanda.org

---------------------------------------------------------
Title: The evolution of planetary nebulae. IV. On the physics of
the luminosity function
Authors: Schönberner, D.; Jacob, R.; Steffen, M.; Sandin, C.
2007A&A...473..467S Altcode: 2007arXiv0708.4292S
Context: The luminosity function of planetary nebulae, in use for
about two decades in extragalactic distance determinations, is still
subject to controversial interpretations. Aims: The physical
basis of the luminosity function is investigated by means of several
evolutionary sequences of model planetary nebulae computed with a 1D
radiation-hydrodynamics code. Methods: The nebular evolution is
followed from the vicinity of the asymptotic-giant branch across the
Hertzsprung-Russell diagram until the white-dwarf domain is reached,
using various central-star models coupled to different initial envelope
configurations. Along each sequence the relevant line emissions of
the nebulae are computed and analysed. Results: Maximum line
luminosities in Hβ and [O iii] 5007 Å are achieved at stellar
effective temperatures of about 65 000 K and 95 000...100 000 K,
respectively, provided the nebula remains optically thick for ionising
photons. In the optically thin case, the maximum line emission occurs
at or shortly after the thick/thin transition. Our models suggest that
most planetary nebulae with hotter (⪆ 45 000 K) central stars are
optically thin in the Lyman continuum, and that their [O iii] 5007 Å
emission fails to explain the bright end of the observed planetary
nebulae luminosity function. However, sequences with central stars
of ⪆0.6 M⊙ and rather dense initial envelopes remain
virtually optically thick and are able to populate the bright end
of the luminosity function. Individual luminosity functions depend
strongly on the central-star mass and on the variation of the nebular
optical depth with time. Conclusions: Hydrodynamical simulations
of planetary nebulae are essential for any understanding of the basic
physics behind their observed luminosity function. In particular,
our models do not support the claim of Marigo et al. (2004, A&amp;A,
423, 995) according to which the maximum 5007 Å luminosity occurs
during the recombination phase well beyond 100 000 K when the stellar
luminosity declines and the nebular models become, at least partially,
optically thick. Consequently, there is no need to invoke relatively
massive central stars of, say &gt; 0.7 M⊙, to account
for the bright end of the luminosity function. Based in parts
on observations made with the NASA/ESA Hubble Space Telescope,
obtained at the Space Science Institute, which is operated by the
Association of the Universities for Research in Astronomy, Inc.,
under NASA contract NAS 5-26555. The data are retrieved from the
ESO/ST-ECF Science Archive Facility.

---------------------------------------------------------
Title: The PEPSI "deep spectrum" project
Authors: Steffen, Matthias; Strassmeier, Klaus G.
2007AN....328..632S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Inter-network regions of the Sun at millimetre wavelengths
Authors: Wedemeyer-Böhm, S.; Ludwig, H. G.; Steffen, M.; Leenaarts,
J.; Freytag, B.
2007A&A...471..977W Altcode: 2007arXiv0705.2304W
Aims:The continuum intensity at wavelengths around 1 mm provides an
excellent way to probe the solar chromosphere and thus valuable input
for the ongoing controversy on the thermal structure and the dynamics of
this layer. The synthetic continuum intensity maps for near-millimetre
wavelengths presented here demonstrate the potential of future
observations of the small-scale structure and dynamics of internetwork
regions on the Sun. Methods: The synthetic intensity/brightness
temperature maps are calculated on basis of three-dimensional
radiation (magneto-)hydrodynamic (MHD) simulations. The assumption
of local thermodynamic equilibrium (LTE) is valid for the source
function. The electron densities are also treated in LTE for most maps
but also in non-LTE for a representative model snapshot. Quantities
like intensity contrast, intensity contribution functions, spatial
and temporal scales are analysed in dependence on wavelength and
heliocentric angle. Results: While the millimetre continuum
at 0.3 mm originates mainly from the upper photosphere, the longer
wavelengths considered here map the low and middle chromosphere. The
effective formation height increases generally with wavelength and
also from disk-centre towards the solar limb. The average intensity
contribution functions are usually rather broad and in some cases they
are even double-peaked as there are contributions from hot shock waves
and cool post-shock regions in the model chromosphere. The resulting
shock-induced thermal structure translates to filamentary brightenings
and fainter regions in between. Taking into account the deviations from
ionisation equilibrium for hydrogen gives a less strong variation of
the electron density and with it of the optical depth. The result is
a narrower formation height range although the intensity maps still
are characterised by a highly complex pattern. The average brightness
temperature increases with wavelength and towards the limb although
the wavelength-dependence is reversed for the MHD model and the NLTE
brightness temperature maps. The relative contrast depends on wavelength
in the same way as the average intensity but decreases towards the
limb. The dependence of the brightness temperature distribution on
wavelength and disk-position can be explained with the differences in
formation height and the variation of temperature fluctuations with
height in the model atmospheres. The related spatial and temporal
scales of the chromospheric pattern should be accessible by future
instruments. Conclusions: Future high-resolution millimetre
arrays, such as the Atacama Large Millimeter Array (ALMA), will
be capable of directly mapping the thermal structure of the solar
chromosphere. Simultaneous observations at different wavelengths
could be exploited for a tomography of the chromosphere, mapping its
three-dimensional structure, and also for tracking shock waves. The
new generation of millimetre arrays will be thus of great value for
understanding the dynamics and structure of the solar atmosphere.

---------------------------------------------------------
Title: Sulphur abundances from the S i near-infrared triplet at
1045 nm
Authors: Caffau, E.; Faraggiana, R.; Bonifacio, P.; Ludwig, H. -G.;
Steffen, M.
2007A&A...470..699C Altcode: 2007arXiv0704.2335C
Context: Unlike silicon and calcium, sulphur is an α-element
that does not form dust. Some of the available observations of
the evolution of sulphur with metallicity indicate an increased
scatter of sulphur-to-iron ratios at low metallicities or even
a bimodal distribution, with some stars showing constant S/Fe at
all metallicities and others showing an increasing S/Fe ratio with
decreasing metallicity. In metal-poor stars S i lines of Multiplet
1 at 920 nm are not yet too weak to permit the measurement of the
sulphur abundance A(S); however, in ground-based observations they
are severely affected by telluric lines. Aims: We investigate
the possibility of measuring sulphur abundances from S iMult. 3 at
1045 nm lines. These lie in the near infrared and are slightly weaker
than those of Mult. 1, but lie in a range not affected by telluric
lines. Methods: We investigated the lines of Mult. 3 in the Sun
(G2V), Procyon (F5V), HD 33256 (F5V), HD 25069 (G9V), and ɛ Eri (HD
22049, K2V). For the Sun and Procyon the analysis was performed with
CO^5BOLD 3D hydrodynamical model atmospheres, while the three other
stars, for which hydrodynamical simulations are not available, were
analysed using 1D model atmospheres. Results: For our sample of
stars we find a global agreement between A(S) from lines of different
multiplets. Conclusions: Our results suggest that the infrared
lines of Mult. 3 are a viable indicator of the sulphur abundance
that, because of the intrinsic strength of this multiplet, should be
suitable for studying the trend of [S/Fe] at low metallicities. Based on data from the UVES Paranal Observatory Project (ESO DDT
Program ID 266.D-5655). Appendix is only available in electronic form
at http://www.aanda.org

---------------------------------------------------------
Title: Spatially resolved spectroscopic studies of planetary nebulae
and their halos
Authors: Sandin, C.; Schönberner, D.; Roth, M. M.; Steffen, M.;
Böhm, P.; Monreal-Iber, A.
2007apn4.confE...6S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Internal Gravity Waves and their Role in the Energetics of
the Solar Atmosphere
Authors: Fleck, Bernard; Straus, T.; Jefferies, S.; McIntosh, S. W.;
Severino, G.; Steffen, M.
2007AAS...210.2410F Altcode: 2007BAAS...39..130F
Internal gravity waves are believed to be excited by convective
overshoot in the solar atmosphere. We compare the results from numerical
simulations of the overshoot region of compressible convection into a
stable photosphere, with observations of the velocity field at several
heights in the solar atmosphere. We find a consistent picture for the
quiet middle/upper photosphere in which internal gravity waves are the
dominant phenomenon at low frequencies (&lt; 2.5 mHz). We estimate the
contribution of these waves to the energy balance in the photosphere
and low chromosphere.

---------------------------------------------------------
Title: 3-D hydrodynamic simulations of convection in A stars
Authors: Kochukhov, O.; Freytag, B.; Piskunov, N.; Steffen, M.
2007IAUS..239...68K Altcode: 2006astro.ph.10111K
Broadening and asymmetry of spectral lines in slowly rotating late
A-type stars provide evidence for high-amplitude convective motions. The
properties of turbulence observed in the A-star atmospheres are not
understood theoretically and contradict results of previous numerical
simulations of convection. Here we describe an ongoing effort to
understand the puzzling convection signatures of A stars with the
help of 3-D hydrodynamic simulations. Our approach combines realistic
spectrum synthesis and non-grey hydrodynamic models computed with the
CO5BOLD code. We discuss these theoretical predictions and confront
them with high-resolution spectra of A stars. Our models have, for
the first time, succeeded in reproducing the observed profiles of weak
spectral lines without introducing fudge broadening parameters.

---------------------------------------------------------
Title: Radiative hydrodynamics models of stellar convection
Authors: Steffen, Matthias
2007IAUS..239...36S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Carbon monoxide in the solar atmosphere. II. Radiative cooling
by CO lines
Authors: Wedemeyer-Böhm, S.; Steffen, M.
2007A&A...462L..31W Altcode: 2006astro.ph.12197W
Aims: The role of carbon monoxide as a cooling agent for the thermal
structure of the mid-photospheric to low-chromospheric layers of the
solar atmosphere in internetwork regions is investigated. Methods:
The treatment of radiative cooling via spectral lines of carbon
monoxide (CO) has been added to the radiation chemo-hydrodynamics code
CO5BOLD. The radiation transport has now been solved in a continuum band
with Rosseland mean opacity and an additional band with CO opacity. The
latter is calculated as a Planck mean over the CO band between 4.4
and 6.2 μm. The time-dependent CO number density is derived from
the solution of a chemical reaction network. Results: The CO
opacity indeed causes additional cooling at the fronts of propagating
shock waves in the chromosphere. There, the time-dependent approach
results in a higher CO number density compared to the equilibrium
case and hence in a larger net radiative cooling rate. The average
gas temperature stratification of the model atmosphere, however, is
only reduced by roughly 100 K. Also the temperature fluctuations and
the CO number density are only affected to small extent. A numerical
experiment without dynamics shows that the CO cooling process works in
principle and drives the atmosphere to a cool radiative equilibrium
state. At chromospheric heights, the radiative relaxation of the
atmosphere to a cool state takes several 1000 s. The CO cooling process
thus would seem to be too slow compared to atmospheric dynamics to be
responsible for the very cool temperature regions observed in the solar
atmosphere. Conclusions: . The hydrodynamical timescales in our
solar atmosphere model are much too short to allow for the radiative
relaxation to a cool state, thus suppressing the potential thermal
instability due to carbon monoxide as a cooling agent. Apparently,
the thermal structure and dynamics of the outer model atmosphere are
instead determined primarily by shock waves.

---------------------------------------------------------
Title: 3D-Spectroscopy of Extragalactic Planetary Nebulae as
Diagnostic Probes for Galaxy Evolution
Authors: Kelz, A.; Monreal-Ibero, A.; Roth, M. M.; Sandin, C.;
Schönberner, D.; Steffen, M.
2007spts.conf..339K Altcode: 2006astro.ph..7498K
In addition to study extragalactic stellar populations in their
integrated light, the detailed analysis of individual resolved
objects has become feasible, mainly for luminous giant stars and for
extragalactic planetary nebulae (XPNe) in nearby galaxies. A recently
started project at the Astrophysical Institute Potsdam (AIP), called
“XPN--Physics”, aims to verify if XPNe are useful probes to measure
the chemical abundances of their parent stellar population. The project
involves theoretical and observational work packages.

---------------------------------------------------------
Title: A First Three-Dimensional Model for the Carbon Monoxide
Concentration in the Solar Atmosphere
Authors: Wedemeyer-Böhm, S.; Kamp, I.; Freytag, B.; Bruls, J.;
Steffen, M.
2006ASPC..354..301W Altcode:
The time-dependent and self-consistent treatment of carbon monoxide
(CO) has been added to the radiation chemo-hydrodynamics code
CO5BOLD. It includes the solution of a chemical reaction network and
the advection of the resulting particle densities with the hydrodynamic
flow field. Here we present a first 3D simulation of the non-magnetic
solar photosphere and low chromosphere, calculated with the upgraded
code. In the resulting model, the highest amount of CO is located in
the cool regions of the reversed granulation pattern in the middle
photosphere. A large fraction of carbon is bound by CO throughout
the chromosphere with exception of hot shock waves where the CO
concentration is strongly reduced. The distribution of carbon monoxide
is very inhomogeneous due to co-existing regions of hot and cool gas
caused by the hydrodynamic flow. High-resolution observations of CO
could thus provide important constraints for the thermal structure of
the solar photosphere and chromosphere.

---------------------------------------------------------
Title: Detection of a Far-Infrared Bow Shock Nebula around R Hya:
The First MIRIAD Results
Authors: Ueta, T.; Speck, A. K.; Stencel, R. E.; Herwig, F.; Gehrz,
R. D.; Szczerba, R.; Izumiura, H.; Zijlstra, A. A.; Latter, W. B.;
Matsuura, M.; Meixner, M.; Steffen, M.; Elitzur, M.
2006ApJ...648L..39U Altcode: 2006astro.ph..7303U
We present the first results of the MIRIAD (MIPS InfraRed Imaging
of AGB Dust shells) project using the Spitzer Space Telescope. The
primary aim of the project is to probe the material distribution in the
extended circumstellar envelopes (CSEs) of evolved stars and recover
the fossil record of their mass-loss history. Hence, we must map the
whole of the CSEs plus the surrounding sky for background subtraction
while avoiding the central star that is brighter than the detector
saturation limit. With our unique mapping strategy, we have achieved
better than 1 MJy sr-1 sensitivity in 3 hr of integration
and successfully detected a faint (&lt;5 MJy sr-1), extended
(~400") far-infrared nebula around the asymptotic giant branch (AGB)
star R Hya. Based on the parabolic structure of the nebula, the
direction of the space motion of the star with respect to the nebula
shape, and the presence of extended Hα emission cospatial to the
nebula, we suggest that the detected far-IR nebula is due to a bow
shock at the interface of the interstellar medium and the AGB wind
of this moving star. This is the first detection of the stellar-wind
bow shock interaction for an AGB star and exemplifies the potential
of Spitzer as a tool to examine the detailed structure of extended
far-IR nebulae around bright central sources.

---------------------------------------------------------
Title: Resonant Oscillation Modes and Background in Realistic
Hydrodynamical Simulations of Solar Surface Convection
Authors: Straus, T.; Severino, G.; Steffen, M.
2006ESASP.617E...4S Altcode: 2006soho...17E...4S
No abstract at ADS

---------------------------------------------------------
Title: Working with VIMOS-IFU data: Searching and characterizing
the faint haloes of planetary nebulae
Authors: Monreal-Ibero, A.; Roth, M. M.; Schönberner, D.; Steffen,
M.; Böhm, P.
2006NewAR..50..426M Altcode:
In this contribution, we present the VIMOS-IFU data analysis of two
planetary nebulae: NGC 3242 and NGC 4361. Due to the technical nature
of the workshop, we emphasize some specific problems of data reduction
and analysis: accuracy in flux calibration, importance of scattered
light and improvement of the S/N ratio. We estimate that accuracy of
our flux calibration is ∼15% in the spectral range 4200-6100 Å. At
∼10″ from the center of a point source, the intensity is ≲0.02%
of the peak value, indicating that scattered light is negligible. Some
examples of science that we are doing are shown. In particular, we
have established an apparent temperature gradient across the halo of
NGC NGC3242.

---------------------------------------------------------
Title: Nod &amp; Shuffle 3D Spectroscopy
Authors: Roth, Martin M.; Cardiel, Nicolas; Cenarro, Javier;
Schönberner, Detlef; Steffen, Matthias
2006ASSL..336...99R Altcode: 2006sda..conf...99R
No abstract at ADS

---------------------------------------------------------
Title: The modelling of the X-ray emission of planetary nebulae
Authors: Schönberner, D.; Steffen, M.; Warmuth, A.
2006IAUS..234..161S Altcode:
We report on our recent efforts to compute the X-ray emission
from the hot, shocked stellar wind gas filling the inner cavity
of planetary nebulae. To this end, we updated our 1D hydrodynamics
code NEBEL by including a module that computes the heat transfer by
thermal conduction across the contact discontinuity separating the
hot shocked wind gas from the much cooler nebular material. Given the
temperature and density structure of the hot bubble the X-ray emission
is computed by means of the CHIANTI code. We find a reasonably close
agreement of the computed X-ray luminosities with recent observations
of Newton-XMM and Chandra. Our simulations also predict how the X-ray
emission depends on the wind luminosity and the stellar parameters.

---------------------------------------------------------
Title: 3D Spectroscopy — a powerful new tool for PN research
Authors: Roth, Martin M.; Schönberner, D.; Steffen, M.; Monreal,
A.; Sandin, C.
2006IAUS..234...17R Altcode:
Historically, technological progress with detectors and instrumentation
has been essential for advances in any field of observational
astronomy, e.g. the advent of CCDs being crucial for high dynamic range
imaging and quantitative spectroscopy of galactic PNe, faint object
spectrophotometry for the discovery of extragalactic PNe to distances
as far as 100 Mpc, etc. The emerging technique of integral field ("3D")
spectroscopy, which is being applied quite successfully to extragalactic
astronomy, has unfortunately hardly been used so far for the study of
PNe. However, 3D spectroscopy has an enormous potential for various
observational problems, ranging from high spatial resolution emission
line mapping in different wavelengths simultaneously, over extremely
high sensitivity spectroscopy of low surface brightness objects like
e.g. PN haloes, to accurate 3D spectrophotometry of extragalactic PNe,
and many others. As an attempt to encourage PN researchers to make
better use of these new opportunities, the presently existing suite
of 3D instruments on 4-8m class telescopes is reviewed, highlighting
some examples of successful 3D observations for the study of PNe.

---------------------------------------------------------
Title: Hydrodynamical interpretation of basic nebular structures
Authors: Steffen, M.; Schönberner, D.
2006IAUS..234..285S Altcode:
Based on existing hydrodynamical simulations, we review our present
understanding of the formation and evolution of planetary nebulae (PNe)
and discuss the relevant processes responsible for the development of
the basic (1D) nebular structures.

---------------------------------------------------------
Title: New observations of the halo radial temperature structure in
NGC 7662
Authors: Sandin, C.; Schönberner, D.; Roth, M. M.; Steffen, M.;
Monreal-Ibero, A.; Böhm, P.; Tripphahn, U.
2006IAUS..234..501S Altcode: 2006astro.ph..9279S
We report on our studies of the physical structure of the planetary
nebula (PN) NGC 7662. Using (3D) Integral Field Spectroscopy we have
been able to measure the electron temperature more accurately and at a
larger number of radial locations than before. Here we briefly present
our method by which we find a strong positive temperature gradient
with increasing radius. According to hydrodynamic models a hot halo,
when compared to the central star, can be the product of the passage
of an ionization front (e.g. Marten 1993). Such a gradient is not
found in equilibrium models, and this finding - when confirmed for
other objects - strongly advocates the use of hydrodynamic models when
modeling PN halos.

---------------------------------------------------------
Title: Crowded Field 3D Spectrophotometry of Extragalactic Planetary
Nebulae
Authors: Roth, Martin M.; Becker, Thomas; Böhm, Petra; Schönberner,
Detlef; Steffen, Matthias; Exter, Katrina
2006pnbm.conf..239R Altcode:
No abstract at ADS

---------------------------------------------------------
Title: On the luminosity function of planetary nebulae
Authors: Schönberner, D.; Jacob, R.; Steffen, M.
2006IAUS..234..505S Altcode:
A new assessment of the physics of the luminosity function of planetary
nebulae is presented, based on our grid of nebular evolutionary
sequences computed with a 1D radiation-hydrodynamics code. The nebular
evolution is followed from the formation stage in the vicinity of the
asymptotic-giant branch across the Hertzsprung-Russell diagram until the
white-dwarf domain is reached, for various central-star models coupled
to different initial envelope configurations. Along each sequence the
relevant line luminosities of the nebulae are computed and analyzed. Our
models predict that reasonably dense initial circumstellar envelopes
with central stars of slightly above 0.6 M⊙ will remain
nearly optically thick and are able to provide the observed 5007 Å
cutoff luminosity. We cannot support the claim of Marigo et al. (2004)
according to which only planetary nebulae with central stars of &gt;
0.7 M⊙ are able to provide sufficient 5007 Å line emission
to account for the bright end of the luminosity function.

---------------------------------------------------------
Title: Simulations of Magnetohydrodynamics and CO Formation from
the Convection Zone to the Chromosphere
Authors: Wedemeyer-Böhm, S.; Schaffenberger, W.; Steiner, O.; Steffen,
M.; Freytag, B.; Kamp, I.
2005ESASP.596E..16W Altcode: 2005ccmf.confE..16W
No abstract at ADS

---------------------------------------------------------
Title: On the Reliability of Planetary Nebulae as Extragalactic Probes
Authors: Schönberner, D.; Jacob, R.; Steffen, M.; Roth, M. M.
2005AIPC..804..269S Altcode:
We report on the first 1D radiation-hydrodynamics simulations aimed
at exploring the metallicity dependance of the evolution of planetary
nebulae. Using scaled mean abundances, based on Galactic disk objects,
we found that for lower metallicities the nebulae become more dilute
and prone to departures from thermal equilibrium because of the reduced
wind power of the central star and the reduced cooling efficiency of
the nebular gas. At selected evolutionary phases we `switched off'
the hydrodynamic terms and computed equilibrium models for the fixed
density structure and radiation field. This enabled us to study how
the electron temperature and the emission of important diagnostic
lines is influenced by hydrodynamics.

---------------------------------------------------------
Title: 3D Spectroscopy of Planetary Nebulae - Diagnostic Tools from
the Milky Way to Nearby Galaxies and Beyond
Authors: Roth, Martin M.; Schönberner, Detlef; Steffen, Matthias;
Monreal, Ana
2005AIPC..804...20R Altcode:
Integral field (“3D”) spectroscopy is an emerging observing
technique in the process of becoming a common user tool, with a
variety of powerful instruments becoming available at most major
observatories. The application of this new method has a number of
advantages over conventional techniques for both extended and point
sources. We describe our experience with 3D spectroscopy applied to
extremely low surface brightness regions in haloes of galactic planetary
nebulae, and to extragalactic planetary nebulae in local group galaxies.

---------------------------------------------------------
Title: Searching and Characterizing the Faint Haloes of Planetary
Nebulae: A Study Case for Integral Field Spectroscopy
Authors: Monreal-Ibero, A.; Roth, M. M.; Schönberner, D.; Steffen,
M.; Böhm, P.
2005AIPC..804..155M Altcode:
We present here the first results of a research project devoted to the
characterization of the physical properties of the planetary nebulae
haloes. We succeeded in determining some properties in the halo of
NGC 3242, but we did not detect any halo feature down to the detection
limit in NGC 4361.

---------------------------------------------------------
Title: Modeling X-ray Emission from Planetary Nebulae
Authors: Steffen, M.; Schönberner, D.; Warmuth, A.; Schwope, A.;
Landi, E.; Perinotto, M.; Bucciantini, N.
2005AIPC..804..161S Altcode:
Recent observations using the Chandra X-ray Observatory and XXM Newton
revealed without doubt diffuse X-ray emission from the shock-heated
wind gas in planetary nebula (PN) interiors. Typical properties
of the emitting gas are temperatures of a few 106 K and
electron densities of the order of 100 cm-3. According
to current hydrodynamical models, the shocked gas becomes too
hot (T ~ 107-108 K) and too tenuous (ne ~
1 cm-3) to produce the observed X-ray emission. However,
the hot gas is confined by the rather cool (T ~ 10 000K) nebular gas,
and thermal heat conduction across the interface between the hot
and cool gas becomes important. It changes the contact discontinuity
into a more extended transition layer covering the temperature range
where the observed X-ray emission is thought to arise. To date, only
similarity solutions for the hydrodynamical problem of PN evolution
with heat conduction have been derived (Zhekov &amp; Perinotto
1996). We present first results from new numerical simulations of the
PN evolution including thermal conduction by electrons. We confront
the X-ray luminosities predicted by these models with those derived
from XMM/Chandra observations.

---------------------------------------------------------
Title: The evolution of planetary nebulae. III. Internal kinematics
and expansion parallaxes
Authors: Schönberner, D.; Jacob, R.; Steffen, M.
2005A&A...441..573S Altcode:
A detailed theoretical study of the basic internal kinematics of
planetary nebulae is presented, based on 1D radiation-hydrodynamics
simulations of circumstellar envelopes around central stars of 0.595
and 0.696 M⊙. By means of observable quantities like
radial surface-brightness distributions and emission-line profiles
computed from the models, a comparison with real objects was performed
and revealed a reasonable agreement. This allowed to draw important
conclusions by investigating the kinematics of these models in
detail. Firstly, it is shown that the determination of kinematical
ages, normally considered to be simple if size and expansion rate of
an object are given, can seriously be flawed. Secondly, the expansion
law of a planetary nebula is different from what is assumed for
deriving spatio-kinematical models. Thirdly and most importantly, our
hydrodynamical models help to correctly use existing angular expansion
measurements for distance determinations. The mere combination of the
angular expansion rates with the spectroscopic expansion velocities
leads always to a serious underestimate of the distance, the degree
of which depends on the evolutionary state of the object. The
necessary correction factor varies between 3 and 1.3. Individual
correction factors can be estimated with an accuracy of about 10%
by matching our hydrodynamical models to real objects. As a result,
revised distances for a few objects with reliable angular expansion
rates are presented. But even these corrected distances are not always
satisfying: they still appear to be inconsistent with other distance
determinations and, even more disturbing, with the accepted theory of
post-asymptotic giant branch evolution. As a byproduct of the angular
expansion measurements, the transition times from the vicinity of
the asymptotic giant branch to the planetary-nebula regime could
be estimated. They appear to be shorter than assumed in the present
evolutionary calculations.

---------------------------------------------------------
Title: Integral Field Spectroscopy of Faint Halos of Planetary Nebulae
Authors: Monreal-Ibero, A.; Roth, M. M.; Schönberner, D.; Steffen,
M.; Böhm, P.
2005ApJ...628L.139M Altcode: 2005astro.ph..6300M
We present the first integral field spectroscopy observations of the
two planetary nebulae NGC 3242 and NGC 4361 with the VIMOS instrument
attached to VLT-UT3. By co-adding a large number of spaxels, we
reach an emission-line detection limit of 5×10-18 ergs
cm-2 s-1 arcsec-2. In the case of NGC
3242, we succeed in determining some properties of the halo. The radial
surface brightness profile in [O III] implies increasing mass loss
before the formation of the planetary nebula. Traces of the mysterious
“rings” are clearly visible. We find for the first time an apparent
temperature gradient across a halo: from about 16,000 K close to the
shell/halo transition to 20,000 K at the halo's outer edge. No line
emission is seen in the suspected halo region of NGC 4361 down to the
sensitivity limit.

---------------------------------------------------------
Title: MIPS Infrared Imaging of AGB Dustshells (MIRIAD): tracing
mass-loss histories in the extremely large shells around evolved stars
Authors: Speck, Angela; Elitzur, Moshe; Gehrz, Robert; Herwig, Falk;
Izumiura, Hideyuki; Latter, William; Matsuura, Mikako; Meixner,
Margaret; Steffen, Matthias; Stencel, Robert; Szczerba, Ryszard;
Ueta, Toshiya; Zijlstra, Albert
2005sptz.prop20258S Altcode:
Evolved intermediate mass stars are major contributors to the
interstellar medium. However, the mechanisms by which they do this
are not well understood. The circumstellar shells of evolved stars
(AGB and post-AGB stars) contain the fossil record of their mass
loss, and therefore have the potential to verify many aspects of
stellar evolution. IRAS and ISO data indicate that huge dust shells
exist around many such objects, extending several parsecs from the
central star. Furthermore, these large dust shells show evidence
for mass-loss variations that correlate with evolutionary changes
in the star itself. Previous observations lacked the sensitivity
and spatial resolution to investigate the full extent and detailed
structure of these large dust shells. Using Spitzer/MIPS's unique
sensitivity and mapping capabilities, we propose to produce far-IR
images of the parsec-sized dust shells around four carefully selected
evolved stars in order to determine the distribution of material in
these circumstellar envelopes. These maps will be the deepest yet
(sensitivity 1MJy/sr) and have the most complete spatial coverage to
date. Crucially, mapping in this level of detail will allow us to:
(a) constrain the masses of the progenitor stars; (b) test theories of
stellar evolution and mass-loss mechanisms; (c) determine the effect
of dust chemistry on mass loss (and therefore on stellar evolution);
(d) determine when the aspherical structures so prevalent in planetary
nebulae actually develop and thus constrain the cause.

---------------------------------------------------------
Title: Science with the STELLA robotic observatory
Authors: Weber, M.; Strassmeier, K. G.; Bartus, J.; Korhonen, H.;
Kővári, Zs.; Oláh, K.; Schwope, A.; Staude, A.; Steffen, M.
2005ESASP.560.1025W Altcode: 2005csss...13.1025W
No abstract at ADS

---------------------------------------------------------
Title: The shock-patterned solar chromosphere in the light of ALMA
Authors: Wedemeyer-Böhm, S.; Ludwig, H. -G.; Steffen, M.; Freytag,
B.; Holweger, H.
2005ESASP.560.1035W Altcode: 2005csss...13.1035W; 2005astro.ph..9747W
Recent three-dimensional radiation hydrodynamic simulations by Wedemeyer
et al. (2004) suggest that the solar chromosphere is highly structured
in space and time on scales of only 1000 km and 20-25 sec, resp.. The
resulting pattern consists of a network of hot gas and enclosed cool
regions which are due to the propagation and interaction of shock
fronts. In contrast to many other diagnostics, the radio continuum at
millimeter wavelengths is formed in LTE, and provides a rather direct
measure of the thermal structure. It thus facilitates the comparison
between numerical model and observation. While the involved time
and length scales are not accessible with todays equipment for that
wavelength range, the next generation of instruments, such as the
Atacama Large Millimeter Array (ALMA), will provide a big step towards
the required resolution. Here we present results of radiative transfer
calculations at mm and sub-mm wavelengths with emphasis on spatial
and temporal resolution which are crucial for the ongoing discussion
about the chromospheric temperature structure.

---------------------------------------------------------
Title: 3D simulation of convection and spectral line formation in
A-type stars
Authors: Steffen, M.; Freytag, B.; Ludwig, H. -G.
2005ESASP.560..985S Altcode: 2005astro.ph..9464S; 2005csss...13..985S
We present first realistic numerical simulations of 3D radiative
convection in the surface layers of main sequence A-type stars with Teff
= 8000 K and 8500 K, log g = 4.4 and 4.0, recently performed with the
CO5BOLD radiation hydrodynamics code. The resulting models are used to
investigate the structure of the H+HeI and the HeII convection zones
in comparison with the predictions of local and non-local convection
theories, and to determine the amount of "overshoot" into the stable
layers below the HeII convection zone. The simulations also predict
how the topology of the photospheric granulation pattern changes from
solar to A-type star convection. The influence of the photospheric
temperature fluctuations and velocity fields on the shape of spectral
lines is demonstrated by computing synthetic line profiles and line
bisectors for some representative examples, allowing us to confront
the 3D model results with observations.

---------------------------------------------------------
Title: The evolution of planetary nebulae. II. Circumstellar
environment and expansion properties
Authors: Schönberner, D.; Jacob, R.; Steffen, M.; Perinotto, M.;
Corradi, R. L. M.; Acker, A.
2005A&A...431..963S Altcode:
We investigate and discuss the expansion properties of planetary nebulae
by means of 1D radiation-hydrodynamics models computed for different
initial envelope configurations and central star evolutionary tracks. In
particular, we study how the expansion depends on the initial density
gradient of the circumstellar envelope and show that it is possible
to derive information on the very last mass-loss episodes during the
star's final evolution along and off the asymptotic giant branch. To
facilitate the comparison of the models with real objects, we have also
computed observable quantities like surface brightness and emission-line
profiles. With the help of newly acquired high-resolution emission-line
profiles for a sample of planetary nebulae we show that models with
initial envelopes based on the assumption of a stationary wind outflow
fail to explain the observed expansion speeds of virtually all of the
observed planetary nebulae. Instead it must be assumed that during the
very last phase of evolution along the final asymptotic giant branch
evolution the mass-loss rate increases in strength, resulting in a much
steeper slope of the circumstellar radial density distribution. Under
these conditions, the expansion properties of the nebular gas differ
considerably from the self-similar solutions found for isothermal
conditions. Furthermore, the mass loss must remain at a rather high
level until the stellar remnant begins to evolve quickly towards the
central star regime. Current theoretical computations of dust-driven
mass-loss which are restricted to rather low temperatures cannot be
applied during the star's departure from the asymptotic giant branch. Based on observations obtained at the 3.5 μm NTT and the 1.2 μm CAT
telescope of the European Southern Observatory, La Silla, and at the
2.6 μm NOT telescope operated on the island of La Palma by NOTSA, in
the Spanish Observatorio del Roque de Los Muchachos of the Institutode
Astrofísica de Canarias. Dedicated to Prof. V. Weidemann on the
occasion of his 80th birthday, October 3, 2004.

---------------------------------------------------------
Title: Numerical simulations of convection in A-stars
Authors: Freytag, Bernd; Steffen, Matthias
2004IAUS..224..139F Altcode:
Radiation hydrodynamics simulations have been used to produce numerical
models of the convective surface layers of a number of stars, including
the Sun and other stars on or above the main-sequence, white dwarfs
of type DA, and red supergiants.

---------------------------------------------------------
Title: Ionization and its Structural Impacts on the Evolution of
Planetary Nebulae
Authors: Schönberner, D.; Steffen, M.; Jacob, R.
2004ASPC..313..283S Altcode: 2004apnw.conf..283S
We review our present knowledge about the formation and evolution
of planetary nebulae and evaluate the relative importance of
photoionization and wind interaction. It turns out that heating by
photoionization drives the expansion of a planetary nebula during
its entire life, while wind interaction accelerates and shapes the
inner regions only during the later stages of evolution. We found
observational evidence that the transition from spherical AGB-wind
structures to more aspherical ones must occur when the star begins to
evolve slowly off the AGB.

---------------------------------------------------------
Title: The evolution of planetary nebulae. I. A
radiation-hydrodynamics parameter study
Authors: Perinotto, M.; Schönberner, D.; Steffen, M.; Calonaci, C.
2004A&A...414..993P Altcode:
We follow hydrodynamically the evolution of spherimodel planetary
nebulae subject to different initial conditions and with various
central stars, investigating how combinations of central-star
mass and asymptotic giant branch mass-loss rate determine the
shape and kinematics of a planetary nebula. With this approach we
aim at constituting a framework useful for the interpretation of
the evolutionary status and previous mass-loss history of observed
individual nebulae, making use of their kinematiproperties and surface
brightness characteristics. In particular, the models are compared with
the observed morphologies and kinematics of double shell nebulae. The
dynamical structure of all the models is characterized by a more or
less complicated shock wave pattern set up by ionization and wind
interaction whose combined action results in general in a typical
double-shell structure. We have found that models with simple initial
structures based on a constant AGB mass-loss rate fail to comply with
observed shell morphologies and surface-brightness distributions. A
reasonable agreement with the observations is only found for a model
where the mass-loss rate is strongly increasing towards the end of the
asymptotic giant-branch evolution. Depending on the central star's
evolutionary speed and the density of the cool wind expelled along
the asymptotic giant-branch, planetary nebulae may never get optically
thin. This is primarily the case for the more massive central stars,
and this fact offers a rather natural explanation for the long standing
problem of the very existence of molecular hydrogen in the immediate
vicinity of hot central stars. We also show that distances to planetary
nebulae based on expansion parallaxes are systematically too small by
a significant amount.

---------------------------------------------------------
Title: Numerical simulation of the three-dimensional structure and
dynamics of the non-magnetic solar chromosphere
Authors: Wedemeyer, S.; Freytag, B.; Steffen, M.; Ludwig, H. -G.;
Holweger, H.
2004A&A...414.1121W Altcode: 2003astro.ph.11273W
Three-dimensional numerical simulations with CO5, a
new radiation hydrodynamics code, result in a dynamic, thermally
bifurcated model of the non-magnetic chromosphere of the quiet Sun. The
3D model includes the middle and low chromosphere, the photosphere,
and the top of the convection zone, where acoustic waves are excited
by convective motions. While the waves propagate upwards, they steepen
into shocks, dissipate, and deposit their mechanienergy as heat in
the chromosphere. Our numerical simulations show for the first time
a complex 3D structure of the chromospheric layers, formed by the
interaction of shock waves. Horizontal temperature cross-sections of
the model chromosphere exhibit a network of hot filaments and enclosed
cool regions. The horizontal pattern evolves on short time-scales of
the order of typically 20-25 s, and has spatial scales comparable to
those of the underlying granulation. The resulting thermal bifurcation,
i.e., the co-existence of cold and hot regions, provides temperatures
high enough to produce the observed chromospheric UV emission and -
at the same time - temperatures cold enough to allow the formation
of molecules (e.g., carbon monoxide). Our 3D model corroborates the
finding by \citet{carlsson94} that the chromospheric temperature rise
of semi-empirical models does not necessarily imply an increase in
the average gas temperature but can be explained by the presence of
substantial spatial and temporal temperature inhomogeneities.

---------------------------------------------------------
Title: On the expansion parallaxes of planetary nebulae.
Authors: Schoenberner, D.; Jacob, R.; Steffen, M.
2004ANS...325..105S Altcode: 2004ANS...325a.105S; 2004ANS...325..P37S
No abstract at ADS

---------------------------------------------------------
Title: Spectroscopy of extragalactic planetary nebulae as tracers
of intermediate age and old stellar populations.
Authors: Roth, M. M.; Schoenberner, D.; Steffen, M.; Becker, T.
2004ANS...325...46R Altcode: 2004ANS...325..F09R; 2004ANS...325a..46R
No abstract at ADS

---------------------------------------------------------
Title: Probing the mass-loss history at the tip of the AGB by
planetary nebulae.
Authors: Schoenberner, D.; Jacob, R.; Hildebrandt, G.; Steffen, M.;
Lehmann, H.; Corradi, R.; Acker, A.
2004ANS...325..104S Altcode: 2004ANS...325..P36S; 2004ANS...325a.104S
No abstract at ADS

---------------------------------------------------------
Title: 3D Simulation of the Solar Granulation: A Comparison of two
Different Hydrodynamics Codes
Authors: Steffen, Matthias; Ludwig, Hans-Günter; Freytag, Bernd
2003ANS...324..174S Altcode: 2003ANS...324..P96S
No abstract at ADS

---------------------------------------------------------
Title: Modelling the Chromospheric Background Pattern of the
Non-magnetic Sun
Authors: Wedemeyer, Sven; Freytag, Bernd; Steffen, Matthias; Ludwig,
Hans-Günter; Holweger, Hartmut
2003ANS...324R..66W Altcode: 2003ANS...324..I07W
No abstract at ADS

---------------------------------------------------------
Title: Long Term Modeling of Mass Loss on the Late AGB
Authors: Simis, Yvonne; Schonberner, Detlef; Steffen, Matthias
2003ANS...324...18S Altcode: 2003ANS...324..D03S; 2003ANS...324b..18S
No abstract at ADS

---------------------------------------------------------
Title: Ionized haloes in planetary nebulae: new discoveries,
literature compilation and basic statistical properties
Authors: Corradi, R. L. M.; Schönberner, D.; Steffen, M.; Perinotto,
M.
2003MNRAS.340..417C Altcode:
We present a comprehensive observational study of haloes around
planetary nebulae (PNe). Deep Hα+[NII] and/or [OIII] narrow-band
images have been obtained for 35 PNe, and faint extended haloes have
been newly discovered in the following 10 objects: Cn 1-5, IC 2165,
IC 2553, NGC 2792, NGC 2867, NGC 3918, NGC 5979, NGC 6578, PB 4, and
possibly IC 1747. New deep images have also been obtained of other
known or suspected haloes, including the huge extended emission around
NGC 3242 and Sh 2-200. In addition, the literature was searched, and
together with the new observations an improved data base containing
some 50 PN haloes has been compiled. The halo sample is illustrated
in an image atlas contained in this paper, and the original images are
made available for use by the scientific community at &lt;externallink
type="url"&gt;http://www.ing. iac.es/~rcorradi/HALOES/&lt;/A&gt;. The haloes have been classified following the predictions of modern
radiation-hydrodynamical simulations that describe the formation and
evolution of ionized multiple shells and haloes around PNe. According
to the models, the observed haloes have been divided into the following
groups: (i) circular or slightly elliptical asymptotic giant branch
(AGB) haloes, which contain the signature of the last thermal pulse
on the AGB; (ii) highly asymmetrical AGB haloes; (iii) candidate
recombination haloes, i.e. limb-brightened extended shells that are
expected to be produced by recombination during the late post-AGB
evolution, when the luminosity of the central star drops rapidly by a
significant factor; (iv) uncertain cases which deserve further study
for a reliable classification; (v) non-detections, i.e. PNe in which
no halo is found to a level of &lt;~10-3 the peak surface
brightness of the inner nebulae. We discuss the properties of the
haloes: detection rate, morphology, location of the central stars in
the Hertzsprung-Russell diagram, sizes, surface brightness profiles,
and kinematical ages. Among the most notable results, we find that, as
predicted by models, ionized AGB haloes are a quite common phenomenon
in PNe, having been found in 60 per cent of elliptical PNe for which
adequately deep images exist. Another 10 per cent show possible
recombination haloes. In addition, using the kinematical ages of
the haloes and inner nebulae, we conclude that most of the PNe with
observed AGB haloes have left the AGB far from a thermal pulse, at a
phase when hydrogen burning is the dominant energy source. We find no
significant differences between the AGB haloes of hydrogen-poor and
hydrogen-rich central stars.

---------------------------------------------------------
Title: Formation and Evolution of Planetary Nebulae: A Radiation
Hydrodynamics Study
Authors: Perinotto, M.; Calonaci, C.; Schönberner, D.; Steffen, M.;
Blöcker, T.
2003IAUS..209..157P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: AGB Mass-Loss History and Haloes Around Planetary Nebulae
Authors: Corradi, R. L. M.; Steffen, M.; Schönberner, D.; Perinotto,
M.
2003IAUS..209..455C Altcode:
No abstract at ADS

---------------------------------------------------------
Title: From Red Giants to White Dwarfs -- A Radiation-Hydrodynamics
Simulation of the Planetary Nebula Stage
Authors: Schönberner, D.; Steffen, M.
2003ASIB..105...19S Altcode: 2003whdw.conf...19S; 2003whsw.conf...19S
Based on 1D radiation-hydrodynamics simulations along the upper AGB we
followed the evolution of AGB wind envelopes towards the white dwarf
domain. Taking into account photoionization and wind interaction in
a realistic manner, the hydrodynamics of the post-AGB phase leads,
if coupled to an evolving 0.6 Modot central star model, to
morphologies very similar to what is observed for spherical/elliptical
planetary nebulae.

---------------------------------------------------------
Title: Structure and Evolution of Planetary Nebula Haloes
Authors: Steffen, M.; Schönberner, D.
2003IAUS..209..439S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Acoustic Waves in the Solar Chromosphere - Numerical
Simulations with COBOLD
Authors: Wedemeyer, S.; Freytag, B.; Steffen, M.; Ludwig, H. -G.;
Holweger, H.
2003IAUS..210P..C1W Altcode:
No abstract at ADS

---------------------------------------------------------
Title: 3-D hydrodynamic simulations of the solar chromosphere
Authors: Wedemeyer, S.; Freytag, B.; Steffen, M.; Ludwig, H. -G.;
Holweger, H.
2003AN....324..410W Altcode:
We present first results of three-dimensional numerical simulations
of the non-magnetic solar chromosphere, computed with the radiation
hydrodynamics code CO5BOLD. Acoustic waves which are
excited at the top of the convection zone propagate upwards into the
chromosphere where the waves steepen into shocks. The interaction of
the waves leads to the formation of complex structures which evolve
on short time scales. Consequently, the model chromosphere is highly
dynamical, inhomogeneous, and thermally bifurcated.

---------------------------------------------------------
Title: The Formation and Evolution of Planetary Nebulae (invited
review)
Authors: Schönberner, D.; Steffen, M.
2003IAUS..209..147S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Granulation Abundance Corrections from Hydrodynamical
Convection Simulations
Authors: Steffen, M.; Holweger, H.
2003IAUS..210P.D15S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Spots on the surface of Betelgeuse -- Results from new 3D
stellar convection models
Authors: Freytag, B.; Steffen, M.; Dorch, B.
2002AN....323..213F Altcode:
The observed irregular brightness fluctuations of the well-known red
supergiant Betelgeuse (alpha Ori, M2 Iab) have been attributed by
M. Schwarzschild (1975) to the changing granulation pattern formed
by only a few giant convection cells covering the surface of this
giant star. The surface structure revealed by modern interferometric
methods appears to be generally consistent with the explanation as
large-scale granular intensity fluctuations. The interferometric
data can be modeled equally well by assuming the presence of a few
(up to 3) unresolved hot or cool spots on a limb-darkened disk. In an
effort to improve our theoretical understanding of the Betelgeuse
phenomena, we have applied a new radiation hydrodynamics code
(CO5BOLD) to the problem of global convection in giant
stars. For this purpose, the "local box" setup usually employed for the
simulation of solar-type surface convection cannot be used. Rather, we
have chosen a radically different approach: the whole star is enclosed
in a cube ("star-in-a-box" setup). The properties of the stellar model
are defined by the prescribed gravitational central potential and by
a special inner boundary condition which replaces the unresolved core,
including the source of nuclear energy production. We present current
results obtained from this novel generation of 3D stellar convection
simulations, proceeding from a toy model ("Mini-Sun") towards the
numerically more demanding supergiant regime. We discuss the basic
observational properties of Betelgeuse in the light of our best model
obtained so far (T_eff = 3300 K, log g = -0.4). Finally, we describe a
first attempt to investigate the interaction of the global convective
flows with magnetic fields based on the kinematic approximation.

---------------------------------------------------------
Title: Line formation in convective stellar
atmospheres. I. Granulation corrections for solar photospheric
abundances
Authors: Steffen, M.; Holweger, H.
2002A&A...387..258S Altcode: 2002astro.ph..3127S
In an effort to estimate the largely unknown effects of photospheric
temperature fluctuations on spectroscopic abundance determinations, we
have studied the problem of LTE line formation in the inhomogeneous
solar photosphere based on detailed 2-dimensional radiation
hydrodynamics simulations of the convective surface layers of the
Sun. By means of a strictly differential 1D/2D comparison of the
emergent equivalent widths, we have derived “granulation abundance
corrections” for individual lines, which have to be applied to
standard abundance determinations based on homogeneous 1D model
atmospheres in order to correct for the influence of the photospheric
temperature fluctuations. In general, we find a line strengthening in
the presence of temperature inhomogeneities as a consequence of the
non-linear temperature dependence of the line opacity. The resulting
corrections are negligible for lines with an excitation potential around
Ei=5 eV, regardless of element and ionization stage. Moderate
granulation effects (Deltagran ~ -0.1 dex) are obtained
for weak, high-excitation lines (Ei &gt;~ 10 eV) of C I,
N I, O I as well as Mg II and Si II. The largest corrections are found
for ground state lines (Ei=0 eV) of neutral atoms with an
ionization potential between 6 and 8 eV like Mg I, Ca I, Ti I, Fe I,
amounting to Delta_gran ~ -0.3 dex in the case of Ti I. For many lines
of practical relevance, the magnitude of the abundance correction may
be estimated from interpolation in the tables and graphs provided with
this paper. The application of abundance corrections may often be an
acceptable alternative to a detailed fitting of individual line profiles
based on hydrodynamical simulations. The present study should be helpful
in providing upper bounds for possible errors of spectroscopic abundance
analyses, and for identifying spectral lines which are least sensitive
to the influence of photospheric temperature inhomogeneities.

---------------------------------------------------------
Title: Planetary Nebulae with Double Shells and Haloes: Insights
from Hydrodynamical Simulations
Authors: Schönberner, D.; Steffen, M.
2002RMxAC..12..144S Altcode:
We combined hydrodynamical simulations with stellar evolutionary
calculations based on prescribed mass-loss rates in order to get a
better understanding of how structure and kinematics of circumstellar
matter develop with time during the late AGB and the following post-AGB
and Planetary Nebula (PN) evolution.

---------------------------------------------------------
Title: Habitable zones in extrasolar planetary systems
Authors: Franck, Siegfried; von Bloh, Werner; Bounama, Christine;
Steffen, Matthias; Schönberner, Detlef; Schellnhuber, Hans-Joachim
2002abqc.book...47F Altcode:
Summarizing we can state that the habitable zone (HZ) is the range
of orbital distances from a star, in which a planet can maintain
liquid water and biological productivity on its surface. The HZ can
be calculated with the help of climatological approaches or within
the framework of Earth system science. According to our model, the
HZ for the present solar system extends between about 0.95 AU and
about 1.2 AU and was broader in the past. For extrasolar systems we
can postulate a distinct HZ for young central stars in the mass range
between about 0.4 and 2 solar masses. The next two parts describe model
calculations for the Sun and for other single main sequence stars,
respectively. In the final we give our main conclusions and point out
several areas for future work.

---------------------------------------------------------
Title: Habitable Zones and the Number of Gaia's Sisters
Authors: Franck, S.; von Bloh, W.; Bounama, C.; Steffen, M.;
Schönberner, D.; Schellnhuber, H. -J.
2002ASPC..269..261F Altcode: 2002esip.conf..261F
No abstract at ADS

---------------------------------------------------------
Title: Spectroscopic influence of temperature inhomogeneities
Authors: Cayrel, Roger; Steffen, Matthias
2002HiA....12..423C Altcode:
The effect of temperature inhomogeneities on the formation of two
oxygen lines and one Fe II line has been investigated based on 2-D
radiation hydrodynamics simulations of convection in the atmospheres
of metal-poor stars. It was expected that the IR O I triplet lines
would be enhanced by the temperature inhomogeneities, more than the
forbidden [O I] line or Fe II lines. The actual computations done on
two snapshots under the assumption of LTE have not confirmed this
expectation, instead suggesting that the main difference between
"classical" and "dynamical" atmospheres of metal-poor dwarfs is the
significantly cooler mean temperature of the dynamical upper photosphere
(τRoss&lt;~0.01).

---------------------------------------------------------
Title: The Evolution of AGB Stars Towards Planetary Nebulae
Authors: Schönberner, Detlef; Steffen, Matthias
2001ASSL..265...85S Altcode: 2001pao..conf...85S
We combined hydrodynamical simulations with stellar evolutionary
calculations based on prescribed mass-loss rates in order to get a
better understanding of how structure and kinematics of circumstellar
matter develop with time during the late AGB and the following
post-AGB evolution. We show (i) that the final density profile of
the circumstellar shells produced by AGB mass loss falls off steeper
than ρ ~r-2, (ii) that stages of high mass-loss rates are
periodically interrupted by shorter phases of much lower outflow rates
and speeds due to the luminosity dips associated with thermal pulses,
and (iii) that during the post-AGB evolution ionization destroys the
initial density and velocity distribution set up at the end of the AGB.

---------------------------------------------------------
Title: Evolution of Thin Gas Shells Along the AGB and Beyond
Authors: Steffen, Matthias; Schönberner, Detlef
2001ASSL..265..131S Altcode: 2001pao..conf..131S
Based on numerical simulations with two different computer codes,
we have studied the time-dependent hydrodynamics of circumstellar
gas/dust shells of AGB stars in their final stages of evolution. We
discuss two different mechanisms capable of producing very thin shells
of greatly enhanced gas density in the dusty outflows from these stars
and illustrate their observable signatures. Our model calculations
demonstrate that the thin shells produced on the AGB survive the
transition to the post-AGB phase and can still be detected in the
haloes of some Planetary Nebulae.

---------------------------------------------------------
Title: Acoustic Energy Generated by Convection: 3-D Numerical
Simulations for the Sun
Authors: Wedemeyer, Sven; Freytag, Bernd; Holweger, Hartmut; Ludwig,
Hans-Günter; Steffen, Matthias
2001AGM....18..P01W Altcode:
Dissipation of acoustic waves may be an efficient heating mechanism for
the lower and middle chromosphere of the quiet Sun. The basic idea is
that turbulent motions at the top of the solar convection zone generate
acoustic waves which propagate upwards and dissipate in the lower and
middle chromosphere, transporting energy into the higher layers. But
still the question remains if this amount of energy is sufficient to
explain the observed temperature increase without invoking magnetic
fields. With a new version of the COBOLD radiation hydrodynamics code
we are able to compute 3-D models extending all the way from the upper
convection zone to the middle chromosphere. First 3-D simulations reveal
a complex, inhomogenous and highly dynamical structure of the lower and
middle chromosphere which evolves on rather short timescales. On small
spatial dimensions very cool regions are present next to a "network"
of hotter matter. The code is being developed further to provide a
more detailed analysis and comparison with observations.

---------------------------------------------------------
Title: Limits of photosynthesis in extrasolar planetary systems for
earth-like planets
Authors: Franck, S.; von Bloh, W.; Bounama, C.; Steffen, M.;
Schönberner, D.; Schellnhuber, H. J.
2001AdSpR..28..695F Altcode:
We present a general modeling scheme for investigating the possibility
of photosynthesis-based life on extrasolar planets. The scheme focuses
on the identification of the habitable zone in main-sequence-star
planetary systems with planets of Earth mass and size. Our definition
of habitability is based on the long-term possibility of photosynthetic
biomass production as a function of mean planetary surface temperature
and atmospheric CO 2-content. All the astrophysical,
climatological, biogeochemical, and geodynamic key processes involved
in the generation of photosynthesis-driven life conditions are taken
into account. Implicitly, a co-genetic origin of the central star and
the orbiting planet is assumed. The numerical solution of an advanced
geodynamic model yields realistic look-up diagrams for determining the
limits of photosynthesis in extrasolar planetary systems, assuming
minimum CO 2 levels set by the demand of C 4
photosynthesis.

---------------------------------------------------------
Title: Extrasolar Planetary Habitable Zones and the Number of Gaias
Authors: von Bloh, W.; Franck, S.; Bounama, C.; Steffen, M.;
Schönberner, D.; Schellnhuber, H. -J.
2001AGM....18..P35V Altcode:
We present a general modelling scheme for assessing the suitability for
life on any Earth-like extrasolar planet by calculating the habitable
zone (HZ) in main-sequence-star planetary systems. Our approach is based
on an integrated Earth system analysis that relates the boundaries
of the HZ to the limits of C4-photosynthesis processes. Within this
model, the evolution of the HZ for any main-sequence-star planetary
system can be calculated straightforwardly, and a convenient filter
can be constructed that picks the candidates for photosynthesis-based
life from all the extrasolar planets discovered by novel observational
methods. These results can be used to determine the average number of
planets per planetary system that are within the HZ. With the help
of a segment of the Drake equation, the number of “Gaias” (i.e.,
extrasolar terrestrial planets with a globally acting biosphere) can
be estimated. Our calculation gives about half a million Gaias in the
Milky Way.

---------------------------------------------------------
Title: Numerical Simulation of Stellar Convection: Comparison with
Mixing-length Theory
Authors: Steffen, M.; Jordan, S.
2000eaa..bookE5198S Altcode:
The energy released by nuclear fusion in the STELLAR INTERIOR
is carried to the surface by two different transport mechanisms,
radiation and convection. In the presence of a temperature gradient,
there is always a net radiative energy flux since more photons travel
from hot to cool regions than in the other direction. If the temperature
gradient exceeds a certain t...

---------------------------------------------------------
Title: On the origin of thin detached gas shells around AGB
stars. Insights from time-dependent hydrodynamical simulations
Authors: Steffen, M.; Schönberner, D.
2000A&A...357..180S Altcode:
We have applied two different computer codes to study the time-dependent
hydrodynamics of circumstellar gas/dust shells of AGB stars in their
final stages of evolution. A two-component radiation hydrodynamics
code is designed to model a stellar wind driven by radiation pressure
on dust grains. Combined with detailed stellar evolution calculations,
this approach allows us to simulate the dynamical response of the AGB
wind envelope and the emergent spectral energy distribution to temporal
changes of the stellar luminosity and mass loss rate. A completely
independent one-component, Godunov-type hydrodynamics code, which is
particularly well suited to resolve shock fronts, is used to check
the results obtained with the numerically more diffusive two-component
code. First, we verify that a presumed short episode of high mass loss
translates into a correspondingly narrow, high-density shell moving
through the circumstellar envelope, provided that the mass loss rate,
and hence the outflow velocity, is essentially constant during the
mass ejection. In principle, this scenario remains a viable explanation
for the existence of the very thin molecular shells recently detected
around some carbon-rich AGB stars. Second, we discovered that an
alternative mechanism producing very thin shells of greatly enhanced
gas density can operate in the dusty outflows from AGB stars: the
interaction of a faster inner wind running into a slower outer wind,
sweeping up matter at the interface between both type of winds. Based
on different numerical simulations and on a simple analytical model,
we show that this mechanism easily leads to the formation of very
thin shells without the need to invoke large variations of the mass
loss rate on very short time scales. Finally, we demonstrate that a
typical helium-shell flash induces both a mass loss `eruption' and a
two-wind interaction due to the increased outflow velocity during the
high mass loss phase, leading to the formation of a thin compressed
gas shell. Very likely, this mechanism is responsible for the origin of
the CO shells found around some semiregular, optically visible carbon
stars, the most prominent example being TT Cygni.

---------------------------------------------------------
Title: 2D Numerical Simulation of Stellar Convection
Authors: Steffen, Matthias
2000astro.ph..3074S Altcode:
The dynamics and thermal structure of the surface layers of stars
with outer convection zones can be studied in some detail by means of
numerical simulations of time-dependent compressible convection. In an
effort to investigate the properties of “stellar granulation” as a
function of spectral type, we have carried out elaborate 2-dimensional
radiation hydrodynamics calculations of surface convection for a variety
of stellar parameters. The main features of these simulations are
reviewed, with particular reference to standard mixing length models.

---------------------------------------------------------
Title: A hydrodynamical study of multiple-shell planetaries . I. NGC
2438
Authors: Corradi, R. L. M.; Schönberner, D.; Steffen, M.; Perinotto,
M.
2000A&A...354.1071C Altcode:
We obtained deep imaging and high-resolution spectroscopy of the
planetary nebula <ASTROBJ>NGC 2438</ASTROBJ>. In addition to the
bright inner rim and the faint halo-like shell already known from
previous imagery, our observations also highlight the existence of
an even fainter large halo of nearly spherical shape. Analysis of
the surface-brightness profiles and the line splitting indicates
a clear ionization stratification within the bright rim, with the
flow velocities increasing with distance from the central star,
reaching 37 km s-1. We compared the surface-brightness
profiles and kinematical data of <ASTROBJ>NGC 2438</ASTROBJ> with
state-of-the-art radiation-hydrodynamics models, and obtained a
surprisingly good qualitative agreement. By means of this modelling
we show that the faint, halo-like shell has formed by recombination
from a once well-developed photoionized shell in response to a
fast luminosity drop of the central star. The large, very faint
halo is interpreted as being the relic of the now fully ionized AGB
wind. Also the velocity/ionization stratification and the thickness of
the bright rim are fully consistent with the prediction of the models
for such an evolved planetary nebula. The properties of <ASTROBJ>NGC
2438</ASTROBJ> can be explained by assuming that the planetary nebula
formation started about 45 000 years after a thermal pulse on the AGB,
and that the remnant star continued to burn hydrogen in a shell until
burning has stopped quite recently. Based on observations obtained at
the 3.5m~NTT telescope of the European Southern Observatory

---------------------------------------------------------
Title: On Carbon Star Evolution in the IRAS Two-Color Diagram
Authors: Szczerba, Ryszard; Steffen, Matthias; Volk, Kevin
2000IAUS..177..581S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Effects of Photospheric Temperature Inhomogeneities on Lithium
abundance Determinations (2D) (Invited Paper)
Authors: Cayrel, R.; Steffen, M.
2000IAUS..198..437C Altcode: 2000astro.ph..3075C
Based on detailed 2D radiation hydrodynamics (RHD) simulations, we have
investigated the effects of photospheric temperature inhomogeneities
induced by convection on spectroscopic determinations of the lithium
abundance. Computations have been performed both for the solar case
and for a metal-poor dwarf. NLTE effects are taken into account,
using a five-level atomic model for LiI. Comparisons are presented
with traditional 1D models having the same effective temperature and
gravity. The net result is that, while LTE results differ dramatically
between 1D and 2D models, especially in the metal-poor case, this
does not remain true when NLTE effects are included: 1D/2D differences
in the inferred NLTE Li abundance are always well below 0.1 dex. The
present computations still assume LTE in the continuum. New computations
removing this assumption are planned for the near future.

---------------------------------------------------------
Title: Radiation Hydrodynamics Simulations of the Solar Chromosphere
Authors: Wedemeyer, Sven; Freytag, Bernd; Steffen, Matthias; Holweger,
Hartmut
2000AGM....17..P01W Altcode:
While heating of the solar corona is commonly attributed to
reconnection of magnetic field lines, the mechanism responsible for
heating the chromosphere of the quiet Sun, away from active regions,
is still under debate1,2. The basic question which we will
address in this contribution i s: Can generation of acoustic waves by
turbulent convection in photospheric and subphotospheric layers explain
the chromospheric emission of the quiet Sun? With a new 3D radiation
hydrodynamics code3 we are able to compute models ex tending
from the upper convection zone to the middle chromosphere. The code
can handle shocks with a minimum of numerical dissipation. Therefore
generation and propagation of acoustic waves can be investigated,
permitting the evaluation of wave dissipation in the chromosphere in
a physically consistent manner. We present first results and discuss
the principal problems and future prospects.

---------------------------------------------------------
Title: On the Transition from AGB Stars to Planetaries: The Spherical
Case
Authors: Schönberner, Detlef; Steffen, Matthias
2000ASPC..199...59S Altcode: 2000apn..conf...59S; 1999astro.ph.12357S
We discuss the basic physical model and the relevant processes
responsible for creating and shaping planetary nebulae out of a cool AGB
wind envelope. We show that a hydrodynamical treatment along the upper
AGB leads quite naturally to more realistic starting configurations
for planetaries with density slopes steeper than r-2. Taking
into account photoionization and wind interaction in a realistic manner,
the hydrodynamics of post-AGB wind envelopes leads to density structures
and velocity fields in close resemblance to observations of spherical
or elliptical planetary nebulae.

---------------------------------------------------------
Title: Determination of habitable zones in extrasolar planetary
systems: Where are Gaia's sisters?
Authors: Franck, Siegfried; von Bloh, Werner; Bounama, Christine;
Steffen, Matthias; Schönberner, Detlef; Schellnhuber, Hans-Joachim
2000JGR...105.1651F Altcode:
A general modeling scheme for assessing the suitability for
life of extrasolar planets is presented. The scheme focuses on
the identification of the “habitable zone” in main sequence star
planetary systems accommodating Earth-like components. Our definition of
habitability is based on the long-term possibility of photosynthetic
biomass production under geodynamic conditions. Therefore all the
pertinent astrophysical, climatological, biogeochemical, and geodynamic
processes involved in the generation of photosynthesis-driven life
conditions are taken into account. Implicitly, a cogenetic origin of
the central star and the orbiting planet is assumed. A geostatic model
version is developed and investigated in parallel for demonstration
purposes. The numerical solution of the advanced geodynamic model yields
realistic lookup diagrams for convenient habitability determination. As
an illustration, the MACHO-98-BLG-35 event is scrutinized. It is shown
that this event is definitely not tantamount to the discovery of one
of Gaia's sisters.

---------------------------------------------------------
Title: Carbon- and Oxygen-Rich Stars in the IRAS Two-Color Diagram:
Results from Hydrodynamical Models of AGB Winds
Authors: Steffen, M.; Szczerba, R.; Men'shchikov, A.; Schönberner, D.
2000IAUS..177..579S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: From the Tip of the AGB Towards a Planetary: A Hydrodynamical
Simulation
Authors: Schönberner, D.; Steffen, M.; Stahlberg, J.; Kifonidis,
K.; Blöcker, T.
2000IAUS..177..469S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Solar p-Mode Background: Observations and Hydrodynamical
Models
Authors: Straus, Th.; Steffen, M.; Severino, G.; Freytag, B.
1999ESASP.448..203S Altcode: 1999mfsp.conf..203S; 1999ESPM....9..203S
No abstract at ADS

---------------------------------------------------------
Title: A calibration of the mixing-length for solar-type stars based
on hydrodynamical simulations. I. Methodical aspects and results
for solar metallicity
Authors: Ludwig, Hans-Günter; Freytag, Bernd; Steffen, Matthias
1999A&A...346..111L Altcode: 1998astro.ph.11179L
Based on detailed 2D numerical radiation hydrodynamics (RHD)
calculations of time-dependent compressible convection, we have
studied the dynamics and thermal structure of the convective surface
layers of solar-type stars. The RHD models provide information about
the convective efficiency in the superadiabatic region at the top of
convective envelopes and predict the asymptotic value of the entropy of
the deep, adiabatically stratified layers (Fig. \ref{f:sstarhd}). This
information is translated into an effective mixing-length parameter
\alphaMLT suitable to construct standard stellar
structure models. We validate the approach by a detailed comparison
to helioseismic data. The grid of RHD models for solar metallicity
comprises 58 simulation runs with a helium abundance of Y=0.28 in the
range of effective temperatures 4300pun {K}&lt;=Teff&lt;=
7100pun {K} and gravities 2.54&lt;={log g}&lt;= 4.74. We find a
moderate, nevertheless significant variation of \alphaMLT
between about 1.3 for F-dwarfs and 1.75 for K-subgiants with a
dominant dependence on Teff (Fig. \ref{f:mlp}). In the close
neighbourhood of the Sun we find a plateau where \alphaMLT
remains almost constant. The internal accuracy of the calibration
of \alphaMLT is estimated to be +/- 0.05 with a possible
systematic bias towards lower values. An analogous calibration of
the convection theory of Canuto &amp;\ Mazzitelli (1991, 1992; CMT)
gives a different temperature dependence but a similar variation of
the free parameter (Fig. \ref{f:mlpcm}). For the first time, values
for the gravity-darkening exponent beta are derived independently of
mixing-length theory: beta = 0.07... 0.10. We show that our findings
are consistent with constraints from stellar stability considerations
and provide compact fitting formulae for the calibrations.

---------------------------------------------------------
Title: Long-term evolution of AGB wind envelopes: Insights from
hydrodynamical models
Authors: Steffen, M.; Schönberner, D.; Szczerba, R.
1999IAUS..191..379S Altcode:
Up to now, hydrodynamical models of dust-driven AGB winds do not
generally take into account the `long-term' changes of the stellar
parameters (on stellar evolution time scales of 103 to
105 yrs), although it is well known that the luminosity and
(very likely) the mass loss rate undergo significant variations when
so called `thermal pulses' occur on the upper AGB. In this review we
demonstrate that time-dependent radiation hydrodynamics calculations
are needed to understand the formation, structure, and spectral energy
distribution of detached dust shells detected by IRAS and ISO. Combined
with appropriate models, these observations can reveal part of the
previous mass loss history on the AGB and allow an empirical check of
presently adopted mass loss laws. Based on insights from hydrodynamical
simulations, we discuss the two competing scenarios that have been put
forward to explain the origin of the very thin molecular shells recently
discovered around some carbon stars. We find that the signature of
a short mass loss `eruption' broadens quickly with time due to the
related velocity gradient across the shell. Hence, this scenario is
not considered a likely explanation of detached CO shells. On the
other hand, the alternative mechanism, interaction of winds, is shown
to be capable of producing very thin shells of greatly enhanced gas
density in the dusty outflows from AGB stars by sweeping up matter at
the interface between both type of winds.

---------------------------------------------------------
Title: A Calibration of the Mixing-Length for Solar-Type Stars Based
on Hydrodynamical Models of Stellar Surface Convection
Authors: Freytag, B.; Ludwig, H. -G.; Steffen, M.
1999ASPC..173..225F Altcode: 1999sstt.conf..225F
No abstract at ADS

---------------------------------------------------------
Title: On the origin of thin detached gas shells around AGB stars:
Insights from time-dependent hydrodynamical simulations
Authors: Steffen, Matthias; Schönberner, Detlef
1999AGAb...15..109S Altcode:
We have applied two different computer codes to study the time-dependent
hydrodynamics of circumstellar gas/dust shells of AGB stars in their
final stages of evolution. We verify that a presumed short episode of
high mass loss translates into a correspondingly narrow, high-density
shell moving through the circumstellar envelope at supersonic speed,
provided that the mass loss rate, and hence the outflow velocity, is
essentially constant during the mass loss `eruption'. In principle,
this scenario remains a viable explanation for the existence of the
very thin molecular shells recently detected around some carbon-rich AGB
stars. We discovered that an alternative mechanism producing very thin
shells of greatly enhanced gas density can operate in the dusty outflows
from carbon-rich AGB stars: the interaction of a faster inner wind
running into a slower outer wind, sweeping up matter at the interface
between both type of winds. We show that this mechanism easily leads
to the formation of very thin shells without the need to invoke large
variations of the mass loss rate on very short time scales. Finally,
we demonstrate that a typical He-shell flash produces both a mass loss
`eruption' and a two-wind interaction due to the increased outflow
velocity during the high mass loss phase, leading to the formation
of a thin compressed gas shell. We propose that this mechanism is
responsible for the origin of the observed very thin CO shell around
the optically visible carbon star TT Cygni.

---------------------------------------------------------
Title: Silicon as a cosmic reference element: a reassessment of the
solar SI abundance
Authors: Wedemeyer, Sven; Holweger, Hartmut; Steffen, Matthias
1999AGAb...15..113V Altcode: 1999AGM....15..P53V; 1999AGM....15..P54W
Silicon is an important reference elements for comparing various
types of cosmic matter with the Sun. The most widely used sources
of solar (photospheric) abundances, the compilation by Anders &amp;
Grevesse (1989) and its updates (e.g. Grevesse &amp; Sauval 1998), are
based on standard abundance analyses employing 1D solar models and,
in most cases, assuming LTE. We report NLTE calculations for Si and
a first attempt to determine the effect of horizontal temperature
inhomogeneities associated with convection on the photospheric
abundance of Si. We combine the result with that obtained previously
for O and Fe (Aellig et al. 1999; Schnabel et al. 1999) and compare
the photospheric Si/Fe, Si/O and Si/H abundance ratios with literature
data for meteorites, the corona and solar wind, energetic particles and
galactic B stars and H ii regions. References: Aellig M.R., Holweger H.,
Bochsler P., et al., 1999, Solar Wind Nine, AIP Conf. Proc. Vol. 471,
255 Anders E., Grevesse N., 1989, Geochim. Comochim. Acta 53, 197
Grevesse N., Sauval A.J., 1998, Space Sci. Rev. 85, 161 Schnabel R.,
Kock M., Holweger H., 1999, A&amp;A 342, 610

---------------------------------------------------------
Title: Line Formation in Convective Stellar Atmospheres
Authors: Steffen, Matthias; Holweger, Hartmut
1999AGb....15..107S Altcode: 1999AGM....15..P40S
Convection affects the temperature structure of a stellar atmosphere
in a twofold way: it influences the mean vertical stratification and
introduces horizontal inhomogeneities. This poses several questions: (i)
What is the “right” alpha_MLT to produce the correct mean temperature
stratification in the framework of mixing-length theory ? (ii) What
errors are introduced, in the context of spectroscopic abundance
determinations, by representing the horizontally averaged spectrum
of an inhomogeneous atmosphere by the spectrum of a plane-parallel
mean stratification? (iii) Is there something like a spectroscopically
equivalent mean stratification of an inhomogeneous atmosphere? In order
to address these questions, we have computed synthetic line profiles for
a sample of fictitious spectral lines of different chemical elements,
based on both mixing-length atmospheres and 2D hydrodynamical solar
granulation models. Through a differential comparison of the equivalent
widths computed from the inhomogeneous atmospheres and different 1D
mean models, respectively, we derive correction factors to be applied
to standard 1D spectroscopic abundance determinations in order to
correct for the effects of stellar `granulation'. We find that the
abundance correction depends sensitively on the ionization stage of
the chemical element and on the excitation potential of the spectral
line considered. In some cases the `granulation correction' can amount
to -0.2 dex.

---------------------------------------------------------
Title: Balmer Line Formation in Convective Stellar Atmospheres
Authors: Steffen, M.; Ludwig, H. -G.
1999ASPC..173..217S Altcode: 1999sstt.conf..217S
No abstract at ADS

---------------------------------------------------------
Title: Formation and Evolution of Planetary Nebulae
Authors: Schönberner, D.; Steffen, M.
1999ASPC..188..281S Altcode: 1999oisc.conf..281S
No abstract at ADS

---------------------------------------------------------
Title: Hydrodynamical models and synthetic spectra of circumstellar
dust shells around AGB stars. II. Time-dependent simulations
Authors: Steffen, Matthias; Szczerba, Ryszard; Schoenberner, Detlef
1998A&A...337..149S Altcode:
We have employed time-dependent two-component hydrodynamics/radiative
transfer calculations to investigate the structure, dynamics and
emergent spectral energy distribution of dusty circumstellar shells
around carbon and oxygen stars in the final stages of their AGB
evolution. These internally consistent, physical models describe a
stellar wind driven by radiation pressure on dust grains and subsequent
momentum transfer to the gas component via collisions. Detailed stellar
evolution calculations, with a prescribed mass loss rate that is a
function of the fundamental stellar parameters, have been used as a
time-dependent inner boundary condition for the numerical solution
of the coupled equations of hydrodynamics and frequency-dependent
radiative transfer governing the structure and temporal evolution of
the circumstellar dust/gas shell. The calculations are based on one
particular evolutionary track for an initial stellar mass M_i=3.0
Msun\ and a final mass M_f=0.605 Msun, but for
different assumptions concerning the composition of the dust grains:
amorphous carbon or “astronomical” silicates. Using our hydrodynamics
code to simulate the dynamical response of the circumstellar wind
shell to the evolutionary changes of the stellar parameters, we
find that the large temporal variations of stellar luminosity and
mass loss rate associated with the final thermal pulses near the
end of the AGB evolution lead to characteristic, time-dependent
signatures in the density structure and emergent energy distribution
of the circumstellar dust shell. We present the resulting “loops”
in the IRAS two-color-diagram, which we find to extend to regions
quite remote from the simple color-color relation defined by steady
state models. These time-dependent hydrodynamical models explain the
existence of carbon and oxygen stars with excess emission near lambda
60 and 100 mu m as a natural consequence of the sharp decrease of the
mass loss rate following a thermal pulse, leading to the development
of a detached dust shell. As an illustration, we present a series
of synthetic spectra and corresponding 100 mu m surface brightness
distributions showing the time-evolution of the circumstellar dust
emission during a thermal pulse cycle, both for a carbon-rich and an
oxygen-rich dust shell. We demonstrate that it is unrealistic to assume
a fixed velocity profile which is independent of mass loss rate: to a
first approximation, the gas velocity is a bimodal function of the mass
loss rate. A short event of high mass loss does not simply translate
into a correspondingly narrow, high-density shell moving through
the circumstellar envelope. Rather, the signature of a short mass
loss peak broadens due to velocity gradients as it moves towards the
outer regions of the wind. Hence, this is hardly a viable scenario to
explain the existence of very thin molecular shells that have recently
been detected around some carbon stars. Our simulations suggest a more
promising mechanism producing thin shells of enhanced gas density in
the outer regions of carbon-rich AGB shells: interaction of winds of
different speed and density.

---------------------------------------------------------
Title: Lithium Depletion in the Sun: A Study of Mixing Based on
Hydrodynamical Simulations
Authors: Blöcker, T.; Holweger, H.; Freytag, B.; Herwig, F.; Ludwig,
H. -G.; Steffen, M.
1998SSRv...85..105B Altcode: 1998astro.ph..6310B
Based on radiation hydrodynamics modeling of stellar convection zones,
a diffusion scheme has been devised describing the downward penetration
of convective motions beyond the Schwarzschild boundary (overshoot)
into the radiative interior. This scheme of exponential diffusive
overshoot has already been successfully applied to AGB stars. Here
we present an application to the Sun in order to determine the time
scale and depth extent of this additional mixing, i.e. diffusive
overshoot at the base of the convective envelope. We calculated the
associated destruction of lithium during the evolution towards and on
the main-sequence. We found that the slow-mixing processes induced by
the diffusive overshoot may lead to a substantial depletion of lithium
during the Sun's main-sequence evolution.

---------------------------------------------------------
Title: IR-Colors for Models of Post-AGB Evolution
Authors: Szczerba, R.; Steffen, M.
1998Ap&SS.262..187S Altcode: 1999Ap&SS.262..187S
No abstract at ADS

---------------------------------------------------------
Title: Hydrodynamical Modelling of the Evolution of Dusty Outflows
from AGB-Stars
Authors: Schönberner, D.; Steffen, M.; Szczerba, R.
1998Ap&SS.255..459S Altcode: 1997Ap&SS.255..459S
No abstract at ADS

---------------------------------------------------------
Title: An improved calibration of the mixing-length based on
simulations of solar-type convection
Authors: Ludwig, H. -G.; Freytag, B.; Steffen, M.
1998IAUS..185..115L Altcode:
Based on detailed 2D numerical radiation hydrodynamics (RHD)
calculations of time-dependent compressible convection, we have studied
the dynamics and thermal structure of the convective surface layers
of stars in the range of effective temperatures and gravities between
4500 pun{K} &lt;= Teff &lt;= 7100 pun{K} and 2.54 &lt;= logg &lt;=
4.74. Although our hydrodynamical models describe only the shallow,
strongly superadiabatic layers at the top of the convective stellar
envelope, we demonstrate that they provide information about the value
of the entropy of the deeper, adiabatically stratified regions. This
quantity can be translated into an effective mixing-length parameter
suitable for constructing standard stellar structure models. We
show that a hydrodynamically calibrated envelope model for the Sun
closely matches the known adiabat and corresponding depth of the solar
convection zone. We determined the dependence of the mixing-length
parameter on Teff, log g, and chemical composition obtaining a moderate
variation over the range studied. We note that the recent description
of convection by Canuto &amp; Mazzitelli extended by including a
variable amount of overshoot does not lead to a smaller variation of
the controlling parameter. We discuss the consistency of our results
with findings derived in the context of the tentative detection of
solar-like oscillations in eta Bootis.

---------------------------------------------------------
Title: A calibration of mixing length theory based on RHD simulations
of solar-type convection
Authors: Ludwig, H. -G.; Freytag, B.; Steffen, M.
1997ASSL..225...59L Altcode: 1997scor.proc...59L
Radiation hydrodynamics (RHD) models provide detailed information
about the dynamics, thermal structure, and convective efficiency
of the superadiabatic region at the top of solar-type convection
zones, and allow an extrapolation of the entropy (s*) in their deep,
adiabatic layers. For the Sun we find a close agreement between s*
inferred from our RHD models and an empirical determination of s*
from helioseismology. In the framework of mixing length theory (MLT),
s* is translated to an effective mixing-length parameter (alpha c)
appropriate to construct global stellar models. The calibration based
on our present set of 2D RHD models shows a moderate variation of
alpha c across the domain of the HRD investigated so far.

---------------------------------------------------------
Title: Hydrodynamical models and synthetic spectra of circumstellar
dust shells around AGB stars
Authors: Steffen, M.; Szczerba, R.; Men'shchikov, A.; Schoenberner, D.
1997A&AS..126...39S Altcode:
We present a sample of hydrodynamical steady state models of
circumstellar gas/dust shells around late type giants together with
computed spectral energy distributions (SEDs). In these models,
the stellar wind is driven by radiation pressure on dust grains and
subsequent momentum transfer to the gas molecules via collisions. Given
the fundamental stellar parameters (M_*, L_*, T_eff), the mass loss rate
(M_dot), and the dust properties, a self-consistent physical model of
the circumstellar gas/dust shell is obtained from the numerical solution
of the coupled equations of hydrodynamics and radiative transfer. The
computed outflow velocities and infrared fluxes of the circumstellar
envelopes can be compared directly with the observed properties of
stars on asymptotic giant branch. Plotting the positions of our steady
state models in different IRAS two-color-diagrams, we confirm that, for
fixed dust properties, all models fall on a simple color-color relation
with M_dot (or optical depth) as the only parameter. Surprisingly, we
find a good agreement between the synthetic spectra resulting from the
self-consistent hydrodynamical approach and those obtained from much
simpler models based on a constant outflow velocity and ignoring drift
of dust relative to the gas. Our models are compared with the results
of similar calculations by \cite[Netzer &amp; Elitzur (1993).]{Net93}
We find significant differences which are probably the result of some
unrealistic approximations in the treatment of radiative transfer
underlying the model calculations of Netzer &amp; Elitzur. Moreover,
our results demonstrate that, in general, gas pressure cannot be
neglected for winds with relatively low expansion velocities (u_e
&lt; 30 km/s). For given stellar parameters and dust properties, the
theoretical minimum (maximum) mass loss rate decreases (increases)
significantly when gas pressure is taken into account.

---------------------------------------------------------
Title: Numerical simulations of stellar surface convection.
Authors: Freytag, B.; Steffen, M.
1997AGAb...13..176F Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Modeling the Long-Term Evolution of Dusty Outflow Outflows
of AGB-Stars
Authors: Steffen, M.; Szczerba, R.
1997Ap&SS.251..131S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: IR-Colors for Models of Post-AGB Evolution
Authors: Szczerba, R.; Steffen, M.
1997Ap&SS.251..149S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: VizieR Online Data Catalog: Models of circumstellar dust shells
(Steffen+ 1997)
Authors: Steffen, M.; Szczerba, R.; Men'shchikov, A.; Schoenberner, D.
1997yCat..41260039S Altcode:
Main properties of the steady state hydrodynamical models of
circumstellar gas/dust shells around late type giants, computed for
different stellar parameters (mass, luminosity, effective temperature)
and dust composition (astronomical silicates, graphite, or amorphous
carbon, with properties given in Tab.1 and Fig.1). Each table lists
the results as a function of the adopted mass loss rate for fixed
stellar parameters and dust properties. Tables 2 - 11 are related to
Figures 2 - 11: Each of the figures shows the velocity structure and
the resulting spectral energy distribution for a subset of models
listed in the respective table. (2 data files).

---------------------------------------------------------
Title: On the Scale of Photospheric Convection
Authors: Freytag, B.; Holweger, H.; Steffen, M.; Ludwig, H. -G.
1997svlt.work..316F Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Through the upper AGB towards a planetary: a hydrodynamical
simulation.
Authors: Schönberner, D.; Steffen, M.; Stahlberg, J.; Kifonidis,
K.; Blöcker, T.
1997seas.conf..146S Altcode: 1997ase..work..146S
The authors present an exploratory investigation of the dynamical
evolution of a dusty stellar wind envelope along the upper AGB and
its transformation into a planetary nebula. They find the existence
of AGB stars with detached shells to be a natural consequence of the
mass loss variations during a thermal pulse. It is also shown that
due to the large dynamical effects caused by the ionizing radiation
field and the fast wind of the central star, it is impossible to
deduce the AGB mass loss history from the planetary's density and
velocity distribution. The structure of the halo, however, is still
determined by the AGB mass loss history. The rapid decline of mass
loss expected in the aftermath of thermal pulses leads to extended
shells of low densities and explains halos with sharp boundaries. The
density structure and velocity field of the model planetary closely
resemble those of observed planetary nebulae.

---------------------------------------------------------
Title: Time-dependent hydrodynamical models of circumstellar dust
shells around carbon- and oxygen-rich AGB stars.
Authors: Steffen, M.; Szczerba, R.; Men'shchikov, A.; Schönberner, D.
1997seas.conf..154S Altcode: 1997ase..work..154S
The authors have employed time-dependent two component hydrodynamics -
radiative transfer calculations to investigate the structure, dynamics
and emergent spectral energy distribution of dusty circumstellar
shells around carbon and oxygen stars in the final stages of their AGB
evolution. In these models, the wind is driven by radiation pressure on
dust grains and subsequent momentum transfer to the gas molecules by
collisions. Detailed stellar evolution calculations with a prescribed
mass loss rate that varies as a function of the fundamental stellar
parameters provide the time-dependent inner boundary conditions for
the numerical solution of the coupled equations of hydrodynamics and
radiative transfer which describe a consistent physical model of the
evolution of the circumstellar shell. Using the hydrodynamical code
to simulate the dynamical response of the circumstellar wind shell
to the evolutionary changes of the stellar parameters, the authors
find that the large temporal variations of L and M˙ associated with
the final thermal pulses near the end of the AGB evolution lead to
characteristic, time-dependent signatures in the density structure
and in the infrared emission of the circumstellar dust.

---------------------------------------------------------
Title: Through the AGB towards a Planetary: A hydrodynamical
simulation
Authors: Steffen, M.; Schonberner, D.; Kifonidis, K.; Stahlberg, J.
1997IAUS..180..368S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Hydrodynamical models of stellar convection. The role of
overshoot in DA white dwarfs, A-type stars, and the Sun.
Authors: Freytag, B.; Ludwig, H. -G.; Steffen, M.
1996A&A...313..497F Altcode:
Based on two-dimensional numerical radiation hydrodynamics
simulations of time-dependent compressible convection, we have
studied the structure and dynamics of a variety of shallow stellar
surface convection zones. Our present grid of models includes
detailed simulations of surface convection in solar-type stars,
main-sequence A-type stars and cool DA white dwarfs, as well as
numerical experiments to study convection and overshoot at the base of
the solar convection zone. Taking into account a realistic equation of
state (including the effects of ionization) and adopting an elaborate
treatment of non-local radiative transfer (with appropriate grey
or frequency-dependent opacities), our simulations are designed to
represent specific stellar objects characterized by T_eff_, logg, and
chemical composition. Contrary to solar-type stars, the A-type stars and
cool DA white dwarfs investigated here have shallow convection zones
which fit into the computational domain together with thick stable
buffer layers on top and below, thus permitting a study of convective
overshoot under genuine conditions. We find that convective motions
extend well beyond the boundary of the convectively unstable region,
with vertical velocities decaying exponentially with depth in the
deeper parts of the lower overshoot region, as expected for linear
g^-^-modes. Even though convective velocities are reduced by orders of
magnitude, they are still able to counteract molecular diffusion. For
a quantitative description of convective mixing in the far overshoot
layers we have derived a depth dependent diffusion coefficient from
the numerical simulations. In combination with otherwise independent
1D diffusion calculations for a trace element, this allows the
determination of the "effective depth" of the overshoot region. For
a typical main-sequence A-type star (T_eff_=7943#1, logg=4.34) the
mass in the overshoot region exceeds the mass in the unstable region
by approximately a factor 10. The amount of overshoot in cool DA white
dwarfs (around T_eff_=12200#1) is even larger: the convectively mixed
mass is increased by roughly a factor 100.

---------------------------------------------------------
Title: On the detection of shocks in the solar granulation.
Authors: Solanki, S. K.; Rueedi, I.; Bianda, M.; Steffen, M.
1996A&A...308..623S Altcode:
We investigate the spectral signature of a hydrodynamic simulation of
solar granulation and compare it with high resolution observations. The
model gives the correct qualitative trend of increasing line width
with decreasing continuum intensity seen by Nesis et al. (1992)
and interpreted by them as a sign of post-shock turbulence. We find,
however, that the profiles in the dark downflow lanes are broader even
when there is no horizontal transonic flow or shock in the vicinity. We
conclude that the observations of Nesis et al. do not provide any firm
evidence for the presence of granular transonic flows. Nevertheless,
the simulation predicts a promising diagnostic of shocked horizontal
flows. We find that at the locations of the shocks the line profiles
are particularly broad, especially near the solar limb. We present
observations of quiet solar regions that show this specific signature
of shocks and thus support the theoretical prediction of transonic
granular flows.

---------------------------------------------------------
Title: Lyapunov exponents for solar surface convection.
Authors: Steffen, M.; Freytag, B.
1995CSF.....5.1965S Altcode:
The authors have carried out detailed 2D numerical radiation
hydrodynamics calculations, specifically designed to model
time-dependent, compressible convection in the surface layers of the
Sun. These simulations, which take into account a realistic equation
of state and use an elaborate scheme to describe multi-dimensional,
non-local, frequency-dependent radiative transfer, allow a direct
comparison with observed photometric and spectroscopic properties of
solar granulation. Their purpose is to enhance the understanding of
the dynamics and thermal structure of convective stellar atmospheres,
and to investigate the generation of acoustic energy by turbulent
convection. The authors briefly present some of the main properties of
their solar convection models. In particular, the authors demonstrate
the chaotic behaviour of solar surface convection, estimating the
magnitude of the two largest Lyapunov exponents, λ1 and
λ2, by analysing the time evolution of three simulations
with slightly different initial conditions. The authors find that
both λ1 and λ2 are positive and of similar
magnitude as expected for a chaotic system of high dimension. The
corresponding characteristic time scale λ1-1
of approximately 320 s is comparable to the convective turnover time.

---------------------------------------------------------
Title: Synthetic spectra computed from hydrodynamical model
atmospheres of DA white dwarfs.
Authors: Steffen, M.; Ludwig, H. -G.; Freytag, B.
1995A&A...300..473S Altcode:
From detailed 2-dimensional numerical radiation hydrodynamics
calculations of time-dependent compressible convection we have obtained
the thermal structure of the convective surface layers of DA white
dwarfs with effective temperatures near the blue edge of the ZZ Ceti
instability strip. Synthetic line profiles of Hbeta_ and the
red wing of Lalpha_ (including the very temperature sensitive
satellite absorption features) computed from two representative
inhomogeneous hydrodynamical models (T_eff_=12200K, 12600K; log g=8.0)
are compared with the spectra resulting from different plane-parallel
model atmospheres. We find that it is possible to represent a given
inhomogeneous atmosphere by a spectroscopically equivalent 1D model,
constructed to have the same frequency-integrated radiative flux as
the respective 2D hydrodynamical model at all depths. Synthetic spectra
computed from this representative 1D model are almost indistinguishable
from the horizontally averaged 2D synthetic spectra of the corresponding
inhomogeneous model. We conclude that in the investigated range of
effective temperature (probably even for the whole range of convective
DAs), spectroscopic analysis based on appropriate 1D atmospheres is
almost unaffected by systematic errors associated with non-linear flux
variations due to the substantial thermal inhomogeneities generated
by photospheric convection in these stars. This work provides the
basis for a well defined comparison between 2D or 3D hydrodynamical
convection models and 1D standard mixing length models.

---------------------------------------------------------
Title: Hydrodynamical model atmospheres: convection and line formation
in the Sun
Authors: Ludwig, H. -G.; Steffen, M.
1995IAUS..176P.235L Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Mixing-Length Parameter for Solar-Type Convection Zones
Inferred from Hydrodynamical Models of the Surface Layers
Authors: Ludwig, H. -G.; Freytag, B.; Steffen, M.; Wagenhuber, J.
1995LIACo..32..213L Altcode: 1995sews.book..213L
No abstract at ADS

---------------------------------------------------------
Title: Numerical Simulations of Convection and Overshoot in the
Envelope of DA White Dwarfs
Authors: Freytag, Bernd; Steffen, Matthias; Ludwig, Hans-Günter
1995LNP...443...88F Altcode: 1995whdw.conf...88F
We present results of realistic 21) numerical radiation hydrodynamics
calculations, simulating the surface convection zones of DA white
dwarfs in the range of effective temperatures from 14 200 K down
to 11400 K. Comparison with mixing length theory (MLT) yields a
conflicting picture: The dynamics of convection is not governed by
up- and downflowing bubbles which dissolve after travelling some
characteristic distance - but by the formation, advection, merging,
and disruption of fast narrow downdrafts in a slowly upstreaming
surrounding. MLT tremendously underestimates the depth of the region
where material is mixed. Nevertheless, it turns out that a mixing
length model with α = 1.5 gives a good fit of the photospheric
temperature structure (T eff = 12 600 K) and that a 1D
temperature stratification suffices to reproduce the mean spectrum
of the 2D simulations, indicating that the photospheric temperature
inhomogeneities are negligible for spectroscopic analysis. In deeper
layers the temperature stratification of our hydrodynamical models
corresponds to larger values of α. Introducing our envelope models into
nonadiabatic pulsation calculations results in a blue edge of the ZZ
Ceti instability strip near T eff = 12 400 K at log g = 8.0.

---------------------------------------------------------
Title: Spectroscopic Effects of T-Inhomogeneities in the Atmospheres
of DA White Dwarfs
Authors: Ludwig, Hans-Günter; Steffen, Matthias
1995LNP...443..128L Altcode: 1995whdw.conf..128L
No abstract at ADS

---------------------------------------------------------
Title: Numerical simulations of surface convection in solar-type stars
Authors: Freytag, B.; Steffen, M.
1995IAUS..176P.111F Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Numerical simulations of convection at the surface of a ZZ
Ceti white dwarf.
Authors: Ludwig, H. -G.; Jordan, S.; Steffen, M.
1994A&A...284..105L Altcode:
We applied two-dimensional hydrodynamics and non-grey radiative transfer
calculations to the surface layers of a hydrogen-rich white dwarf
(spectral type DA) with T_eff_=12600K and log(g)=8.0, corresponding to
a position in the HR-diagram slightly cooler than the hot boundary of
the ZZ Ceti instability strip. In our simulations the entire convection
zone including the overshoot layers is embedded in the computational box
so that we obtain a complete and detailed model of convection for this
representative object. We address the important question to what extent
models based on mixing length theory (MLT) are able to predict the
physical properties of convection. We find a rapidly (timescale ~100ms)
evolving flow pattern with fast concentrated downdrafts surrounded by
slow broad upflows of warmer material. Convection carries up to 30%
of the total flux and excites internal gravity waves by dynamical
processes associated with the merging of downdrafts. The mean entropy
gradient is reversed with respect to MLT predictions in the deeper
layers of the convection zone. Strong overshoot occurs at its upper
and lower boundary. A synthetic spectrum calculated from the mean
photospheric temperature stratification can be fitted satisfactorily
with a MLT model adopting α=1.5. At greater depth the temperature
profile approaches a model with α=4. The total depth of the convective
layers is rather small compared to values suggested by studies of the
excitation mechanism for the pulsations of DAs.

---------------------------------------------------------
Title: Acoustic flux generation in the solar convection zone
Authors: Steffen, M.
1994smf..conf..294S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Shocks in the solar photosphere and their spectroscopic
signature
Authors: Steffen, M.; Freytag, B.; Holweger, H.
1994smf..conf..298S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Acoustic flux generation in the solar convection zone.
Authors: Steffen, M.
1993AGAb....9..118S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Depth of the Solar Convection Zone Inferred from
Hydrodynamical Models of the Surface Layers
Authors: Steffen, M.
1993ASPC...40..300S Altcode: 1993ist..proc..300S
No abstract at ADS

---------------------------------------------------------
Title: First numerical simulations of convection at the surface of
a ZZ Ceti white dwarf
Authors: Ludwig, H. -G.; Jordan, S.; Steffen, M.
1993ASIC..403..471L Altcode: 1993wdao.conf..471L
No abstract at ADS