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Author name code: fabbian
ADS astronomy entries on 2022-09-14
author:"Fabbian, Damian"
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Title: Do MURaM and STAGGER Simulations of Solar Faculae Match
Observational Signatures from Magnetic Structures?
Authors: Cubas Armas, Melania; Fabbian, Damian
2021ApJ...923..207C Altcode:
We compare results of simulations of solar facular-like conditions
performed using the numerical codes MURaM and STAGGER. Both simulation
sets have a similar setup, including the initial condition of ≍200
G vertical magnetic flux. After interpolating the output physical
quantities to constant optical depth, we compare them and test
them against inversion results from solar observations. From the
snapshots, we compute the monochromatic continuum in the visible
and infrared, and the full Stokes vector of the Fe I spectral line
pair around 6301-6302 Å. We compare the predicted spectral lines
(at the simulation resolution and after smearing to the HINODE SP/SOT
resolution) in terms of their main parameters for the Stokes I line
profiles, and of their area and amplitude asymmetry for the Stokes V
profiles. The codes produce magnetoconvection with similar appearance
and distribution in temperature and velocity. The results also closely
match the values from recent relevant solar observations. Although
the overall distribution of the magnetic field is similar in both
radiation-magnetohydrodynamic (RMHD) simulation sets, a detailed
analysis reveals substantial disagreement in the field orientation,
which we attribute to the differing boundary conditions. The resulting
differences in the synthetic spectra disappear after spatial smearing
to the resolution of the observations. We conclude that the two sets
of simulations provide robust models of solar faculae. Nevertheless,
we also find differences that call for caution when using results from
RMHD simulations to interpret solar observational data.
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Title: The CARMENES search for exoplanets around M dwarfs. Not-so-fine
hyperfine-split vanadium lines in cool star spectra
Authors: Shan, Y.; Reiners, A.; Fabbian, D.; Marfil, E.; Montes,
D.; Tabernero, H. M.; Ribas, I.; Caballero, J. A.; Quirrenbach, A.;
Amado, P. J.; Aceituno, J.; Béjar, V. J. S.; Cortés-Contreras, M.;
Dreizler, S.; Hatzes, A. P.; Henning, Th.; Jeffers, S. V.; Kaminski,
A.; Kürster, M.; Lafarga, M.; Morales, J. C.; Nagel, E.; Pallé, E.;
Passegger, V. M.; Rodriguez-López, C.; Schweitzer, A.; Zechmeister, M.
2021A&A...654A.118S Altcode: 2021arXiv210812442S
Context. M-dwarf spectra are complex and notoriously difficult to model,
posing challenges to understanding their photospheric properties and
compositions in depth. Vanadium (V) is an iron-group element whose
abundance supposedly closely tracks that of iron, but has origins that
are not completely understood. <BR /> Aims: Our aim is to characterize
a series of neutral vanadium atomic absorption lines in the 800-910
nm wavelength region of high signal-to-noise, high-resolution,
telluric-corrected M-dwarf spectra from the CARMENES survey. Many
of these lines are prominent and exhibit a distinctive broad and
flat-bottom shape, which is a result of hyperfine structure (HFS). We
investigate the potential and implications of these HFS split lines
for abundance analysis of cool stars. <BR /> Methods: With standard
spectral synthesis routines, as provided by the spectroscopy software
iSpec and the latest atomic data (including HFS) available from the
VALD3 database, we modeled these striking line profiles. We used them
to measure V abundances of cool dwarfs. <BR /> Results: We determined
V abundances for 135 early M dwarfs (M0.0 V to M3.5 V) in the CARMENES
guaranteed time observations sample. They exhibit a [V/Fe]-[Fe/H]
trend consistent with that derived from nearby FG dwarfs. The tight
(±0.1 dex) correlation between [V/H] and [Fe/H] suggests the potential
application of V as an alternative metallicity indicator in M dwarfs. We
also show hints that neglecting to model HFS could partially explain
the temperature correlation in V abundance measurements observed in
previous studies of samples involving dwarf stars with T<SUB>eff</SUB>
≲ 5300 K. <BR /> Conclusions: Our work suggests that HFS can impact
certain absorption lines in cool photospheres more severely than in
Sun-like ones. Therefore, we advocate that HFS should be carefully
treated in abundance studies in stars cooler than ~5000 K. On the other
hand, strong HFS split lines in high-resolution spectra present an
opportunity for precision chemical analyses of large samples of cool
stars. The V-to-Fe trends exhibited by the local M dwarfs continue
to challenge theoretical models of V production in the Galaxy. <P
/>Full Table A.1 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/A118">http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/654/A118</A>
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Title: VizieR Online Data Catalog: Vanadium measurements for 135 M
dwarfs (Shan+, 2021)
Authors: Shan, Y.; Reiners, A.; Fabbian, D.; Marfil, E.; Montes,
D.; Tabernero, H. M.; Ribas, I.; Caballero, J. A.; Quirrenbach, A.;
Amado, P. J.; Aceituno, J.; Bejar, V. J. S.; Cortes-Contreras, M.;
Dreizler, S.; Hatzes, A. P.; Henning, T.; Jeffers, S. V.; Kaminski,
A.; Kuerster, M.; Lafarga, M.; Morales, J. C.; Nagel, E.; Palle, E.;
Passegger, V. M.; Rodriguez-Lopez, C.; Schweitzer, A.; Zechmeister, M.
2021yCat..36540118S Altcode:
Table A1 presents the measured V abundances ([V/H]) for 135
nearby early-M dwarfs from the CARMENES GTO sample. Two sets
of measurements are given, which are based on two independently
measured sets of fundamental stellar parameters for this sample
(Schweitzer et al., 2019A&A...625A..68S, Cat. J/A+A/625/A68 and
Marfil et al., submitted). Rotational velocities from Reiners et
al. (2018A&A...612A..49R, Cat. J/A+A/612/A49) and disk kinematic
membership designations (Cortes-Contreras+ in prep) are also
included. <P />(1 data file).
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Title: The Lines are Not Fine: Measuring Vanadium Abundances in M
dwarfs from Hyperfine-Split Lines
Authors: Shan, Yutong; Reiners, Ansgar; Fabbian, Damian; Marfil,
Emilio; Montes, David; Tabernero, Hugo M.; Ribas, Ignasi; Caballero,
Jose A.; Quirrenbach, Andreas; Amado, Pedro J.; Aceituno, J.; Bejar,
Victor J. S.; Cortes-Contreras, Miriam; Dreizler, Stefan; Hatzes,
Artie P.; Henning, Thomas; Jeffers, Sandra V.; Kaminski, Adrian;
Kürster, Martin; Lafarga, Marina; Morales, Juan Carlos; Nagel,
Evangelos; Rodriguez-Lopez, Cristina; Passegger, Vera M.; Schweitzer,
Andreas; Zechmeister, Mathias
2021csss.confE.160S Altcode:
Cool star atmospheres present challenges to chemical abundance
studies. To date, only a handful of elements have been quantified for
a handful of M dwarfs. In high-resolution spectra from the CARMENES
survey, we identify a series of dramatically hyperfine-split vanadium
features between 800 and 910 nm, which have strong and clean profiles
throughout the early M-dwarf range. These 'bucket-shaped' line regions
can be well-modeled with standard model atmospheres combined with
the latest atomic data from VALD. From these line regions, we measure
vanadium abundances for 140 nearby early M dwarfs in the CARMENES GTO
sample and confirm that they follow the same trend with metallicity
as the FG-type stars in the solar neighborhood, i.e., significantly
above predictions from galactic chemical evolution models. Exhibiting
a tight correlation with iron, vanadium abundances show promise
as a potential metallicity indicator for M dwarfs. We also present
evidence that several well-known chemical studies of K dwarfs have
systematically overestimated their vanadium abundances largely as a
result of neglecting to model hyperfine structure, a bias that worsens
with decreasing temperature. Our work highlights opportunities for
robust chemical analysis of cool stars afforded by high-quality spectra
redward of visible.
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Title: Accurate Short-Characteristics Radiative Transfer in A
Numerical Tool for Astrophysical RESearch (ANTARES)
Authors: Kostogryz, Nadiia M.; Kupka, Friedrich; Piskunov, Nikolai;
Fabbian, Damian; Krüger, Daniel; Gizon, Laurent
2021SoPh..296...46K Altcode:
We aim to improve the accuracy of radiative energy transport in
three-dimensional radiation hydrodynamical simulations in ANTARES
(A Numerical Tool for Astrophysical RESearch). We implement in the
ANTARES short-characteristics numerical schemes a modification of
the Bézier interpolant solver. This method yields a smoother surface
structure in simulations of solar convection and reduces the artifacts
appearing due to the limited number of rays along which the integration
is done. Reducing such artifacts leads to increased stability of the
code. We show that our new implementation achieves a better agreement
of the temperature structure and its gradient with a semi-empirical
model derived from observations, as well as of synthetic spectral-line
profiles with the observed solar spectrum.
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Title: The ANTARES code: recent developments and applications
Authors: Kupka, Friedrich; Zaussinger, Florian; Fabbian, Damian;
Krüger, Daniel
2020JPhCS1623a2016K Altcode:
ANTARES (A Numerical Tool for Astrophysical RESearch) is a
multi-purpose numerical tool to solve different variants of the
equations of hydrodynamics as they appear in problems of astrophysics,
geophysics, and engineering sciences and which require the construction
of detailed numerical simulation models. A presentation of the current
feature set of the code with a focus on recent add-ons is given here
in addition to a summary on several results from recent applications of
ANTARES to solar physics, the physics of planets, and basic convection
studies including the damping of pressure modes (solar oscillations)
in numerical simulations of convection at the solar surface and the
coupling of layers in numerical simulations of sheared and non-sheared
double-diffusive convection.
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Title: Comparing Radiative Transfer Codes and Opacity Samplings for
Solar Irradiance Reconstructions
Authors: Criscuoli, Serena; Rempel, Matthias; Haberreiter, Margit;
Pereira, Tiago M. D.; Uitenbroek, Han; Fabbian, Damian
2020SoPh..295...50C Altcode:
Some techniques developed to reproduce solar irradiance variations make
use of synthetic radiative fluxes of quiet and magnetic features. The
synthesis of radiative fluxes of astronomical objects is likely
to be affected by uncertainties resulting from approximations and
specific input employed for the synthesis. In this work we compare
spectra obtained with three radiative transfer codes with the
purpose of investigating differences in reproducing solar irradiance
variations. Specifically, we compare spectral synthesis produced in
non-local thermodynamic equilibrium obtained with COSI and RH using
1-D atmosphere models. We also compare local thermodynamic equilibrium
syntheses emerging from 3-D MURaM simulations of the solar atmosphere
obtained with two sets of opacity tables generated with the ATLAS9
package and with the RH code, and test the effects of opacity sampling
on the emergent spectra. We find that, although the different codes
and methodologies employed to synthesize the spectrum reproduce overall
the observed solar spectrum with a similar degree of accuracy, subtle
differences in quiet Sun spectra may translate into larger differences
in the computation of the contrasts of magnetic features, which,
in turn, critically affect the estimates of solar variability.
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Title: On long-duration 3D simulations of stellar convection using
ANTARES
Authors: Kupka, F.; Fabbian, D.; Krüger, D.; Kostogryz, N.; Gizon, L.
2020IAUGA..30..373K Altcode:
We present initial results from three-dimensional (3-D) radiation
hydrodynamical simulations for the Sun and targeted Sun-like stars. We
plan to extend these simulations up to several stellar days to study
p-mode excitation and damping processes. The level of variation of
irradiance on the time scales spanned by our 3-D simulations will
be studied too. Here we show results from a first analysis of the
computational data we produced so far.
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Title: Shine BRITE: shedding light on stellar variability through
advanced models
Authors: Fabbian, D.; Kupka, F.; Krüger, D.; Kostogryz, N. M.;
Piskunov, N.
2020svos.conf..155F Altcode: 2020arXiv200201560F
The correct interpretation of the large amount of complex data from
next-generation (in particular, space-based) observational facilities
requires a very strong theoretical underpinning. One can predict
that, in the near future, the use of atmospheric models obtained with
three-dimensional (3-D) radiation magneto-hydrodynamics (RMHD) codes,
coupled with advanced radiative transfer treatment including non-local
thermodynamic equilibrium (non-LTE) effects and polarisation, will
become the norm. In particular, stellar brightness variability in cool
stars (i.e., spectral types F-- M) can be caused by several different
effects besides pulsation. In this review we have briefly discussed
some published results, and mentioned aspects of recent progress. It
then attempted to peek into what the future may hold for understanding
this important aspect of the lives of stars.
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Title: On the Challenges of synthetizing solar and stellar spectra
for Irradiance reconstructions
Authors: Criscuoli, Serena; Rempel, Matthias D.; Haberreiter, Margit;
Pereira, Tiago; Uitenbroek, Han; Fabbian, Damian
2019AAS...23421702C Altcode:
Syntheses of solar and stellar spectra strongly depend on the adopted
approximations and atomic and molecular databases. We compare LTE and
NLTE syntheses of solar spectra obtained with widely used radiative
transfer codes, utilizing both 3D-MHD simulations and 1D-static
atmosphere models. We show that although different codes reproduce
reasonably well the observed spectrum, subtle differences may translate
into discrepancies of several tens of percents in the estimate of
solar and stellar spectral irradiance variability.
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Title: Comparison of Parameters from Three-Dimensional
Magnetoconvection Simulations of the Solar Photosphere
Authors: Cubas Armas, M.; Fabbian, D.; Vitas, N.
2019ASPC..526..195C Altcode:
We present preliminary results from a comparison of parameters
derived from three-dimensional magnetohydrodynamic simulations of a
plage obtained with the MURaM and STAGGER codes. We first compared the
computed atmospheric physical parameters stored in the output temporal
snapshots. Subsequently, we used the same snapshots as input to perform
spectral synthesis calculations using the NICOLE code. We compared
the synthetic Fe I 630.1 nm and 630.2 nm spectral lines in terms
of central intensity, equivalent width, full width at half maximum,
as well as area and amplitude asymmetry of the Stokes V profile.
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Title: Magnetic Flux Density in 3D MHD Simulations and Observations
Authors: Beck, C.; Fabbian, D.; Rezaei, R.; Puschmann, K. G.
2019ASPC..526..191B Altcode:
We compare the polarization signals induced in three-dimensional (3D)
magneto-hydrodynamical (MHD) simulations by the Zeeman effect in the
presence of photospheric magnetic fields to those in observations
at disc centre. We consider quantities determined from Stokes vector
profiles of observations of photospheric spectral lines in the visible
and near-infrared, and in corresponding synthetic spectra obtained
from numerical 3D MHD simulations with an average magnetic flux
density of 20-200 G. We match the spatial resolution of observations
by degrading the spectra of the simulations. We find that the total
unsigned vertical magnetic flux density in the simulation should
be less than 50 G to reproduce the observed polarization signals in
the quiet Sun internetwork. A value of ∼30 G best agrees with all
observations we employed.
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Title: The variability of magnetic activity in solar-type stars
Authors: Fabbian, D.; Simoniello, R.; Collet, R.; Criscuoli, S.;
Korhonen, H.; Krivova, N. A.; Oláh, K.; Jouve, L.; Solanki, S. K.;
Alvarado-Gómez, J. D.; Booth, R.; García, R. A.; Lehtinen, J.;
See, V.
2017AN....338..753F Altcode:
No abstract at ADS
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Title: The Polarization Signature of Photospheric Magnetic Fields
in 3D MHD Simulations and Observations at Disk Center
Authors: Beck, C.; Fabbian, D.; Rezaei, R.; Puschmann, K. G.
2017ApJ...842...37B Altcode: 2017arXiv170506812B
Before using three-dimensional (3D) magnetohydrodynamical (MHD)
simulations of the solar photosphere in the determination of elemental
abundances, one has to ensure that the correct amount of magnetic
flux is present in the simulations. The presence of magnetic flux
modifies the thermal structure of the solar photosphere, which affects
abundance determinations and the solar spectral irradiance. The amount
of magnetic flux in the solar photosphere also constrains any possible
heating in the outer solar atmosphere through magnetic reconnection. We
compare the polarization signals in disk-center observations of the
solar photosphere in quiet-Sun regions with those in Stokes spectra
computed on the basis of 3D MHD simulations having average magnetic
flux densities of about 20, 56, 112, and 224 G. This approach allows
us to find the simulation run that best matches the observations. The
observations were taken with the Hinode SpectroPolarimeter (SP),
the Tenerife Infrared Polarimeter (TIP), the Polarimetric Littrow
Spectrograph (POLIS), and the GREGOR Fabry-Pèrot Interferometer
(GFPI), respectively. We determine characteristic quantities of full
Stokes profiles in a few photospheric spectral lines in the visible
(630 nm) and near-infrared (1083 and 1565 nm). We find that the
appearance of abnormal granulation in intensity maps of degraded
simulations can be traced back to an initially regular granulation
pattern with numerous bright points in the intergranular lanes
before the spatial degradation. The linear polarization signals in
the simulations are almost exclusively related to canopies of strong
magnetic flux concentrations and not to transient events of magnetic
flux emergence. We find that the average vertical magnetic flux density
in the simulation should be less than 50 G to reproduce the observed
polarization signals in the quiet-Sun internetwork. A value of about 35
G gives the best match across the SP, TIP, POLIS, and GFPI observations.
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Title: The missing 'M' ingredient in 3D photospheric simulations
for solar abundances
Authors: Fabbian, Damian
2017psio.confE..28F Altcode:
No abstract at ADS
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Title: Lower solar atmosphere and magnetism at ultra-high spatial
resolution
Authors: Collet, Remo; Criscuoli, Serena; Ermolli, Ilaria; Fabbian,
Damian; Guerreiro, Nuno; Haberreiter, Margit; Peck, Courtney; Pereira,
Tiago M. D.; Rempel, Matthias; Solanki, Sami K.; Wedemeyer-Boehm, Sven
2016arXiv161202348C Altcode:
We present the scientific case for a future space-based telescope
aimed at very high spatial and temporal resolution imaging of the
solar photosphere and chromosphere. Previous missions (e.g., HINODE,
SUNRISE) have demonstrated the power of observing the solar photosphere
and chromosphere at high spatial resolution without contamination from
Earth's atmosphere. We argue here that increased spatial resolution
(from currently 70 km to 25 km in the future) and high temporal cadence
of the observations will vastly improve our understanding of the
physical processes controlling solar magnetism and its characteristic
scales. This is particularly important as the Sun's magnetic field
drives solar activity and can significantly influence the Sun-Earth
system. At the same time a better knowledge of solar magnetism can
greatly improve our understanding of other astrophysical objects.
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Title: Spectroscopy at the Solar Limb: II. Are Spicules Heated to
Coronal Temperatures?
Authors: Beck, C.; Rezaei, R.; Puschmann, K. G.; Fabbian, D.
2016SoPh..291.2281B Altcode: 2016arXiv160606132B; 2016SoPh..tmp..132B
Spicules of the so-called type II were suggested to be relevant for
coronal heating because of their ubiquity on the solar surface and
their eventual extension into the corona. We investigate whether solar
spicules are heated to transition-region or coronal temperatures and
reach coronal heights (≫6 Mm) using multiwavelength observations
of limb spicules in different chromospheric spectral lines (Ca II H,
Hε , Hα , Ca II IR at 854.2 nm, He I at 1083 nm) taken with slit
spectrographs and imaging spectrometers. We determine the line width
of spectrally resolved line profiles in individual spicules and
throughout the field of view, and estimate the maximal height that
different types of off-limb features reach. We derive estimates of
the kinetic temperature and the non-thermal velocity from the line
width of spectral lines from different chemical elements. We find that
most regular, i.e. thin and elongated, spicules reach a height of at
most about 6 Mm above the solar limb. The majority of features found
at larger heights are irregularly shaped with a significantly larger
lateral extension, of up to a few Mm, than spicules. Both individual and
average line profiles in all spectral lines show a decrease in their
line width with height above the limb with very few exceptions. The
kinetic temperature and the non-thermal velocity decrease with height
above the limb. We find no indications that the spicules in our data
reach coronal heights or transition-region or coronal temperatures.
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Title: How different are the Liège and Hamburg atlases of the
solar spectrum?
Authors: Doerr, H. -P.; Vitas, N.; Fabbian, D.
2016A&A...590A.118D Altcode: 2016arXiv160403748D
Context. The high-fidelity solar spectral atlas prepared by
<A HREF="http://adsabs.harvard.edu/abs/1973apds.book.....D">
http://adsabs.harvard.edu/abs/1973apds.book.....D</A>
Delbouille et al. (Liège atlas, 1973) and the atlas by
<A HREF="http://adsabs.harvard.edu/abs/1999SoPh..184..421N">
http://adsabs.harvard.edu/abs/1999SoPh..184..421N</A> Neckel (Hamburg
atlas, 1999, Sol. Phys., 184, 421) are widely recognised as the most
important collection of reference spectra of the Sun at disc centre in
the visible wavelength range. The two datasets serve as fundamental
resources for many researchers, in particular for chemical abundance
analyses. But despite their similar published specifications (spectral
resolution and noise level), the shapes of the spectral lines in the
two atlases differ significantly and systematically. <BR /> Aims:
Knowledge of any instrumental degradations is imperative to fully
exploit the information content of spectroscopic data. We seek to
investigate the magnitude of these differences and explain the possible
sources. We provide the wavelength-dependent correction parameters that
need to be taken into account when the spectra are to be compared with
synthetic data, for instance. <BR /> Methods: A parametrically degraded
version of the Hamburg spectrum was fitted to the Liège spectrum. The
parameters of the model (wavelength shift, broadening, intensity
scaling, and intensity offset) represent the different characteristics
of the respective instruments, observational strategies, and data
processing. <BR /> Results: The wavelength scales of the Liège and
Hamburg atlases differ on average by 0.5 mÅ with a standard deviation
of ± 2 mÅ, except for a peculiar region around 5500 Å. The continuum
levels are offset by up to 18% below 5000 Å, but remain stably at a
0.8% difference towards the red. We find no evidence for spectral stray
light in the Liège spectrum. Its resolving power is almost independent
of wavelength but limited to about 216 000, which is between two to
six times lower than specified. When accounting for the degradations
determined in this work, the spectra of the two atlases agree to within
a few parts in 10<SUP>3</SUP>. <P />The fit parameters displayed in
Fig. 2 and derived data 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://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/590/A118">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/590/A118</A>
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Title: VizieR Online Data Catalog: Differences of atlases of solar
spectrum (Doerr+, 2016)
Authors: Doerr, H. -P.; Vitas, N.; Fabbian, D.
2016yCat..35900118D Altcode:
A parametrically degraded version of the Hamburg spectrum was fitted
to the Liege spectrum. The parameters of the model (wavelength shift,
broadening, intensity scaling, intensity offset) represent the different
characteristics of the respective instruments, observational strategies,
and data processing. The fits were carried out for all identified solar
lines in the line list provided by Pierce and Breckinridge (1973, The
Kitt-Peak Table of Solar Spectrum Wavelengths, Vol. Contribution No. 559
(Kitt Peak National Observatory); hereafter PB73) for an interval of
±15pm around the line cores. From an initial testrun we found that
the offset parameter (stray-light) is compatible with zero. The final
fits were carried out with only three free parameters. The results
from that run are provided in the file 'fitres.dat'. <P />A second
file (liegepar.dat) contains derived parameters (spectral resolving
power, position of the continuum) for the Liege spectrum that can be
used in studies that compare Liege data to other data (e.g. synthetic
spectra). <P />For instance, synthetic spectra have to be degraded to
match the spectral resolving power of the Liege spectrum for a valid
comparison. Our analysis showed that a Gaussian convolution kernel with
a full-width at half-maximum (FWHM) of lambda/R (lambda: wavelength; R:
spectral resolving power) is a very good approximation to the actual
instrumental profile of the Liege atlas. <P />Likewise, the continuum
of the Liege atlas at a particular wavelength can be matched to the
continuum of the Hamburg atlas by division with the parameter 'C'
from liegepar.dat. The correction parameters in liegepar.dat should
be interpolated to the desired wavelength range before being applied
in any data analysis. <P />We want to stress that the parameters
provided here result from the fitting-procedure as described in the
paper, with no further consistency checks or corrections applied. Some
fits are affected by nearby telluric blends. We recommend to apply an
outlier-rejection (e.g. a median filter) and/or smoothing before using
the data. The results need to be carefully checked. <P />(2 data files).
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Title: Constraining Solar/Stellar Activity and Magnetically-Driven
Variability
Authors: Fabbian, Damian; Simoniello, Rosaria
2016IBVS.6161....1F Altcode:
No abstract at ADS
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Title: Continuum Intensity and [O I] Spectral Line Profiles in Solar
3D Photospheric Models: The Effect of Magnetic Fields
Authors: Fabbian, D.; Moreno-Insertis, F.
2015ApJ...802...96F Altcode: 2015arXiv150106916F
The importance of magnetic fields in three-dimensional (3D)
magnetoconvection models of the Sun’s photosphere is investigated
in terms of their influence on the continuum intensity at different
viewing inclination angles and on the intensity profile of two [O i]
spectral lines. We use the RH numerical radiative transfer code to
perform a posteriori spectral synthesis on the same time series of
magnetoconvection models used in our publications on the effect of
magnetic fields on abundance determination. We obtain a good match of
the synthetic disk-center continuum intensity to the absolute continuum
values from the Fourier Transform Spectrometer (FTS) observational
spectrum; the match of the center-to-limb variation synthetic data
to observations is also good, thanks, in part, to the 3D radiation
transfer capabilities of the RH code. The different levels of magnetic
flux in the numerical time series do not modify the quality of the
match. Concerning the targeted [O i] spectral lines, we find, instead,
that magnetic fields lead to nonnegligible changes in the synthetic
spectrum, with larger average magnetic flux causing both of the lines
to become noticeably weaker. The photospheric oxygen abundance that
one would derive if instead using nonmagnetic numerical models would
thus be lower by a few to several centidex. The inclusion of magnetic
fields is confirmed to be important for improving the current modeling
of the Sun, here in particular in terms of spectral line formation
and of deriving consistent chemical abundances. These results may shed
further light on the still controversial issue regarding the precise
value of the solar oxygen abundance.
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Title: The Tenth Data Release of the Sloan Digital Sky Survey:
First Spectroscopic Data from the SDSS-III Apache Point Observatory
Galactic Evolution Experiment
Authors: Ahn, Christopher P.; Alexandroff, Rachael; Allende Prieto,
Carlos; Anders, Friedrich; Anderson, Scott F.; Anderton, Timothy;
Andrews, Brett H.; Aubourg, Éric; Bailey, Stephen; Bastien, Fabienne
A.; Bautista, Julian E.; Beers, Timothy C.; Beifiori, Alessandra;
Bender, Chad F.; Berlind, Andreas A.; Beutler, Florian; Bhardwaj,
Vaishali; Bird, Jonathan C.; Bizyaev, Dmitry; Blake, Cullen H.;
Blanton, Michael R.; Blomqvist, Michael; Bochanski, John J.; Bolton,
Adam S.; Borde, Arnaud; Bovy, Jo; Shelden Bradley, Alaina; Brandt,
W. N.; Brauer, Dorothée; Brinkmann, J.; Brownstein, Joel R.; Busca,
Nicolás G.; Carithers, William; Carlberg, Joleen K.; Carnero, Aurelio
R.; Carr, Michael A.; Chiappini, Cristina; Chojnowski, S. Drew; Chuang,
Chia-Hsun; Comparat, Johan; Crepp, Justin R.; Cristiani, Stefano;
Croft, Rupert A. C.; Cuesta, Antonio J.; Cunha, Katia; da Costa, Luiz
N.; Dawson, Kyle S.; De Lee, Nathan; Dean, Janice D. R.; Delubac,
Timothée; Deshpande, Rohit; Dhital, Saurav; Ealet, Anne; Ebelke,
Garrett L.; Edmondson, Edward M.; Eisenstein, Daniel J.; Epstein,
Courtney R.; Escoffier, Stephanie; Esposito, Massimiliano; Evans,
Michael L.; Fabbian, D.; Fan, Xiaohui; Favole, Ginevra; Femenía
Castellá, Bruno; Fernández Alvar, Emma; Feuillet, Diane; Filiz
Ak, Nurten; Finley, Hayley; Fleming, Scott W.; Font-Ribera, Andreu;
Frinchaboy, Peter M.; Galbraith-Frew, J. G.; García-Hernández, D. A.;
García Pérez, Ana E.; Ge, Jian; Génova-Santos, R.; Gillespie, Bruce
A.; Girardi, Léo; González Hernández, Jonay I.; Gott, J. Richard,
III; Gunn, James E.; Guo, Hong; Halverson, Samuel; Harding, Paul;
Harris, David W.; Hasselquist, Sten; Hawley, Suzanne L.; Hayden,
Michael; Hearty, Frederick R.; Herrero Davó, Artemio; Ho, Shirley;
Hogg, David W.; Holtzman, Jon A.; Honscheid, Klaus; Huehnerhoff,
Joseph; Ivans, Inese I.; Jackson, Kelly M.; Jiang, Peng; Johnson,
Jennifer A.; Kinemuchi, K.; Kirkby, David; Klaene, Mark A.; Kneib,
Jean-Paul; Koesterke, Lars; Lan, Ting-Wen; Lang, Dustin; Le Goff,
Jean-Marc; Leauthaud, Alexie; Lee, Khee-Gan; Lee, Young Sun; Long,
Daniel C.; Loomis, Craig P.; Lucatello, Sara; Lupton, Robert H.;
Ma, Bo; Mack, Claude E., III; Mahadevan, Suvrath; Maia, Marcio
A. G.; Majewski, Steven R.; Malanushenko, Elena; Malanushenko,
Viktor; Manchado, A.; Manera, Marc; Maraston, Claudia; Margala,
Daniel; Martell, Sarah L.; Masters, Karen L.; McBride, Cameron K.;
McGreer, Ian D.; McMahon, Richard G.; Ménard, Brice; Mészáros,
Sz.; Miralda-Escudé, Jordi; Miyatake, Hironao; Montero-Dorta,
Antonio D.; Montesano, Francesco; More, Surhud; Morrison, Heather
L.; Muna, Demitri; Munn, Jeffrey A.; Myers, Adam D.; Nguyen, Duy
Cuong; Nichol, Robert C.; Nidever, David L.; Noterdaeme, Pasquier;
Nuza, Sebastián E.; O'Connell, Julia E.; O'Connell, Robert W.;
O'Connell, Ross; Olmstead, Matthew D.; Oravetz, Daniel J.; Owen,
Russell; Padmanabhan, Nikhil; Palanque-Delabrouille, Nathalie; Pan,
Kaike; Parejko, John K.; Parihar, Prachi; Pâris, Isabelle; Pepper,
Joshua; Percival, Will J.; Pérez-Ràfols, Ignasi; Dotto Perottoni,
Hélio; Petitjean, Patrick; Pieri, Matthew M.; Pinsonneault, M. H.;
Prada, Francisco; Price-Whelan, Adrian M.; Raddick, M. Jordan; Rahman,
Mubdi; Rebolo, Rafael; Reid, Beth A.; Richards, Jonathan C.; Riffel,
Rogério; Robin, Annie C.; Rocha-Pinto, H. J.; Rockosi, Constance
M.; Roe, Natalie A.; Ross, Ashley J.; Ross, Nicholas P.; Rossi,
Graziano; Roy, Arpita; Rubiño-Martin, J. A.; Sabiu, Cristiano G.;
Sánchez, Ariel G.; Santiago, Basílio; Sayres, Conor; Schiavon,
Ricardo P.; Schlegel, David J.; Schlesinger, Katharine J.; Schmidt,
Sarah J.; Schneider, Donald P.; Schultheis, Mathias; Sellgren, Kris;
Seo, Hee-Jong; Shen, Yue; Shetrone, Matthew; Shu, Yiping; Simmons,
Audrey E.; Skrutskie, M. F.; Slosar, Anže; Smith, Verne V.; Snedden,
Stephanie A.; Sobeck, Jennifer S.; Sobreira, Flavia; Stassun, Keivan
G.; Steinmetz, Matthias; Strauss, Michael A.; Streblyanska, Alina;
Suzuki, Nao; Swanson, Molly E. C.; Terrien, Ryan C.; Thakar, Aniruddha
R.; Thomas, Daniel; Thompson, Benjamin A.; Tinker, Jeremy L.; Tojeiro,
Rita; Troup, Nicholas W.; Vandenberg, Jan; Vargas Magaña, Mariana;
Viel, Matteo; Vogt, Nicole P.; Wake, David A.; Weaver, Benjamin A.;
Weinberg, David H.; Weiner, Benjamin J.; White, Martin; White, Simon
D. M.; Wilson, John C.; Wisniewski, John P.; Wood-Vasey, W. M.;
Yèche, Christophe; York, Donald G.; Zamora, O.; Zasowski, Gail;
Zehavi, Idit; Zhao, Gong-Bo; Zheng, Zheng; Zhu, Guangtun
2014ApJS..211...17A Altcode: 2013arXiv1307.7735A
The Sloan Digital Sky Survey (SDSS) has been in operation since 2000
April. This paper presents the Tenth Public Data Release (DR10) from
its current incarnation, SDSS-III. This data release includes the first
spectroscopic data from the Apache Point Observatory Galaxy Evolution
Experiment (APOGEE), along with spectroscopic data from the Baryon
Oscillation Spectroscopic Survey (BOSS) taken through 2012 July. The
APOGEE instrument is a near-infrared R ~ 22,500 300 fiber spectrograph
covering 1.514-1.696 μm. The APOGEE survey is studying the chemical
abundances and radial velocities of roughly 100,000 red giant star
candidates in the bulge, bar, disk, and halo of the Milky Way. DR10
includes 178,397 spectra of 57,454 stars, each typically observed three
or more times, from APOGEE. Derived quantities from these spectra
(radial velocities, effective temperatures, surface gravities, and
metallicities) are also included. DR10 also roughly doubles the number
of BOSS spectra over those included in the Ninth Data Release. DR10
includes a total of 1,507,954 BOSS spectra comprising 927,844 galaxy
spectra, 182,009 quasar spectra, and 159,327 stellar spectra selected
over 6373.2 deg<SUP>2</SUP>.
---------------------------------------------------------
Title: APOGEE-2: The Second Phase of the Apache Point Observatory
Galactic Evolution Experiment in SDSS-IV
Authors: Sobeck, Jennifer; Majewski, S.; Hearty, F.; Schiavon, R. P.;
Holtzman, J. A.; Johnson, J.; Frinchaboy, P. M.; Skrutskie, M. F.;
Munoz, R.; Pinsonneault, M. H.; Nidever, D. L.; Zasowski, G.; Garcia
Perez, A.; Fabbian, D.; Meza Cofre, A.; Cunha, K. M.; Smith, V. V.;
Chiappini, C.; Beers, T. C.; Steinmetz, M.; Anders, F.; Bizyaev, D.;
Roman, A.; Fleming, S. W.; Crane, J. D.; SDSS-IV/APOGEE-2 Collaboration
2014AAS...22344006S Altcode:
The second phase of the Apache Point Observatory Galactic Evolution
Experiment (APOGEE-2), a part of the Sloan Digital Sky Survey IV
(SDSS-IV), will commence operations in 2014. APOGEE-2 represents a
significant expansion over APOGEE-1, not only in the size of the stellar
sample, but also in the coverage of the sky through observations in both
the Northern and Southern Hemispheres. Observations on the 2.5m Sloan
Foundation Telescope of the Apache Point Observatory (APOGEE-2N) will
continue immediately after the conclusion of APOGEE-1, to be followed
by observations with the 2.5m du Pont Telescope of the Las Campanas
Observatory (APOGEE-2S) within three years. Over the six-year lifetime
of the project, high resolution (R∼22,500), high signal-to-noise
(≥100) spectroscopic data in the H-band wavelength regime (1.51-1.69
μm) will be obtained for several hundred thousand stars, more than
tripling the total APOGEE-1 sample. Accurate radial velocities and
detailed chemical compositions will be generated for target stars in
the main Galactic components (bulge, disk, and halo), open/globular
clusters, and satellite dwarf galaxies. The spectroscopic follow-up
program of Kepler targets with the APOGEE-2N instrument will be
continued and expanded. APOGEE-2 will significantly extend and enhance
the APOGEE-1 legacy of scientific contributions to understanding the
origin and evolution of the elements, the assembly and formation history
of galaxies like the Milky Way, and fundamental stellar astrophysics.
---------------------------------------------------------
Title: DR10 SDSS-III release of APOGEE data
Authors: Shetrone, Matthew D.; Allende-Prieto, C.; Beers, T. C.;
Cunha, K. M.; Fabbian, D.; Feuillet, D.; Frinchaboy, P. M.; Garcia
Perez, A.; Johnson, J.; Majewski, S. R.; Nidever, D. L.; Pinsonneault,
M. H.; Smith, V. V.; Zasowski, G.; SDSS-III/APOGEE Collaboration
2014AAS...22344002S Altcode:
SDSS-III's newest release is Data Release 10 (DR10). DR10 contains the
first spectra of the APO Galactic Evolution Experiment (APOGEE). APOGEE
is the first high-resolution (R ~ 22,500), high signal-to-noise ratio
(S/N >= 100 per resolution element), H-band (1.51 - 1.70 um) survey
of all Galactic stellar populations (bulge, bar, disks, halo) with a
uniform set of stellar tracers and spectral diagnostics. This poster
will briefly describe the stellar sample included in DR10, review the
data made available in DR10, consisting of fully calibrated, 1-d spectra
radial velocities and the by-products of the APOGEE Stellar Parameters
and Chemical Abundance Pipeline (ASPCAP): effective temperature, surface
gravity, metallicity, and alpha, carbon and nitrogen abundances. We
will also present the web tools that are available to the public and
highlight the most critical warning and bad data flags.
---------------------------------------------------------
Title: Ages of Solar Neighborhood Stars Using APOGEE
Authors: Feuillet, Diane; Holtzman, J. A.; Girardi, L.; Allende-Prieto,
C.; Beers, T. C.; Cunha, K. M.; Fabbian, D.; Frinchaboy, P. M.;
Hayden, M. R.; Majewski, S.
2014AAS...22315209F Altcode:
The SDSS-III Apache Point Observatory Galactic Evolution Experiment
(APOGEE) is a high resolution (R ~23,000) near-IR (H-band) spectroscopic
survey of 100,000 Milky Way stars designed to chemically trace the
formation and evolution of Galactic stellar populations. In addition
to the primary survey, the APOGEE spectrograph has been fitted with
10 fibers from the robotically controlled NMSU 1 m telescope to
maximize the use of this instrument when not on sky with the Sloan
2.5 m telescope. This allows for single object observations with this
high resolution NIR spectrograph. Using this new capability provided
by the 1 m, we are conducting a survey of bright stars (H < 8)
with accurate Hipparcos parallax measurements (μ_err < 10%),
which are not accessible to the main APOGEE survey. These data can be
reduced and analyzed in the same way as main survey data, resulting
in detailed chemical information for hundreds of nearby stars. The
atmospheric parameters combined with the Hipparcos distances allow
for age estimates of these stars. We present initial age estimates
from isochrone matching to Padova isochrones, and an age-metallicity
relation for the current sample.
---------------------------------------------------------
Title: Thermodynamic fluctuations in solar photospheric
three-dimensional convection simulations and observations
(Corrigendum)
Authors: Beck, C.; Fabbian, D.; Moreno-Insertis, F.; Puschmann, K. G.;
Rezaei, R.
2013A&A...559C...1B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Target Selection for the Apache Point Observatory Galactic
Evolution Experiment (APOGEE)
Authors: Zasowski, G.; Johnson, Jennifer A.; Frinchaboy, P. M.;
Majewski, S. R.; Nidever, D. L.; Rocha Pinto, H. J.; Girardi, L.;
Andrews, B.; Chojnowski, S. D.; Cudworth, K. M.; Jackson, K.; Munn, J.;
Skrutskie, M. F.; Beaton, R. L.; Blake, C. H.; Covey, K.; Deshpande,
R.; Epstein, C.; Fabbian, D.; Fleming, S. W.; Garcia Hernandez, D. A.;
Herrero, A.; Mahadevan, S.; Mészáros, Sz.; Schultheis, M.; Sellgren,
K.; Terrien, R.; van Saders, J.; Allende Prieto, C.; Bizyaev, D.;
Burton, A.; Cunha, K.; da Costa, L. N.; Hasselquist, S.; Hearty,
F.; Holtzman, J.; García Pérez, A. E.; Maia, M. A. G.; O'Connell,
R. W.; O'Donnell, C.; Pinsonneault, M.; Santiago, B. X.; Schiavon,
R. P.; Shetrone, M.; Smith, V.; Wilson, J. C.
2013AJ....146...81Z Altcode: 2013arXiv1308.0351Z
The Apache Point Observatory Galactic Evolution Experiment (APOGEE) is
a high-resolution infrared spectroscopic survey spanning all Galactic
environments (i.e., bulge, disk, and halo), with the principal goal
of constraining dynamical and chemical evolution models of the Milky
Way. APOGEE takes advantage of the reduced effects of extinction
at infrared wavelengths to observe the inner Galaxy and bulge at
an unprecedented level of detail. The survey's broad spatial and
wavelength coverage enables users of APOGEE data to address numerous
Galactic structure and stellar populations issues. In this paper
we describe the APOGEE targeting scheme and document its various
target classes to provide the necessary background and reference
information to analyze samples of APOGEE data with awareness of the
imposed selection criteria and resulting sample properties. APOGEE's
primary sample consists of ~10<SUP>5</SUP> red giant stars, selected
to minimize observational biases in age and metallicity. We present the
methodology and considerations that drive the selection of this sample
and evaluate the accuracy, efficiency, and caveats of the selection and
sampling algorithms. We also describe additional target classes that
contribute to the APOGEE sample, including numerous ancillary science
programs, and we outline the targeting data that will be included in
the public data releases.
---------------------------------------------------------
Title: Thermodynamic fluctuations in solar photospheric
three-dimensional convection simulations and observations
Authors: Beck, C.; Fabbian, D.; Moreno-Insertis, F.; Puschmann, K. G.;
Rezaei, R.
2013A&A...557A.109B Altcode: 2013arXiv1306.6093B
Context. Numerical three-dimensional (3D) radiative
(magneto-)hydrodynamical [(M)HD] simulations of solar convection
are nowadays used to understand the physical properties of the solar
photosphere and convective envelope, and, in particular, to determine
the Sun's photospheric chemical abundances. To validate this approach,
it is important to check that no excessive thermodynamic fluctuations
arise as a consequence of the partially incomplete treatment of
radiative transfer causing radiative damping that is too modest. <BR
/> Aims: We investigate the realism of the thermodynamics in recent
state-of-the-art 3D convection simulations of the solar atmosphere
carried out with the Stagger code. <BR /> Methods: We compared the
characteristic properties of several Fe i lines (557.6 nm, 630 nm, 1565
nm) and one Si i line at 1082.7 nm in solar disc-centre observations
of different spatial resolution with spectra synthesized from 3D
convection simulations. The observations were taken with ground-based
(Echelle spectrograph, Göttingen Fabry-Pérot Interferometer (GFPI),
POlarimetric LIttrow Spectrograph, Tenerife Infrared Polarimeter, all
at the Vacuum Tower Telescope on Tenerife) and space-based instruments
(Hinode/Spectropolarimeter). We degraded the synthetic spectra to
the spatial resolution of the observations, based on the distribution
of the continuum intensity I<SUB>c</SUB>. We estimated the spectral
degradation to be applied to the simulation results by comparing atlas
spectra with averaged observed spectra. In addition to deriving a set
of line parameters directly from the intensity profiles, we used the
SIR (Stokes Inversion based on Response functions) code to invert
the spectra. <BR /> Results: The spatial degradation kernels yield
a similar generic spatial stray-light contamination of about 30%
for all instruments. The spectral stray light inside the different
spectrometers is found to be between 2% and 20%. Most of the line
parameters from the observational data are matched by the degraded
HD simulation spectra. The inversions predict a macroturbulent
velocity v<SUB>mac</SUB> below 10 m s<SUP>-1</SUP> for the HD
simulation spectra at full spatial resolution, whereas they yield
v<SUB>mac</SUB> ≲ 1000 m s<SUP>-1</SUP> at a spatial resolution of
0.″3. The temperature fluctuations in the inversion of the degraded
HD simulation spectra do not exceed those from the observational data
(of the order of 100-200 K rms for -2 ⪉ log τ<SUB>500 nm</SUB>
⪉ -0.5). The comparison of line parameters in spatially averaged
profiles with the averaged values of line parameters in spatially
resolved profiles indicates a significant change in (average) line
properties on a spatial scale between 0.″13 and 0.″3. <BR />
Conclusions: Up to a spatial resolution of 0.″3 (GFPI spectra),
we find no indications of excessive thermodynamic fluctuations
in the 3D HD simulation. To definitely confirm that simulations
without spatial degradation contain fully realistic thermodynamic
fluctuations requires observations at even higher spatial resolution
(i.e. <0.″13). <P />Appendices A and B are available in electronic
form at <A href="http://www.aanda.org">http://www.aanda.org</A>
---------------------------------------------------------
Title: Solar Fe abundance and magnetic fields. Towards a consistent
reference metallicity
Authors: Fabbian, D.; Moreno-Insertis, F.; Khomenko, E.; Nordlund, Å.
2012A&A...548A..35F Altcode: 2012arXiv1209.2771F
<BR /> Aims: We investigate the impact on Fe abundance determination of
including magnetic flux in series of 3D radiation-magnetohydrodynamics
(MHD) simulations of solar convection, which we used to synthesize
spectral intensity profiles corresponding to disc centre. <BR />
Methods: A differential approach is used to quantify the changes
in theoretical equivalent width of a set of 28 iron spectral lines
spanning a wide range in wavelength, excitation potential, oscillator
strength, Landé factor, and formation height. The lines were computed
in local thermodynamic equilibrium (LTE) using the spectral synthesis
code LILIA. We used input magnetoconvection snapshots covering 50 min
of solar evolution and belonging to series having an average vertical
magnetic flux density of ⟨ B<SUB>vert</SUB> ⟩ = 0,50,100, and
200 G. For the relevant calculations we used the Copenhagen Stagger
code. <BR /> Results: The presence of magnetic fields causes both a
direct (Zeeman-broadening) effect on spectral lines with non-zero
Landé factor and an indirect effect on temperature-sensitive
lines via a change in the photospheric T - τ stratification. The
corresponding correction in the estimated atomic abundance ranges
from a few hundredths of a dex up to |Δlog ɛ(Fe)<SUB>⊙</SUB>|
~ 0.15 dex, depending on the spectral line and on the amount of
average magnetic flux within the range of values we considered. The
Zeeman-broadening effect gains relatively more importance in the
IR. The largest modification to previous solar abundance determinations
based on visible spectral lines is instead due to the indirect effect,
i.e., the line-weakening caused by a warmer stratification as seen on
an optical depth scale. Our results indicate that the average solar
iron abundance obtained when using magnetoconvection models can be ~
0.03-0.11 dex higher than when using the simpler hydrodynamics (HD)
convection approach. <BR /> Conclusions: We demonstrate that accounting
for magnetic flux is important in state-of-the-art solar photospheric
abundance determinations based on 3D convection simulations.
---------------------------------------------------------
Title: Stray-light contamination and spatial deconvolution of
slit-spectrograph observations
Authors: Beck, C.; Rezaei, R.; Fabbian, D.
2011A&A...535A.129B Altcode: 2011arXiv1109.2421B
Context. Stray light caused by scattering on optical surfaces
and in the Earth's atmosphere degrades the spatial resolution
of observations. Whereas post-facto reconstruction techniques
are common for 2D imaging and spectroscopy, similar options for
slit-spectrograph data are rarely applied. <BR /> Aims: We study the
contribution of stray light to the two channels of the POlarimetric
LIttrow Spectrograph (POLIS) at 396 nm and 630 nm as an example of
a slit-spectrograph instrument. We test the performance of different
methods of stray-light correction and spatial deconvolution to improve
the spatial resolution post-facto. <BR /> Methods: We model the stray
light as having two components: a spectrally dispersed component and
a "parasitic" component of spectrally undispersed light caused by
scattering inside the spectrograph. We used several measurements to
estimate the two contributions: a) observations with a (partly) blocked
field of view (FOV); b) a convolution of the FTS spectral atlas; c)
imaging of the spider mounting in the pupil plane; d) umbral profiles;
and e) spurious polarization signal in telluric spectral lines. The
measurements with a partly blocked FOV in the focal plane allowed us
to estimate the spatial point spread function (PSF) of POLIS and the
main spectrograph of the German Vacuum Tower Telescope (VTT). We then
used the obtained PSF for a deconvolution of both spectroscopic and
spectropolarimetric data and investigated the effect on the spectra. <BR
/> Results: The parasitic contribution can be directly and accurately
determined for POLIS, amounting to about 5% (0.3%) of the (continuum)
intensity at 396 nm (630 nm). The spectrally dispersed stray light is
less accessible because of its many contributing sources. We estimate
a lower limit of about 10% across the full FOV for the dispersed stray
light from umbral profiles. In quiet Sun regions, the stray-light level
from the close surroundings (d < 2”) of a given spatial point is
about 20%. The stray light reduces to below 2% at a distance of 20”
from a lit area for both POLIS and the main spectrograph. The spatial
deconvolution using the PSF obtained improves the spatial resolution
and increases the contrast, with a minor amplification of noise. <BR
/> Conclusions: A two-component model of the stray-light contributions
seems to be sufficient for a basic correction of observed spectra. The
instrumental PSF obtained can be used to model the off-limb stray light,
to determine the stray-light contamination accurately for observation
targets with large spatial intensity gradients such as sunspots,
and also to improve the spatial resolution of observations post-facto.
---------------------------------------------------------
Title: (1) The effect of magnetic fields on solar abundance
determinations (2) The solar photosphere in 3D. This time from
observations
Authors: Fabbian, D.; Socas-Navarro, H.
2010iac..talk..232F Altcode: 2010iac..talk..186F
No abstract at ADS
---------------------------------------------------------
Title: Solar Abundance Corrections Derived Through Three-dimensional
Magnetoconvection Simulations
Authors: Fabbian, D.; Khomenko, E.; Moreno-Insertis, F.; Nordlund, Å.
2010ApJ...724.1536F Altcode: 2010arXiv1006.0231F
We explore the effect of the magnetic field when using realistic
three-dimensional convection experiments to determine solar element
abundances. By carrying out magnetoconvection simulations with a
radiation-hydro code (the Copenhagen stagger code) and through a
posteriori spectral synthesis of three Fe I lines, we obtain evidence
that moderate amounts of mean magnetic flux cause a noticeable
change in the derived equivalent widths compared with those for a
non-magnetic case. The corresponding Fe abundance correction for a
mean flux density of 200 G reaches up to ~0.1 dex in magnitude. These
results are based on space- and time-averaged line profiles over a time
span of 2.5 solar hours in the statistically stationary regime of the
convection. The main factors causing the change in equivalent widths,
namely the Zeeman broadening and the modification of the temperature
stratification, act in different amounts and, for the iron lines
considered here, in opposite directions; yet, the resulting |Δlog
epsilon<SUB>sun</SUB>(Fe)| coincides within a factor of 2 in all
of them, even though the sign of the total abundance correction
is different for the visible and infrared lines. We conclude that
magnetic effects should be taken into account when discussing precise
values of the solar and stellar abundances and that an extended study
is warranted.
---------------------------------------------------------
Title: HD 172189: another step in furnishing one of the best
laboratories known for asteroseismic studies
Authors: Creevey, O. L.; Uytterhoeven, K.; Martín-Ruiz, S.; Amado,
P. J.; Niemczura, E.; van Winckel, H.; Suárez, J. C.; Rolland,
A.; Rodler, F.; Rodríguez-López, C.; Rodríguez, E.; Raskin, G.;
Rainer, M.; Poretti, E.; Pallé, P.; Molina, R.; Moya, A.; Mathias,
P.; Le Guillou, L.; Hadrava, P.; Fabbian, D.; Garrido, R.; Decin,
L.; Cutispoto, G.; Casanova, V.; Broeders, E.; Arellano Ferro, A.;
Aceituno, F.
2009A&A...507..901C Altcode: 2009arXiv0909.3435C
HD 172189 is a spectroscopic eclipsing binary system with a
rapidly-rotating pulsating δ Scuti component. It is also a member of
the open cluster IC 4756. These combined characteristics make it an
excellent laboratory for asteroseismic studies. To date, HD 172189 has
been analysed in detail photometrically but not spectroscopically. For
this reason we have compiled a set of spectroscopic data to determine
the absolute and atmospheric parameters of the components. We
determined the radial velocities (RV) of both components using four
different techniques. We disentangled the binary spectra using KOREL,
and performed the first abundance analysis on both disentangled
spectra. By combining the spectroscopic results and the photometric
data, we obtained the component masses, 1.8 and 1.7 M<SUB>⊙</SUB>,
and radii, 4.0 and 2.4 R<SUB>⊙</SUB>, for inclination i = 73.2°,
eccentricity e = 0.28, and orbital period Π = 5.70198 days. Effective
temperatures of 7600 K and 8100 K were also determined. The measured v
sin i are 78 and 74 km s<SUP>-1</SUP>, respectively, giving rotational
periods of 2.50 and 1.55 days for the components. The abundance analysis
shows [Fe/H] = -0.28 for the primary (pulsating) star, consistent with
observations of IC 4756. We also present an assessment of the different
analysis techniques used to obtain the RVs and the global parameters.
---------------------------------------------------------
Title: The C/O ratio at low metallicity: constraints on early chemical
evolution from observations of Galactic halo stars
Authors: Fabbian, D.; Nissen, P. E.; Asplund, M.; Pettini, M.;
Akerman, C.
2009A&A...500.1143F Altcode: 2008arXiv0810.0281F
Aims: We present new measurements of the abundances of carbon and
oxygen derived from high-excitation C i and O i absorption lines in
metal-poor halo stars, with the aim of clarifying the main sources
of these two elements in the early stages of the chemical enrichment
of the Galaxy. <BR />Methods: We target 15 new stars compared to our
previous study, with an emphasis on additional C/O determinations
in the crucial metallicity range -3 ⪉ [Fe/H]⪉ -2. The stellar
effective temperatures were estimated from the profile of the Hβ
line. Departures from local thermodynamic equilibrium were accounted
for in the line formation for both carbon and oxygen. The non-LTE
effects are very strong at the lowest metallicities but, contrary
to what has sometimes been assumed in the past due to a simplified
assessment, of different degrees for the two elements. In addition,
for the 28 stars with [Fe/H] < -1 previously analysed, stellar
parameters were re-derived and non-LTE corrections applied in the same
fashion as for the rest of our sample, giving consistent abundances
for 43 halo stars in total. <BR />Results: The new observations and
non-LTE calculations strengthen previous suggestions of an upturn in
C/O towards lower metallicity (particularly for [O/H] ⪉ -2). The
C/O values derived for these very metal-poor stars are, however,
sensitive to excitation via the still poorly quantified inelastic
H collisions. While these do not significantly affect the non-LTE
results for C i, they greatly modify the O i outcome. Adopting the
H collisional cross-sections estimated from the classical Drawin
formula leads to [C/O] ≈ 0 at [O/H] ≈ -3. To remove the upturn
in C/O, near-LTE formation for O i lines would be required, which
could only happen if the H collisional efficiency with the Drawin
recipe is underestimated by factors of up to several tens of times,
a possibility which we consider unlikely. <BR />Conclusions: The high
C/O values derived at the lowest metallicities may be revealing the
fingerprints of Population III stars or may signal rotationally-aided
nucleosynthesis in more normal Population II stars. <P />Based on data
collected with the European Southern Observatory's Very Large Telescope
(VLT) at the Paranal, Chile (programmes No. 67.D-0106 and 73.D-0024)
and with the Magellan Telescope at Las Campanas Observatory, Chile.
---------------------------------------------------------
Title: Neutral oxygen spectral line formation revisited with new
collisional data: large departures from LTE at low metallicity
Authors: Fabbian, D.; Asplund, M.; Barklem, P. S.; Carlsson, M.;
Kiselman, D.
2009A&A...500.1221F Altcode: 2009arXiv0902.4472F
Aims: A detailed study is presented, including estimates of the
impact on elemental abundance analysis, of the non-local thermodynamic
equilibrium (non-LTE) formation of the high-excitation neutral oxygen
777 nm triplet in model atmospheres representative of stars with
spectral types F to K. <BR />Methods: We have applied the statistical
equilibrium code MULTI to a number of plane-parallel MARCS atmospheric
models covering late-type stars (4500 ≤ T_eff ≤ 6500 K, 2 ≤ log
g ≤ 5 [cgs], and -3.5 ≤ [Fe/H] ≤ 0). The atomic model employed
includes, in particular, recent quantum-mechanical electron collision
data. <BR />Results: We confirm that the O i triplet lines form under
non-LTE conditions in late-type stars, suffering negative abundance
corrections with respect to LTE. At solar metallicity, the non-LTE
effect, mainly attributed in previous studies to photon losses in the
triplet itself, is also driven by an additional significant contribution
from line opacity. At low metallicity, the very pronounced departures
from LTE are due to overpopulation of the lower level (3s ^5S^o) of the
transition. Large line opacity stems from triplet-quintet intersystem
electron collisions, a form of coupling previously not considered or
seriously underestimated. The non-LTE effects generally become severe
for models (both giants and dwarfs) with higher T_eff. Interestingly,
in metal-poor turn-off stars, the negative non-LTE abundance corrections
tend to rapidly become more severe towards lower metallicity. When
neglecting H collisions, they amount to as much as |Δlog ɛ_O|
~ 0.9 dex and ~1.2 dex, respectively at [Fe/H] = -3 and [Fe/H]
= -3.5. Even when such collisions are included, the LTE abundance
remains a serious overestimate, correspondingly by |Δlog ɛ_O| ~ 0.5
dex and ~0.9 dex at such low metallicities. Although the poorly known
inelastic hydrogen collisions thus remain an important uncertainty,
the large metallicity-dependent non-LTE effects seem to point to
a resulting “low” (compared to LTE) [O/Fe] in metal-poor halo
stars. <BR />Conclusions: Our results may be important in solving
the long-standing [O/Fe] debate. When applying the derived non-LTE
corrections, the LTE oxygen abundance inferred from the 777 nm permitted
triplet will be decreased substantially at low metallicity. If the
classical Drawin formula is employed for O+H collisions, the derived
[O/Fe] trend becomes almost flat below [Fe/H] ~ -1, in better agreement
with recent literature estimates generally obtained from other oxygen
abundance indicators. A value of [O/Fe] ⪉ +0.5 may therefore be
appropriate, as suggested by standard theoretical models of type II
supernovae nucleosynthetic yields. If neglecting impacts with H atoms
instead, [O/Fe] decreases towards lower [Fe/H], which would open new
questions. Our tests using ATLAS model atmospheres show that, though
non-LTE corrections for metal-poor dwarfs are smaller (by ~0.2 dex
when adopting efficient H collisions) than in the MARCS case, our
main conclusions are preserved, and that the LTE approach tends to
seriously overestimate the O abundance at low metallicity. However,
in order to finally reach consistency between oxygen abundances from
the different available spectral features, it is of high priority to
reduce the large uncertainty regarding H collisions, to undertake a
full investigation of the interplay of non-LTE and 3D effects, and to
clarify the issue of the temperature scale at low metallicity.
---------------------------------------------------------
Title: Chemical compositions of stars in the light of non-LTE spectral
line formation: The evolution of carbon and oxygen in the Galaxy
Authors: Fabbian, Damian
2008PhDT.......131F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: [C/O] Observations in Low-[Fe/H] Halo Stars
Authors: Fabbian, D.; Nissen, P. E.; Asplund, M.; Akerman, C. J.;
Pettini, M.
2008psa..conf...45F Altcode:
We have observed 15 halo stars to determine the [C/O] behaviour at low
[Fe/H]. Making use of our recent non-LTE calculations, which show
that the high excitation C and O lines used in previous studies in
the literature are affected by very significant departures from LTE,
we aim to obtain accurate [C/O] ratios down to [Fe/H]∼ —3.2, which
will enable us to shed light on the possible presence of an upturn of
[C/O] at low metallicities.
---------------------------------------------------------
Title: Sulphur Abundances in Metal-poor Stars
Authors: Nissen, P. E.; Akerman, C.; Asplund, M.; Fabbian, D.;
Pettini, M.
2008psa..conf...51N Altcode:
We report on sulphur abundances in halo stars as derived from near-IR
UVES spectra. The importance of removing telluric lines and residual
CCD fringing patterns by using early B-type stars as calibrators is
emphasized. Comparison of data from the weak λ8694.6 and the stronger
λ9212.9, 9237.5 pair of S I lines provides important constraints on
non-LTE effects. We do not confirm the high sulphur abundances reported
by others for some metal-poor stars; our results instead indicate that
sulphur behaves like other typical α-capture elements with a plateau at
[S/Fe] ∼ +0.3 dex in the Galactic halo.
---------------------------------------------------------
Title: Chemical compositions of stars in the light of non-LTE spectral
line formation: the evolution of carbon and oxygen in the Galaxy
Authors: Fabbian, D.
2008PhDT........12F Altcode:
Non-LTE effects affect the formation of many spectral features
in the spectra of late-type stars. This thesis investigates the
effect on carbon and oxygen abundances. The non-LTE calculations
have been performed with the code MULTI, using MARCS atmospheric
models with 4500<Teff<6500 K, 2<log g<5 [cgs], and
-3.5<[Fe/H]<0. The large negative non-LTE abundance corrections
found for high-excitation IR lines of both C I and O I previously used
in the literature, are due to dilution of the line source function (due
to photon losses in the lines) at solar metallicity, and to increased
line opacity in non-LTE for metal-poor stars. The tendency of the
non-LTE abundance corrections to become more important when moving to
very low metallicity has important implications in terms of galactic
chemical evolution. The results are applied to high-quality observation
for a sample of 43 turnoff halo stars with -3.24<[Fe/H]<-1.10. At
low-metallicity, in particular due to decreasing Si background opacity,
the non-LTE corrections affecting the 777 nm oxygen triplet are found to
be more significant than those affecting the C I lines. The presence of
a [C/O] upturn at low [Fe/H], here for the first time based on a solid
non-LTE analysis, is confirmed, pointing to significant C yields from
early generations of stars. This could be tracing the products of so-far
elusive earlier Population III stars, or may signal rotationally-aided
nucleosynthesis in more normal Population II stars.
---------------------------------------------------------
Title: Sulphur and zinc abundances in Galactic halo stars revisited
Authors: Nissen, P. E.; Akerman, C.; Asplund, M.; Fabbian, D.; Kerber,
F.; Kaufl, H. U.; Pettini, M.
2007A&A...469..319N Altcode: 2007astro.ph..2689N
Aims:Based on a new set of sulphur abundances in very metal-poor stars
and an improved analysis of previous data, we aim at resolving current
discrepancies on the trend of S/Fe vs. Fe/H and thereby gain better
insight into the nucleosynthesis of sulphur. The trends of Zn/Fe and
S/Zn will also be studied. <BR />Methods: High resolution VLT/UVES
spectra of 40 main-sequence stars with -3.3 < [Fe/H] < -1.0 are
used to derive S abundances from the weak λ 8694.6 S I line and the
stronger λ λ 9212.9,9237.5 pair of S I lines. For one star, the S
abundance is also derived from the S I triplet at 1.046 μm recently
observed with the VLT infrared echelle spectrograph CRIRES. Fe and
Zn abundances are derived from lines in the blue part of the UVES
spectra, and effective temperatures are obtained from the profile
of the Hβ line. <BR />Results: Comparison of sulphur abundances
from the weak and strong S I lines provides important constraints on
non-LTE effects. The high sulphur abundances reported by others for
some metal-poor stars are not confirmed; instead, when taking non-LTE
corrections into account, the Galactic halo stars distribute around a
plateau at [S/Fe] ~ +0.2 dex with a scatter of 0.07 dex only. [Zn/Fe]
is close to zero for metallicities in the range -2.0 < [Fe/H]
< -1.0 but increases to a level of [Zn/Fe] ~ +0.1 to +0.2 dex in
the range -2.7 < [Fe/H] < -2.0. At still lower metallicities
[Zn/Fe] rises steeply to a value of [Zn/Fe] ~ +0.5 dex at [Fe/H] =
-3.2. <BR />Conclusions: The trend of S/Fe vs. Fe/H corresponds to
the trends of Mg/Fe, Si/Fe, and Ca/Fe and indicates that sulphur in
Galactic halo stars has been made by α-capture processes in massive
SNe. The observed scatter in S/Fe is much smaller than predicted from
current stochastic models of the chemical evolution of the early Galaxy,
suggesting that either the models or the calculated yields of massive
SNe should be revised. We also examine the behaviour of S/Zn and find
that departures from the solar ratio are significantly reduced at
all metallicities if non-LTE corrections to the abundances of these
two elements are adopted. This effect, if confirmed, would reduce the
usefulness of the S/Zn ratio as a diagnostic of past star-formation
activity, but would bring closer together the values measured in damped
Lyman-alpha systems and in Galactic stars. <P />Based on observations
collected at the European Southern Observatory at Paranal, Chile
(programmes No. 67.D-0106, 73.D-0024 and CRIRES science verification
program 60.A-9072). Table 1 and Appendices are only available in
electronic form at http://www.aanda.org
---------------------------------------------------------
Title: Sulphur Abundances in Metal-Poor Stars First Result from
CRIRES Science Verification
Authors: Nissen, Poul Erik; Asplund, Martin; Fabbian, Damian; Kerber,
Florian; Käufl, Hans Ulrich; Pettini, Max
2007Msngr.128...38N Altcode:
Sulphur is the tenth most abundant element in the Universe and plays an
im-portant role in studies of the chemical enrichment and star formation
history of distant galaxies. Due to the lack of suitable sulphur lines
in the visible part of stellar spectra there is, however, still no
agreement on the abundance of sulphur in Galactic metal poor stars,
and we are therefore uncertain about the nucleosynthetic origin of
sulphur. New observations of infrared sulphur lines with the cryogenic
high resolution infrared echelle spectrograph (CRIRES) at ESO's VLT
are helping to solve this problem.
---------------------------------------------------------
Title: The non-LTE line formation of neutral carbon in late-type stars
Authors: Fabbian, D.; Asplund, M.; Carlsson, M.; Kiselman, D.
2006A&A...458..899F Altcode: 2006astro.ph..8284F
Aims.We investigate the non-Local Thermodynamic Equilibrium (non-LTE)
line formation of neutral carbon in late-type stars in order to remove
some of the potential systematic errors in stellar abundance analyses
employing C i features.<BR /> Methods: .The statistical equilibrium
code MULTI was used on a grid of plane-parallel 1D MARCS atmospheric
models.<BR /> Results: .Within the parameter space explored, the
high-excitation C i lines studied are stronger in non-LTE due to
the combined effect of line source function drop and increased line
opacity due to overpopulation of the lower level for the transitions
considered; the relative importance of the two effects depends on
the particular combination of T<SUB>{eff</SUB>}, log g, [Fe/H] and
[C/Fe] and on the analysed C i line. As a consequence, the non-LTE
abundance corrections are negative and can be substantially so,
for example ∼ -0.4 dex in halo turn-off stars at [Fe/H]∼ -3. The
magnitude of the non-LTE corrections is rather insensitive to whether
inelastic H collisions are included or not.<BR /> Conclusions: .Our
results have implications on studies of nucleosynthetic processes and
on Galactic chemical evolution models. When applying our calculated
corrections to recent observational data, the upturn in [C/O] at low
metallicity might still be present (thus apparently still necessitating
contributions from massive Pop. III stars for the carbon production),
but at a lower level and possibly with a rather shallow trend of ∼
-0.2 dex/dex below [O/H]∼ -1.
---------------------------------------------------------
Title: Ci Non-LTE Spectral Line Formation in Late-Type Stars
Authors: Fabbian, D.; Asplund, M.; Carlsson, M.
2006cams.book...52F Altcode:
We present the results from our non-LTE investigation for neutral
carbon, which was carried out to remove potential systematic errors
in stellar abundance analyses. The calculations were performed for
late-type stars and give substantial negative non-LTE abundance
corrections. When applied to observations of extremely metal-poor
stars, which within the LTE framework seem to suggest a possible [C/O]
uprise at low metallicities (Akerman et al. 2004), these improvements
will have important implications, enabling us to understand if the
standard chemical evolution model is adequate, with no need to invoke
signatures by Pop. III stars for the carbon nucleosynthesis.
---------------------------------------------------------
Title: Abundance Anomalies in Hot Horizontal Branch Stars of the
Galactic Globular Cluster NGC 1904
Authors: Recio-Blanco, A.; Fabbian, D.; Gratton, R. G.
2006cams.book..151R Altcode:
We present abundance measurements, based on high resolution optical
spectroscopic data obtained with the UVES at VLT, for 10 stars in
the blue horizontal branch (BHB) of the Galactic globular cluster NGC
1904 (M79). In agreement with previous findings for other clusters,
we obtain normal abundances for stars cooler than Te. ~ 11000 K, and
largely anomalous abundances for hotter stars: large He depletions
and overabundances of Fe, Ti, Cr, P and Mn. The abundances of Mg, Si
and Ca are roughly normal, in the hot stars as well as in the cooler
ones. This abundance pattern can be attributed to the onset of diffusion
and to radiation pressure in the stable atmospheres of hot HB stars. A
possibly related discontinuity in the stellar rotation rate seems also
to occur at Te. ~ 11000 K.
---------------------------------------------------------
Title: Abundance anomalies in hot horizontal branch stars of the
galactic globular cluster NGC1904
Authors: Fabbian, D.; Recio-Blanco, A.; Gratton, R. G.; Piotto, G.
2005A&A...434..235F Altcode: 2005astro.ph..1321F
We present abundance measurements, based on high-resolution optical
spectroscopic data obtained with the Ultraviolet and Visual Echelle
Spectrograph mounted on Kueyen (Very Large Telescope UT2), for ten
stars in the extended blue horizontal branch of the Galactic globular
cluster NGC 1904 (M79). In agreement with previous findings for other
clusters, we obtain normal abundances for stars cooler than T_eff∼
11 000 K, and largely anomalous abundances for hotter stars: large He
depletions, and overabundances of Fe, Ti, Cr, P and Mn. The abundances
of Mg, Si and Ca are roughly normal, in the hot stars as well as in the
cooler ones. This abundance pattern can be attributed to the onset of
diffusion and to radiation pressure in the stable atmospheres of hot
horizontal branch stars.
---------------------------------------------------------
Title: Effective temperatures and lithium abundances of halo
turnoff stars
Authors: Nissen, P. E.; Akerman, C.; Asplund, M.; Fabbian, D.;
Pettini, M.
2005IAUS..228..101N Altcode:
Effective temperatures of 30 turnoff stars with -3.2 {[Fe/H]{
have been derived from the profiles of Balmer lines in high S/N,
VLT/UVES spectra. While the systematic error of T<SUB>eff</SUB> may
be of the order of 100K, the differential values of T<SUB>eff</SUB>
are determined with a one-sigma precision of ∼25K. These precise
T<SUB>eff</SUB> values are used in a study of the slope and dispersion
of the Li abundance as a function of [Fe/H]. A small, but significant
cosmic dispersion in A(Li) appears to be present exemplified by the
two very metal-poor stars G64-12 and G64-37.
---------------------------------------------------------
Title: CI non LTE spectral line formation in late-type stars
Authors: Fabbian, D.; Asplund, M.; Carlsson, M.; Kiselman, D.
2005IAUS..228..255F Altcode: 2005astro.ph..8063F
We present non-Local Thermodynamic Equilibrium (non-LTE) calculations
for neutral carbon spectral line formation, carried out for a grid
of model atmospheres covering the range of late-type stars. The
results of our detailed calculations suggest that the carbon
non-LTE corrections in these stars are higher than usually adopted,
remaining substantial even at low metallicity. For the most metal-poor
stars in the sample of Akerman et al (2004), the effects are of the
order of Δlogɛ<SUB>C</SUB> ≃ -0.35ldots-0.45 (when neglecting H
collisions). Applying our results to those observations, the apparent
[C/O] upturn seen in their LTE analysis is no longer present, thus
revealing no need to invoke contributions from Pop. III stars to the
carbon nucleosynthesis.
---------------------------------------------------------
Title: VizieR Online Data Catalog: Abundances of BHB stars in NGC 1904
(Fabbian+, 2005)
Authors: Fabbian, D.; Recio-Blanco, A.; Gratton, R. G.; Piotto, G.
2005yCat..34340235F Altcode:
The abundance measurements are based on high-resolution optical
spectroscopical data, obtained during 2 observing runs with the
Ultraviolet and Visual Echelle Spectrograph mounted on Kueyen
(Very Large Telescope UT2). Ten target stars in the extended blue
horizontal branch of the Galactic globular cluster NGC 1904 (M79)
have been analysed. <P />(2 data files).
---------------------------------------------------------
Title: Chemical and Rotational Properties of Cluster Hot HB Stars.
Authors: Recio-Blanco, Alejandra; Piotto, Giampaolo; Gratton, Raffaele;
Fabbian, Damian; Aparicio, Antonio
2003IAUJD...4E..11R Altcode:
In this contribution we analize the chemical and rotational properties
of a sample of hot HB stars in the globular clusters M15 M79 M80 and
NGC 2808 with effective temperatures T<SUB>eff</SUB> between 7000 K
and 25000 K. To this purpose we have used high resolution spectroscopy
with UVES+VLT. Our study reveals the existence of a discontinuity in
the rotational rate distribution with temperature detected also by
Behr et al. 1999 and 2000 in M13 and M15. Stars with Teff< 11000
K show a range of vsini values up to 30 km/s. All the stars with Teff
> 11000 K however are slow rotators with vsini < 12 km/s. This
discontinuity seems to coincide with the onset of diffusive processes
in the stellar atmosphere which cause abundance anomalies in the hotter
stars: overabundances of metals and helium depletion. This is confirmed
also by the chemical analysis performed in 10 of the M79 targets in
which we have found evidences of radiative levitation of metals and
gravitational settling of helium for stars with temperatures hotter
than 11000 K.
