lanz

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Author name code: lanz
ADS astronomy entries on 2022-09-14
author:"Lanz, Thierry M."

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Title: Properties of Primitive Galaxies
Authors: Heap, Sara R.; Hubeny, I.; Bouret, J. -C.; Lanz, T.;
Brinchmann, J.
2022arXiv220704553H Altcode:
We report on a study of 9 nearby star-forming, very low-metallicity
galaxies observed by Hubble's COS far-UV spectrograph that can serve
as templates of high-z galaxies to be observed by JWST. We find
that the nebular spectra of these primitive galaxies show evidence
of irradiation by X-ray emitters. Following Thuan et al. (2004),
we identify the sources of X-ray emission as massive X-ray binaries
containing a massive accreting stellar black hole. We further find
that the lower the metallicity, the higher the probability of strong
X-irradiation. Following Heger et al. (2003), we suggest that these
accreting black holes are produced by direct collapse of stars having
initial masses greater than $\sim50\, M_\odot$. Our models of young
star clusters with an embedded stellar black hole produce effects on
the surrounding gaseous medium that are consistent with the observed
spectra. We conclude that primitive galaxies are qualitatively different
from more metal-rich galaxies in showing evidence of hard radiation
that can best be explained by the presence of one or more embedded
stellar black holes.

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Title: High Resolution Spectroscopy of the Unusual Spectral Feature
in the Central Compact Object PSR J0821-4300
Authors: Gotthelf, Eric; Bogdanov, Slavko; Paerels, Frits; Groger,
John; Helfand, David; Halpern, Jules; Hubeny, Ivan; Lanz, Thierry
2022HEAD...1911006G Altcode:
We present Chandra LETG high-resolution X-ray spectroscopy of the
unusual line feature seen in the blackbody emission from the central
compact object (CCO) in supernova remnant (SNR) Puppis~A. This feature
is likely associated with one of the antipodal hot-spots on the NS
surface and may provide the first detection of atomic photospheric
absorption lines, allowing a model independent, direct measurement of
the mass-radius relation, a strong constrain on the equation of state
of nuclear matter. If generated by cyclotron resonance scattering or
possibly from accretion of supernova debris, the line emission may
provide a localized measurement of the magnetic field, important for
reconciling the observed bright surface hot-spots unprecedented for
a neutron star with such a weak (2.8E-10 G) dipole magnetic field.

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Title: The Photospheric X-ray Spectrum of the Neutron Star in Puppis A
Authors: Groger, John; Paerels, Frits; Gotthelf, Eric; Halpern, Jules;
Helfand, David; Bogdanov, Slavko; Hubeny, Ivan; Lanz, Thierry
2022HEAD...1910107G Altcode:
We have collected 352.6 ksec (as of September 7, 2021) of a 400 ksec
Chandra observation of the central compact object PSR J0821-4300
in the supernova remnant Puppis A using the Low Energy Transmission
Grating Spectrometer. We have clearly detected photospheric radiation
from the neutron star with a temperature of approximately 2 million
degrees. We anticipate that we will have the sensitivity to detect
absorption features from highly-ionized metals that may be present at
the surface of the star. In this poster, we discuss our preliminary
analysis of the spectral data.

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Title: The Neutron Star in Puppis A: Atomic Photospheric Spectroscopy
at Last?
Authors: Paerels, Frits; Groger, John John; Helfand, David; Bogdanov,
Slavko; Gotthelf, Eric; Halpern, Jules; Hubeny, Ivan; Lanz, Thierry
2022HEAD...1910106P Altcode:
We have accumulated 400 ksec of exposure time with the Chandra Low
Energy Transmission Grating Spectrometer on the neutron star in the
supernova remnant Puppis A. We clearly detect photospheric radiation
from a ~ 2 million degree star. We anticipate that the spectrum will
have sufficient sensitivity for us to detect atomic absorption features
from highly ionized mid-Z atoms, which may be present at the surface
of the star. We discuss the data and our preliminary spectroscopic
analysis.

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Title: TLUSTY and SYNSPEC Users's Guide IV: Upgraded Versions 208
and 54
Authors: Hubeny, Ivan; Allende Prieto, Carlos; Osorio, Yeisson;
Lanz, Thierry
2021arXiv210402829H Altcode:
We present a brief description of the newly upgraded versions of the
programs TLUSTY, version 208, and SYNSPEC, version 54. TLUSTY is used
to generate model stellar atmospheres or accretion disks, and SYNSPEC
produces detailed synthetic spectra and/or opacity tables. This paper
complements published guides that provide a detailed description
of earlier versions of the codes, TLUSTY205, and SYNSPEC51. The
main upgrades include the flexible construction of opacity tables
in SYNSPEC, and their use in producing hybrid models with TLUSTY},
with important species treated in NLTE, while the bulk of opacity of
atomic and molecular lines and continua are considered in LTE using
a pre-calculated opacity table. There is also a number of additional
changes and upgrades that increase the versatility and flexibility of
these codes.

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Title: Massive stars in the Small Magellanic Cloud. Evolution,
rotation, and surface abundances
Authors: Bouret, J. -C.; Martins, F.; Hillier, D. J.; Marcolino,
W. L. F.; Rocha-Pinto, H. J.; Georgy, C.; Lanz, T.; Hubeny, I.
2021A&A...647A.134B Altcode: 2021arXiv210109269B
Context. The evolution of massive stars depends on several physical
processes and parameters. Metallicity and rotation are among the most
important, but their quantitative effects are not well understood. Aims: To complement our earlier study on main-sequence stars,
we study the evolutionary and physical properties of evolved O stars
in the Small Magellanic Cloud (SMC). We focus in particular on their
surface abundances to further investigate the efficiency of rotational
mixing as a function of age, rotation, and global metallicity. 
Methods: We analysed the UV and optical spectra of 13 SMC O-type giants
and supergiants using the stellar atmosphere code CMFGEN to derive
photospheric and wind properties. We compared the inferred properties to
theoretical predictions from evolution models. For a more comprehensive
analysis, we interpret the results together with those we previously
obtained for O-type dwarfs. Results: Most dwarfs of our sample
lie in the early phases of the main sequence. For a given initial mass,
giants are farther along the evolutionary tracks, which confirms that
they are indeed more evolved than dwarfs. Supergiants have higher
initial masses and are located past the terminal-age main-sequence in
each diagram. We find no clear trend of a mass discrepancy, regardless
of the diagram that was used to estimate the evolutionary mass. Surface
CNO abundances are consistent with nucleosynthesis from the CNO
cycle. Comparisons to theoretical predictions reveal that the initial
mixture is important when the observed trends in the N/C versus N/O
diagram are to be reproduced. A trend for stronger chemical evolution
for more evolved objects is observed. Above about 30 M⊙,
more massive stars are on average more chemically enriched at a given
evolutionary phase. Below 30 M⊙, the trend vanishes. This
is qualitatively consistent with evolutionary models. A principal
component analysis of the abundance ratios for the whole (dwarfs and
evolved stars) sample supports the theoretical prediction that massive
stars at low metallicity are more chemically processed than their
Galactic counterparts. Finally, models including rotation generally
reproduce the surface abundances and rotation rates when different
initial rotational velocities are considered. Nevertheless, for some
objects, a stronger braking and/or more efficient mixing is required. This research is based on observations made with the NASA/ESA Hubble
Space Telescope obtained from the Space Telescope Science Institute,
which is operated by the Association of Universities for Research in
Astronomy, Inc., under NASA contract NAS 5-26555. These observations
are associated with programmes GO 7437, GO 9434, and GO 11625. Based on data products from observations made with ESO Telescopes
at the La Silla Paranal Observatory under programmes ID 67.D-0238,
70.D-0164, 074.D-0109, 079.D-0073, and 079.D-0562.

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Title: Supernovae Chemical Yields in Magellanic Cloud Environments
Authors: Peters, G. J.; Lanz, T.; Bouret, J.; Proffitt, C. R.; Adelman,
S. J.; Hubeny, I.
2020AAS...23511025P Altcode:
Recently there has been interest in the abundance of Mn and other
Fe group elements as diagnostics for determining the progenitors of
SNe Ia and their role in the chemical evolution of a galaxy. We have
combined recent spectroscopic observations from the Cosmic Origins
Spectrograph (COS) on the Hubble Space Telescope with archival data from
the Far Ultraviolet Spectroscopic Explorer (FUSE) and ESO's VLT/UVES to
determine the abundances of the Fe group elements (Ti, V, Cr, Mn, Fe,
Co, &amp; Ni) in main sequence B stars in the Magellanic Clouds. Here we
report results for NGC 1818-D1 (LMC) and AV 304 (SMC). The analysis was
carried through using the Hubeny/Lanz NLTE programs TLUSTY/SYNSPEC. The
COS observations were secured with the G130M, G160M, G185M, and G225M
gratings. Combined with the FUSE data, we have achieved spectral
coverage in the UV from 950 to 2400 A. Measurable lines from the Fe
group, except for a very few multiplets of Fe II, III are not found
in optical spectra. The following stellar parameters were adopted:
Teff = 24700 K, log g = 4.0 cm/s2, Vturb =
0 km/s, and v sin i = 30 km/s for NGC 1818-D1, and Teff =
27500 K, log g = 3.7 cm/s2, Vturb = 1 km/s, and v sin i =
8 km/s for AV 304. Both stars show Fe group abundances about 0.2-0.3
dex smaller than their lighter elements when compared to solar values
and the LMC/SMC averages. However Ti, an alpha-process element, appears
to have an abundance more in line with LMC/SMC values. [Mn/Fe] ranges
from 0.2 dex (NGC 1818-D1) to 0.1 dex (AV 304) with an uncertainty of
0.2 dex, and implies that the progenitors that produced their Fe group
material probably had Chandrasekhar masses (cf. Seitenzahl et al. 2013,
A&amp;A, 559, L5). Support from STScI grants HST-GO-14081.002 and
HST-GO-13346.022, and USC's Women in Science and Engineering (WiSE)
program are greatly appreciated.

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Title: New insight on accretion shocks onto young stellar
objects. Chromospheric feedback and radiation transfer
Authors: de Sá, L.; Chièze, J. -P.; Stehlé, C.; Hubeny, I.; Lanz,
T.; Cayatte, V.
2019A&A...630A..84D Altcode:
Context. Material accreted onto classical T Tauri stars is expected
to form a hot quasi-periodic plasma structure that radiates
in X-rays. Simulations of this phenomenon only partly match
observations. They all rely on a static model for the chromosphere
and on the assumption that radiation and matter are decoupled. 
Aims: We explore the effects of a shock-heated chromosphere and of
the coupling between radiation and hydrodynamics on the structure
and dynamics of the accretion flow. Methods: We simulated
accretion columns that fall onto a stellar chromosphere using the
1D ALE code AstroLabE. This code solves the hydrodynamics equations
along with the first two moment equations for radiation transfer,
with the help of a dedicated opacity table for the coupling between
matter and radiation. We derive the total electron and ion densities
from collisional-radiative model. Results: The chromospheric
acoustic heating affects the duration of the cycle and the structure
of the heated slab. In addition, the coupling between radiation
and hydrodynamics leads to a heating of the accretion flow and of
the chromosphere: the whole column is pushed up by the inflating
chromosphere over several times the steady chromosphere thickness. These
last two conclusions are in agreement with the computed monochromatic
intensity. Acoustic heating and radiation coupling affect the amplitude
and temporal variations of the net X-ray luminosity, which varies
between 30 and 94% of the incoming mechanical energy flux, depending
on which model is considered.

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Title: Cosmic Evolution Through UV Spectroscopy (CETUS) Probe-Class
Mission Concept
Authors: Danchi, William; Arenberg, J.; Bartoszyk, A.; Bezanson, R.;
Bianchi, L.; Bowen, D.; Burge, J.; Cenko, B.; Choi, M.; Danchi, W.;
Dodson, K.; Driver, S.; Durbeck, G.; Dwek, E.; Eckles, W.; Fleming,
B.; France, K.; Gatkine, P.; Gezari, S.; Gong, Q.; Greene, J.; Heap,
S.; Heckman, T.; Hodges-Kluck, E.; Hull, T.; Kendrick, S.; King,
R.; Kutyrev, A.; Lanz, T.; MacKenty, J.; McCandliss, S.; Mehle, G.;
Mentzell, E.; Moseley, S.; Nikzad, S.; Purves, L.; Rauscher, B.;
Rhee, M.; Riall, S.; Rigby, J.; Roederer, I.; Schur, N.; Seipel, S.;
Sheikh, D.; Siegmund, O.; Simmons, R.; Smith, W.; Spergel, D.; Stark,
D.; Stoneking, E.; Szalay, A.; Tompkins, S.; Trump, J.; Vandewel, A.;
Valente, M.; Valerga, J.; Veilleux, S.; Waczynski, A.; Whitaker, K.;
Woodruff, R.; Wyse, R.
2019BAAS...51g..84D Altcode: 2019astro2020U..84D
CETUS is a concept for an affordable, Probe-Class mission based on a
1.5-m-aperture space telescope with a wide FOV covering UV wavelengths
from 100 to 400 nm. The set of science instruments consists of a
wide-field camera, a wide-field Multi-Object Spectrograph, and a
Point/slit Source echelle Spectrograph with high spectral resolution.

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Title: The Probe-class mission concept, Cosmic Evolution Through UV
Surveys (CETUS)
Authors: Heap, Sara; Hull, Tony; Kendrick, Steve; Woodruff, Bob;
Arenberg, Jonathan; Baes, Maarten; Bezanson, Rachel; Bianchi,
Luciana; Bowen, David; Cenko, Brad; Chiang, Yi-Kuan; Cochrane, Rachel;
Corcoran, Mike; Crowther, Paul; Driver, Simon; Danchi, Bill; Dwek,
Eli; Fleming, Brian; France, Kevin; Gatkine, Pradip; Gezari, Suvi;
Hayward, Chris; Hayes, Matthew; Heckman, Tim; Hodges-Kluck, Edmund;
Kutyrev, Alexander; Lanz, Thierry; MacKenty, John; McCandliss, Steve;
Moseley, Harvey; Neiner, Coralie; Pacifici, Camilla; Rafelski, Marc;
Rauscher, Bernie; Rigby, Jane; Roederer, Ian; Spergel, David; Stark,
Dan; Szalay, Alexander; Terrazas, Bryan; Trump, Jonathan; van der
Wel, Arjun; Veilleux, Sylvain; Whitaker, Kate; Wold, Isak; Wyse,
Rosemary; Burge, Jim; Dodson, Kelly; Eckles, Chip; Fleming, Brian;
MacKenty, John; McCandliss, Steve; Mehle, Greg; Nikzad, Shouleh;
Purves, Lloyd; Quijada, Manuel; Siegmund, Ossy; Sheik, Dave; Vallerga,
John; Valente, Marty
2019BAAS...51g.159H Altcode: 2019astro2020U.159H
CETUS is a 1.5-m, wide-field UV observatory that will be a worthy
successor to Hubble. Its distinguishing characteristics include
multi-object slit spectroscopy, long-slit spectroscopy, spectroscopy in
the Lyman-UV, prompt-response observations, and detection of low-surface
brightness objects. These new capabilities ensure future discoveries.

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Title: Stars and Stellar Black Holes in the Low-metallicity Galaxy
I Zw 18
Authors: Heap, S. R.; Hubeny, I.; Lanz, T. M.
2019ASPC..519..267H Altcode:
I Zw 18 is a star-forming dwarf galaxy having a very low metal content,
O/H ∼ 1/50 solar (Skillman &amp; Kennicutt 1993). While galaxies
with such low metallicity are rare in the low-redshift universe,
they are likely to be common in galaxies at cosmic dawn. Thus,
I Zw 18 is a “living” template for z &gt; 6 galaxies. We have
obtained HST/COS far-UV spectra of the northwest star cluster in I
Zw 18 and have compared them to SYNSPEC model spectra by Lanz &amp;
Hubeny (2003, 2007) in order to determine the properties of the stellar
population. We have also compared the observed spectra of I Zw 18-NW to
the CLOUDSPEC models (Hubeny et al. 2000) of the stellar cluster with
an embedded ultra-luminous X-ray source (ULX). This comparison reveals
feedback of the stellar black hole in the form of photoionization and
heating. Such models can be used as starting points to explore the
physical conditions in which stars and black holes form and evolve in
an extremely low-metallicity environment at high redshift.

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Title: The Radial Oxygen Abundance Gradient from OB Stars in the
Outer Galactic Disk
Authors: Daflon, Simone; Bragança, Gustavo A.; Lanz, Thierry; Cunha,
Katia; Hubeny, Ivan; Bensby, Thomas; McMillan, Paul J.; Garmany,
Kate; Glaspey, John W.; Borges Fernandes, Marcelo; Oey, Sally
2019ASPC..519..213D Altcode:
Metallicity gradients are important constraints for models of chemical
evolution of the Galaxy. We present the current radial gradient of
oxygen abundances using a sample of main-sequence OB stars located
in the outer Galactic Disk, considering non-NLTE abundances, and
distances based on GAIA DR2 results. We compare the obtained gradient
with results from other young objects of the Galactic Disk.

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Title: 3D Spectral Radiative Transfer and Perspectives for
Spectroscopic Diagnostics
Authors: Ibgui, Laurent; Hubeny, Ivan; Lanz, Thierry; González,
Matthias; Stehlé, Chantal; Orlando, Salvatore; Colombo, Salvatore
2019ASPC..519...21I Altcode:
We present the features of the three-dimensional (3D) radiative
transfer code IRIS, which synthesizes spectra emitted from hydrodynamics
structures. We discuss our first application of IRIS to the analysis
of a laboratory radiative shock. We demonstrate, with the help
of the radiation hydrodynamics (RHD) code HERACLES, the existence
of a radiative precursor. We validate the gray approximated model
M1 used by HERACLES. We show a couple of synthesized X-UV
spectra of such a shock. We finally open up our discussion to the future
possibilities of spectroscopic diagnostics of accreting streams onto
the surface of Classical T Tauri stars (CTTSs).

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Title: Hot UV-bright stars of galactic globular clusters
Authors: Moehler, S.; Landsman, W. B.; Lanz, T.; Miller Bertolami,
M. M.
2019A&A...627A..34M Altcode: 2019arXiv190506718M
Context. We have performed a census of the UV-bright population in
78 globular clusters using wide-field UV telescopes. This population
includes a variety of phases of post-horizontal branch (HB) evolution,
including hot post-asymptotic giant branch (AGB) stars, and post-early
AGB stars. There are indications that old stellar systems like globular
clusters produce fewer post-(early) AGB stars than currently predicted
by evolutionary models, but observations are still scarce. Aims:
We wish to derive effective temperatures, surface gravities, and helium
abundances of the luminous hot UV-bright stars in these clusters to
determine their evolutionary status and compare the observed numbers
to predictions from evolutionary theory. Methods: We obtained
FORS2 spectroscopy of eleven of these UV-selected objects (covering a
range of -2.3 &lt; [Fe/H] &lt; -1.0), which we (re-)analysed together
with previously observed data. We used model atmospheres of different
metallicities, including super-solar ones. Where possible, we verified
our atmospheric parameters using UV spectrophotometry and searched
for metal lines in the optical spectra. We calculated evolutionary
sequences for four metallicity regimes and used them together with
information about the HB morphology of the globular clusters to
estimate the expected numbers of post-AGB stars. Results:
We find that metal-rich model spectra are required to analyse stars
hotter than 40 000 K. Seven of the eleven new luminous UV-bright
stars are post-AGB or post-early AGB stars, while two are evolving
away from the HB, one is a foreground white dwarf, and another is a
white dwarf merger. Taking into account published information on other
hot UV-bright stars in globular clusters, we find that the number
of observed hot post-AGB stars generally agrees with the predicted
values, although the numbers are still low. Conclusions:
Spectroscopy is clearly required to identify the evolutionary status
of hot UV-bright stars. For hotter stars, metal-rich model spectra
are required to reproduce their optical and UV spectra, which may
affect the flux contribution of hot post-AGB stars to the UV spectra
of evolved populations. While the observed numbers of post-AGB and
post-early AGB stars roughly agree with the predictions, our current
comparison is affected by low number statistics. The extracted
spectra, their best-fitting model spectra, and the evolutionary tracks
used in this paper are available at 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/627/A34">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/627/A34</A>Based
on observations with the ESO Very Large Telescope at Paranal
Observatory, Chile (proposal ID 089.C-0210).

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Title: Radiation Feedback in Accretion Shocks on Young Stars
Authors: de Sá, Lionel; Stehlé, Chantal; Chièze, Jean-Pierre;
Hubeny, Ivan; Lanz, Thierry; Colombo, Salvatore; Cayatte, Véronique;
Ibgui, Laurent; Orlando, Salvatore
2019ASPC..519..281D Altcode:
The first stages of pre-main sequence stars evolution are governed
by exchanges of mass and momentum between the proto-star and its
accretion disk. These quantities remain uncertain due to numerous
unanswered questions concerning the topology of the accretion flow,
its temperature, and its dynamics. In this work, we first present 1D
simulations of accretion columns falling onto a a stellar chromosphere;
the goal is to inspect the feedback of the dynamics of the chromosphere
on the accretion flow. Then, we analyze the effect of radiation
absorption and emission on the flow. The simulations take benefit of
a newly generated base of opacities computed by SYNSPEC code for the
conditions of the present astrophysical process. We finally present
our perspectives on this topic.

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Title: Radiative Signatures from the Cosmos
Authors: Werner, K.; Stehle, C.; Rauch, T.; Lanz, T.
2019ASPC..519.....W Altcode:
No abstract at ADS

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Title: The Galactic Abundance Gradient for the Fe Group Elements in
Early B Stars
Authors: Peters, Geraldine J.; Bouret, Jean-Claude; Lanz, Thierry;
Proffitt, Charles R.
2019hst..prop15869P Altcode:
We propose HST spectroscopic observations of twelve sharp-lined early
B main-sequence stars with galactocentric distances ranging from 3-14
kpc in order to determine the metallicity gradient for the Fe group
elements (Ti, V, Cr, Mn, Fe, Co, Ni) in our galaxy. This will be the
first attempt to establish abundance gradients for all of the Fe-peak
elements found in these young stars, that were formed &lt;100 Myr
ago. The result will be an assessment of the chemical evolution of our
galaxy. Whereas the light elements are mostly delivered to the ISM by
core-collapse supernovae (CCSNe), the Fe group elements are believed
to primarily come from low/intermediate mass binaries containing white
dwarfs that undergo SNe Ia explosions. According to some references,
a single SNe Ia can deliver as much as 0.5 solar masses of pure Fe and
Mn to the ISM compared with about 0.07 solar masses from a CCSNe. The
older stellar populations were formed from an ISM that was enriched
primarily by CCSNe, but the Fe group elements in B stars probably
owe their origin mostly to SN Ia. Recently there has been a great
deal of interest in using the observed Mn/Fe ratio to determine the
progenitors of SNe Ia. Since the SNe models predict that Mn/Fe varies
with metallicity, a good determination of the Mn abundance across our
galaxy would also be useful for SN science. For each Fe group element we
will determine its abundance relative to H and Fe. The unique spectra
will have legacy value, as strong lines from the Fe group elements
(except for a very few features of Fe itself) are found only in the
FUV/NUV. Both COS and STIS will be used. The analysis will be carried
through with TLUSTY/SYNSPEC.

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Title: VizieR Online Data Catalog: GC hot UV-bright stars model
spectra (Moehler+, 2019)
Authors: Moehler, S.; Landsman, W. B.; Lanz, T.; Miller Bertolami,
M. M.
2019yCat..36270034M Altcode:
Optical medium- and low-resolution spectra (flux-calibrated and
normalized) of 13 UV bright stars in globular clusters are presented
together with the atmospheric parameters derived from them. The
following globular clusters have been observed: NGC5139, NGC6121,
NGC6656, NGC6712, NGC6779, NGC7099. The flux-calibrated spectra
were corrected to laboratory wavelengths and their continua were
normalized. Effective temperatures, surface gravities, and helium
abundances were derived from line profile fits and the corresponding
best-fitting model spectra are also presented here. We also present
post-helium core burning evolutionary sequences calculated low mass
stars for [M/H] = -2.3, -2.0, -1.5, -1.0. Grid of post-horizontal
branch evolutionary sequences for four different metallicities. Models
at the Zero Age Horizontal Branch (ZAHB) were constructing by removing
mass at the tip of the Red Giant Branch (RGB). Details about the
initial model on the main sequence are provided in table B.1 of
the article. Each file contains all the sequences with a given
initial metallicity and are named as CDSFeH-1.0.dat,
CDSFeH-1.5.dat, CDSFeH-2.0.dat, and
CDSFeH-2.3.dat (corresponding to metallicities [Fe/H]=-1,
-1.5, -2, -2.3, see Table 8 of the Article) All sequences are
presented at similar timesteps Sequences are presented until
their luminosity drops to 10 solar luminosities. (11 data files).

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Title: Radial abundance gradients in the outer Galactic disk as
traced by main-sequence OB stars
Authors: Bragança, G. A.; Daflon, S.; Lanz, T.; Cunha, K.; Bensby,
T.; McMillan, P. J.; Garmany, C. D.; Glaspey, J. W.; Borges Fernandes,
M.; Oey, M. S.; Hubeny, I.
2019A&A...625A.120B Altcode: 2019arXiv190404340B
Context. Elemental abundance gradients in galactic disks are important
constraints for models of how spiral galaxies form and evolve. However,
the abundance structure of the outer disk region of the Milky Way
is poorly known, which hampers our understanding of the spiral
galaxy that is closest to us and that can be studied in greatest
detail. Young OB stars are good tracers of the present-day chemical
abundance distribution of a stellar population and because of their
high luminosities they can easily be observed at large distances,
making them suitable to explore and map the abundance structure and
gradients in the outer regions of the Galactic disk. Aims: Using
a sample of 31 main-sequence OB stars located between galactocentric
distances 8.4-15.6 kpc, we aim to probe the present-day radial abundance
gradients of the Galactic disk. Methods: The analysis is
based on high-resolution spectra obtained with the MIKE spectrograph
on the Magellan Clay 6.5-m telescope on Las Campanas. We used a
non-NLTE analysis in a self-consistent semi-automatic routine based
on TLUSTY and SYNSPEC to determine atmospheric parameters and chemical
abundances. Results: Stellar parameters (effective temperature,
surface gravity, projected rotational velocity, microturbulence, and
macroturbulence) and silicon and oxygen abundances are presented for 28
stars located beyond 9 kpc from the Galactic center plus three stars
in the solar neighborhood. The stars of our sample are mostly on the
main-sequence, with effective temperatures between 20 800-31 300 K,
and surface gravities between 3.23-4.45 dex. The radial oxygen and
silicon abundance gradients are negative and have slopes of -0.07
dex kpc-1 and -0.09 dex kpc-1, respectively,
in the region 8.4 ≤ RG ≤ 15.6 kpc. Conclusions:
The obtained gradients are compatible with the present-day oxygen and
silicon abundances measured in the solar neighborhood and are consistent
with radial metallicity gradients predicted by chemodynamical models
of Galaxy Evolution for a subsample of young stars located close to
the Galactic plane. Based on data obtained with the Magellan Clay
telescope at the Las Campanas observatory and the ESO/MPI telescope
at La Silla under the ESO-ON agreement.

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Title: New insight on Young Stellar Objects accretion shocks --
a claim for NLTE opacities
Authors: de Sá, Lionel; Chièze, Jean-Pierre; Stehlé, Chantal;
Hubeny, Ivan; Lanz, Thierry; Cayatte, Véronique
2019arXiv190409156D Altcode:
Context. Accreted material onto CTTSs is expected to form a hot
quasi-periodic plasma structure that radiates in X-rays. Simulations
of this phenomenon only partly match with observations. They all rely
on a static model for the chromosphere model and on the assumption
that radiation and matter are decoupled. Aims. We explore the effects
on the structure and on the dynamics of the accretion flow of both a
shock-heated chromosphere and of the coupling between radiation and
hydrodynamics. Methods. We simulate accretion columns falling onto
a stellar chromosphere using the 1D ALE code AstroLabE. This code
solves the hydrodynamics equations along with the two first momenta
equations for radiation transfer, with the help of a dedicated opacity
table for the coupling between matter and radiation. We derive the
total electron and ions densities from collisional-radiative NLTE
ionisation equilibrium. Results. The chromospheric acoustic heating
has an impact on the duration of the cycle and on the structure
of the heated slab. In addition, the coupling between radiation
and hydrodynamics leads to a heating of the accretion flow and the
chromosphere, inducing a possible unburial of the whole column. These
two last conclusions are in agreement with the computed monochromatic
intensity. Both effects (acoustic heating and radiation coupling)
have an influence on the amplitude and temporal variations of the
net X-ray luminosity, which varies between 30 and 94% of the incoming
mechanical energy flux, depending on the model considered.

---------------------------------------------------------
Title: Modelling the Accretion on Young Stars, Recent Results and
Perspectives
Authors: de Sá, L.; Stehlé, C.; Chièze, J. P.; Hubeny, I.; Lanz,
T.; Colombo, S.; Ibgui, L.; Orlando, S.
2019ASSP...55...29D Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Abundances of the Fe Group Elements in AV 304, an Abundance
Standard in the Small Magellanic Cloud
Authors: Peters, Geraldine J.; Lanz, Thierry; Bouret, Jean-Claude;
Proffitt, Charles R.; Adelman, Saul J.; Hubeny, Ivan
2018AAS...23231704P Altcode:
AV 304 is a B0.5 IV field star in the Small Magellanic Cloud with
ultra-sharp spectral lines that has emerged as an abundance standard. We
have combined recent spectroscopic observations from the Cosmic Origins
Spectrograph (COS) on the Hubble Space Telescope with archival data from
the Far Ultraviolet Spectroscopic Explorer (FUSE) and ESO’s VLT/UVES
to determine the abundances of the Fe group elements (Ti, V, Cr, Mn, Fe,
Co, &amp; Ni). The analysis was carried through using the Hubeny/Lanz
NLTE programs TLUSTY/SYNSPEC. The COS observations were secured with the
G130M, G160M, G185M, and G225M gratings. Combined with the FUSE data, we
have achieved spectral coverage in the UV from 950 to 2400 A. Measurable
lines from the Fe group, except for a very few multiplets of Fe II, III
are not observed in optical spectra. The following stellar parameters
were found: Teff = 27500±500 K, log g = 3.7±0.1 cm/s2,
Vturb= 1±1 km/s, and v sin i = 8 ±2 km/s. The Fe abundance appears
to be only slightly lower than the mean depletion in the SMC, but the
other Fe group elements are underabundant by 0.3 dex or more. This study
confirmed the low abundance of nitrogen (-1.25 dex relative to the solar
value) that was reported by Peters &amp; Adelman (ASP Conf. Series, 348,
p. 136, 2006). Whereas the light elements are delivered to the ISM by
core-collapse supernovae (CCSNe), the Fe group elements are believed
to come mostly from low/intermediate mass binaries containing white
dwarfs that undergo SNe Ia explosions. A single SNe Ia can deliver
0.5 solar masses of pure Fe (and maybe Mn) to the ISM compared with
about 0.07 solar masses from a CCSNe. It appears that there is very
little processed material from its interior in the atmosphere of AV
304 and that the star did not form from an interstellar cloud that
was enriched by material from earlier supernova activity. Support from
STScI grants HST-GO-14081.002 and HST-GO-13346.022, and USC’s Women
in Science and Engineering (WiSE) program is greatly appreciated.

---------------------------------------------------------
Title: Mid-infrared observations of O-type stars: spectral morphology
Authors: Marcolino, W. L. F.; Bouret, J. -C.; Lanz, T.; Maia, D. S.;
Audard, M.
2017MNRAS.470.2710M Altcode: 2017arXiv170504244M
We present mid-infrared (mid-IR) observations for a sample of 16 O-type
stars. The data were acquired with the NASA Spitzer Space Telescope,
using the IRS instrument at moderate resolution (R ∼ 600), covering
the range of ∼10-37 μm. Our sample includes early, mid and late O
supergiants and dwarfs. We explore for the first time their mid-IR
spectral morphology in a quantitative way. We use NLTE expanding
atmosphere models to help with line identifications, analyse profile
contributions and line-formation regions. The O supergiants present a
rich emission line spectra. The most intense features are from hydrogen
- 6 α, 7 α and 8 α - that have non-negligible contributions of He I
or He II lines, depending on the spectral type. The spectrum of early
O supergiants is a composite of H I and He II lines, He I lines being
absent. On the other hand, late O supergiants present features composed
mainly by H I and He I lines. All emission lines are formed throughout
the stellar wind. We found that O dwarfs exhibit a featureless mid-IR
spectrum. Two stars of our sample exhibit very similar mid-IR features,
despite having a very different optical spectral classification. The
analysis of O-type stars based on mid-IR spectra alone to infer
spectral classes or to estimate physical parameters may thus be prone
to substantial errors. Our results may therefore inform spectroscopic
observations of massive stars located in heavily obscured regions and
help establish an initial framework for observations of massive stars
using the Mid-Infrared Instrument on the James Webb Space Telescope.

---------------------------------------------------------
Title: Uncrowding R 136 from VLT/SPHERE extreme adaptive optics
Authors: Khorrami, Z.; Vakili, F.; Lanz, T.; Langlois, M.; Lagadec,
E.; Meyer, M. R.; Robbe-Dubois, S.; Abe, L.; Avenhaus, H.; Beuzit,
J. L.; Gratton, R.; Mouillet, D.; Origné, A.; Petit, C.; Ramos, J.
2017A&A...602A..56K Altcode: 2017arXiv170302876K
This paper presents the sharpest near-IR images of the massive cluster
R 136 to date, based on the extreme adaptive optics of the SPHERE
focal instrument implemented on the ESO Very Large Telescope and
operated in its IRDIS imaging mode.The crowded stellar population
in the core of the R 136 starburst compact cluster remains still to
be characterized in terms of individual luminosities, age, mass and
multiplicity. SPHERE/VLT and its high contrast imaging possibilities
open new windows to make progress on these questions.Stacking-up
a few hundreds of short exposures in J and Ks spectral bands over
a field of view (FoV) of 10.9″ × 12.3″ centered on the R 136a1
stellar component, enabled us to carry a refined photometric analysis
of the core of R 136. We detected 1110 and 1059 sources in J and
Ks images respectively with 818 common sources. Thanks to better
angular resolution and dynamic range, we found that more than 62.6%
(16.5%) of the stars, detected both in J and Ks data, have neighbours
closer than 0.2” (0.1”). The closest stars are resolved down to
the full width at half maximum (FWHM) of the point spread function
(PSF) measured by Starfinder. Among resolved and/or detected sources
R 136a1 and R 136c have optical companions and R 136a3 is resolved as
two stars (PSF fitting) separated by 59 ± 2 mas. This new companion
of R 136a3 presents a correlation coefficient of 86% in J and 75% in
Ks. The new set of detected sources were used to re-assess the age and
extinction of R 136 based on 54 spectroscopically stars that have been
recently studied with HST slit-spectroscopy (Crowther et al. &lt;xref
ref-type="bibr" rid="R11"&gt;2016&lt;/xref&gt;, MNRAS, 458, 624)
of the core of this cluster. Over 90% of these 54 sources identified
visual companions (closer than 0.2”). We found the most probable age
and extinction for these sources are 1.8+1.2-0.8
Myr, AJ = (0.45 ± 0.5) mag and AK = (0.2 ± 0.5)
mag within the photometric and spectroscopic error-bars. Additionally,
using PARSEC evolutionary isochrones and tracks, we estimated the
stellar mass range for each detected source (common in J and K data) and
plotted the generalized histogram of mass (MF with error-bars). Using
SPHERE data, we have gone one step further and partially resolved
and studied the initial mass function covering mass range of (3-300)
M⊙ at the age of 1 and 1.5 Myr. The density in the core
of R 136 (0.1-1.4 pc) is estimated and extrapolated in 3D and larger
radii (up to 6 pc). We show that the stars in the core are still
unresolved due to crowding, and the results we obtained are upper
limits. Higher angular resolution is mandatory to overcome these
difficulties. Based on data collected at the European Southern
Observatory, Chile, Guaranteed Time Observation 095.D-0309(K). The catalogue 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/602/A56">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/602/A56</A>

---------------------------------------------------------
Title: TLUSTY User's Guide II: Reference Manual
Authors: Hubeny, Ivan; Lanz, Thierry
2017arXiv170601935H Altcode:
This is the second part of a three-volume guide to TLUSTY and
SYNSPEC. It presents a detailed reference manual for TLUSTY, which
contains a detailed description of basic physical assumptions and
equations used to model an atmosphere, together with an overview of
the numerical methods to solve these equations.

---------------------------------------------------------
Title: A brief introductory guide to TLUSTY and SYNSPEC
Authors: Hubeny, Ivan; Lanz, Thierry
2017arXiv170601859H Altcode:
This is the first of three papers that present a detailed guide for
working with the codes {\sc tlusty} and {\sc synspec} to generate
model stellar atmospheres or accretion disks, and to produce detailed
synthetic spectra. In this paper, we present a very brief manual
intended for casual users who intend to use these codes for simple,
well defined tasks. This paper does not present any background theory,
or a description of the adopted numerical approaches, but instead uses
simple examples to explain how to employ these codes. In particular,
it shows how to produce a simple model atmosphere from the scratch,
or how to improve an existing model by considering more extended model
atoms. This paper also presents a brief guide to the spectrum synthesis
program {\sc synspec}.

---------------------------------------------------------
Title: Towards a Sharper Picture of R136 with SPHERE Extreme
Adaptive Optics
Authors: Khorrami, Z.; Vakili, F.; Lanz, T.; Langlois, M.; Lagadec,
E.; Meyer, M. R.; Gratton, R.; Beuzit, J. -L.; Mouillet, D.
2017Msngr.168...32K Altcode:
The SPHERE extreme adaptive optics instrument was used to observe
the central core of the Large Magellanic Cloud, R136, in the
near-infrared. This challenging observation demonstrated the
capabilities of SPHERE for imaging distant clusters. More than one
thousand sources have been detected in Ks- and J-band images in the
small field of view of IRDIS covering almost 2.7 × 3.1 pc of the core
of R136. Based on isochrone fitting of the colour-magnitude diagram,
ages of 1 and 1.5 Myr for the inner 3-arcsecond core and the outer core
of R136 fit our data best. The mass function slope is -0.96 ± 0.22 over
the mass range of 3 to 300 M⊙. Using SPHERE data, we have
gone one step further in partially resolving the core of R136, but this
is certainly not the final step and higher resolution is still required.

---------------------------------------------------------
Title: TLUSTY User's Guide III: Operational Manual
Authors: Hubeny, Ivan; Lanz, Thierry
2017arXiv170601937H Altcode:
This paper presents a detailed operational manual for TLUSTY. It
provides a guide for understanding the essential features and the basic
modes of operation of the program. To help the user, it is divided
into two parts. The first part describes the most important input
parameters and available numerical options. The second part covers
additional details and a comprehensive description of all physical
and numerical options, and a description of all input parameters,
many of which needed only in special cases.

---------------------------------------------------------
Title: VizieR Online Data Catalog: R136 JKs photometry from VLT/SPHERE
EAO (Khorrami+, 2017)
Authors: Khorrami, Z.; Vakili, F.; Lanz, T.; Langlois, M.; Lagadec,
E.; Meyer, M. R.; Robbe-Dubois, S.; Abe, L.; Avenhaus, H.; Beuzit,
J. L.; Gratton, R.; Mouillet, D.; Origne, A.; Petit, C.; Ramos, J.
2017yCat..36020056K Altcode:
The SPHERE/IRDIS catalog of the common sources between J and Ks-band
data on R136. The ID, Xpix and Ypix are the identification and pixel
position in the IRDIS K and J image. σK and σJ are the total error
(combination of PSF-fitting error, residual errors and the calibration
error) in Ks and J images. CK and CJ are the Correlation coefficients
between the input PSF and the star, in Ks and J data. (1 data
file).

---------------------------------------------------------
Title: Masses of the Planetary Nebula Central Stars in the Galactic
Globular Cluster System from HST Imaging and Spectroscopy
Authors: Jacoby, George H.; De Marco, Orsola; Davies, James;
Lotarevich, I.; Bond, Howard E.; Harrington, J. Patrick; Lanz, Thierry
2017ApJ...836...93J Altcode: 2017arXiv170103516J
The globular cluster (GC) system of our Galaxy contains four planetary
nebulae (PNe): K 648 (or Ps 1) in M15, IRAS 18333-2357 in M22, JaFu 1 in
Pal 6, and JaFu 2 in NGC 6441. Because single-star evolution at the low
stellar mass of present-epoch GCs was considered incapable of producing
visible PNe, their origin presented a puzzle. We imaged the PN JaFu
1 with the Hubble Space Telescope (HST) to obtain photometry of its
central star (CS) and high-resolution morphological information. We
imaged IRAS 18333-2357 with better depth and resolution, and we
analyzed its archival HST spectra to constrain its CS temperature
and luminosity. All PNe in Galactic GCs now have quality HST data,
allowing us to improve CS mass estimates. We find reasonably consistent
masses between 0.53 and 0.58 M ⊙ for all four objects,
though estimates vary when adopting different stellar evolutionary
calculations. The CS mass of IRAS 18333-2357, though, depends
strongly on its temperature, which remains elusive due to reddening
uncertainties. For all four objects, we consider their CS and nebula
masses, their morphologies, and other incongruities to assess the
likelihood that these objects formed from binary stars. Although
generally limited by uncertainties (∼0.02 M ⊙) in
post-AGB tracks and core mass versus luminosity relations, the high-mass
CS in K 648 indicates a binary origin. The CS of JaFu 1 exhibits
compact, bright [O III] and Hα emission, like EGB 6, suggesting a
binary companion or disk. Evidence is weaker for a binary origin of
JaFu 2. Based, in part, on observations made with the NASA/ESA
Hubble Space Telescope, obtained [from the Data Archive] at the Space
Telescope Science Institute, which is operated by the Association of
Universities for Research in Astronomy, Inc., under NASA contract NAS
5-26555. These observations are associated with program GO-11558.

---------------------------------------------------------
Title: Probing Supernovae Chemical Yields in Low Metallicity
Environments with UV Spectroscopy of Magellanic Cloud B-type Stars
Authors: Lanz, Thierry; Adelman, Saul J.; Bouret, Jean-Claude;
Gulliver, Austin F.; Hill, Graham; Hubeny, Ivan; Peters, Geraldine J.;
Pickering, Juliet C.; Proffitt, Charles R.
2016hst..prop14081L Altcode:
Spectrum synthesis studies of the UV spectra of sharp-lined
main sequence B stars provide astronomers with some of the best
determinations of the abundances of the light, Fe group, and neutron
capture elements. B stars are therefore best-suited to study the
chemical evolution of the Magellanic Clouds. But the HST archive
is virtually devoid of high resolution spectra of such objects. We
propose FUV and NUV observations with the COS G130M, G160M, G185M,
and G225M gratings. The four program stars have been observed with
the FUSE spacecraft, hence this project will produce continuous
high-resolution spectral coverage from 950 to 2400 A and provide a
permanent archive of fundamental spectra from which ground-breaking
studies of the Magellanic Clouds can be performed in the decades
to come. This limited program aims at producing an extragalactic,
low-metallicity counterpart to the bright star library of early B
stars that is currently being obtained as part of the HST Cycle 21
Treasury program "Advanced Spectral Library II: Hot Stars" (GO 13346,
PI T. Ayres). Spectral lines from most Fe group and s-process
elements are found only in the UV region in B stars and information
on their abundances is important for studying the chemical evolution
of a galaxy, computing opacities for stellar evolution calculations,
and assessing the validity of theoretical calculations of explosive
nucleosynthesis. Comparing the derived abundances of iron-peak and
heavier elements in galactic and Magellanic Cloud B main sequence
stars will provide an empirical probe of chemical yields ejected by
evolved stars and supernovae in different environments.

---------------------------------------------------------
Title: Grown-up stars physics with MATISSE
Authors: Millour, F.; Hron, J.; Chiavassa, A.; Weigelt, G.; Soulain,
A.; Khorrami, Z.; Meilland, A.; Nardetto, N.; Paladini, C.; Domiciano
de Souza, A.; Niccolini, G.; Hofmann, K. -H.; Schertl, D.; Stee, P.;
Bendjoya, P.; Thévenin, F.; Vakili, F.; Berio, P.; Lanz, T.; Matter,
A.; Cruzalèbes, P.; Petrov, R.; Lopez, B.
2016SPIE.9907E..3QM Altcode: 2016arXiv160702393M
MATISSE represents a great opportunity to image the environment around
massive and evolved stars. This will allow one to put constraints
on the circumstellar structure, on the mass ejection of dust and its
reorganization, and on the dust-nature and formation processes. MATISSE
measurements will often be pivotal for the understanding of large
multiwavelength datasets on the same targets collected through
many high-angular resolution facilities at ESO like sub-millimeter
interferometry (ALMA), near-infrared adaptive optics (NACO, SPHERE),
interferometry (PIONIER, GRAVITY), spectroscopy (CRIRES), and
mid-infrared imaging (VISIR). Among main sequence and evolved stars,
several cases of interest have been identified that we describe in
this paper.

---------------------------------------------------------
Title: An overview of the mid-infrared spectro-interferometer MATISSE:
science, concept, and current status
Authors: Matter, A.; Lopez, B.; Antonelli, P.; Lehmitz, M.; Bettonvil,
F.; Beckmann, U.; Lagarde, S.; Jaffe, W.; Petrov, R.; Berio, P.;
Millour, F.; Robbe-Dubois, S.; Glindemann, A.; Bristow, P.; Schoeller,
M.; Lanz, T.; Henning, T.; Weigelt, G.; Heininger, M.; Morel, S.;
Cruzalebes, P.; Meisenheimer, K.; Hofferbert, R.; Wolf, S.; Bresson,
Y.; Agocs, T.; Allouche, F.; Augereau, J. -C.; Avila, G.; Bailet, C.;
Behrend, J.; van Belle, G.; Berger, J. -P.; van Boekel, R.; Bourget,
P.; Brast, R.; Clausse, J. -M.; Connot, C.; Conzelmann, R.; Csepany,
G.; Danchi, W. C.; Delbo, M.; Dominik, C.; van Duin, A.; Elswijk, E.;
Fantei, Y.; Finger, G.; Gabasch, A.; Gonté, F.; Graser, U.; Guitton,
F.; Guniat, S.; De Haan, M.; Haguenauer, P.; Hanenburg, H.; Hofmann,
K. -H.; Hogerheijde, M.; ter Horst, R.; Hron, J.; Hummel, C.; Isderda,
J.; Ives, D.; Jakob, G.; Jasko, A.; Jolley, P.; Kiraly, S.; Kragt,
J.; Kroener, T.; Kroes, G.; Kuindersma, S.; Labadie, L.; Laun, W.;
Leinert, C.; Lizon, J. -L.; Lucuix, C.; Marcotto, A.; Martinache,
F.; Martinot-Lagarde, G.; Mauclert, N.; Mehrgan, L.; Meilland, A.;
Mellein, M.; Menardi, S.; Merand, A.; Neumann, U.; Nussbaum, E.;
Ottogalli, S.; Palsa, R.; Panduro, J.; Pantin, E.; Percheron, I.;
Phan Duc, T.; Pott, J. -U.; Pozna, E.; Roelfsema, R.; Rupprecht, G.;
Schertl, D.; Schmidt, C.; Schuil, M.; Spang, A.; Stegmeier, J.; Tromp,
N.; Vakili, F.; Vannier, M.; Wagner, K.; Venema, L.; Woillez, J.
2016SPIE.9907E..0AM Altcode: 2016arXiv160802350M
MATISSE is the second-generation mid-infrared spectrograph and imager
for the Very Large Telescope Interferometer (VLTI) at Paranal. This
new interferometric instrument will allow significant advances by
opening new avenues in various fundamental research fields: studying
the planet-forming region of disks around young stellar objects,
understanding the surface structures and mass loss phenomena affecting
evolved stars, and probing the environments of black holes in active
galactic nuclei. As a first breakthrough, MATISSE will enlarge the
spectral domain of current optical interferometers by offering the L
and M bands in addition to the N band. This will open a wide wavelength
domain, ranging from 2.8 to 13 μm, exploring angular scales as small
as 3 mas (L band) / 10 mas (N band). As a second breakthrough, MATISSE
will allow mid-infrared imaging - closure-phase aperture-synthesis
imaging - with up to four Unit Telescopes (UT) or Auxiliary Telescopes
(AT) of the VLTI. Moreover, MATISSE will offer a spectral resolution
range from R ∼ 30 to R ∼ 5000. Here, we present one of the main
science objectives, the study of protoplanetary disks, that has driven
the instrument design and motivated several VLTI upgrades (GRA4MAT
and NAOMI). We introduce the physical concept of MATISSE including
a description of the signal on the detectors and an evaluation of
the expected performances. We also discuss the current status of
the MATISSE instrument, which is entering its testing phase, and the
foreseen schedule for the next two years that will lead to the first
light at Paranal.

---------------------------------------------------------
Title: Science with MATISSE
Authors: Wolf, Sebastian; Lopez, Bruno; Augereau, Jean-Charles;
Delbo, Marco; Dominik, Carsten; Henning, Thomas; Hofmann, Karl-Heinz;
Hogerheijde, Michiel; Hron, Josef; Jaffe, Walter; Lanz, Thierry;
Meisenheimer, Klaus; Millour, Florentin; Pantin, Eric; Petrov, Roman;
Schertl, Dieter; van Boekel, Roy; Weigelt, Gerd; Chiavassa, Andrea;
Juhasz, Attila; Matter, Alexis; Meilland, Anthony; Nardetto, Nicolas;
Paladini, Claudia
2016SPIE.9907E..3SW Altcode: 2016arXiv160906152W
We present an overview of the scientific potential of MATISSE, the
Multi Aperture mid-Infrared SpectroScopic Experiment for the Very
Large Telescope Interferometer. For this purpose we outline selected
case studies from various areas, such as star and planet formation,
active galactic nuclei, evolved stars, extrasolar planets, and solar
system minor bodies and discuss strategies for the planning and analysis
of future MATISSE observations. Moreover, the importance of MATISSE
observations in the context of complementary high-angular resolution
observations at near-infrared and submillimeter/millimeter wavelengths
is highlighted.

---------------------------------------------------------
Title: A Definitive Test of Rotational Mixing in Massive Stars
Authors: Proffitt, Charles R.; Brott, Ines; Cunha, Katia; Dufton,
Philip; Langer, Norbert; Lanz, Thierry; Lennon, Daniel J.; Simon-Diaz,
Sergio; de Mink, Selma E.
2016hst..prop14673P Altcode:
CNO processed surface material found in OB stars may originate either
from internal mixing or from binary interaction, but incomplete boron
depletion is an unambiguous sign of internal mixing. Existing boron
observations indeed suggest that internal mixing occurs in some stars
at a level that is consistent with the low end of the efficiency
range predicted by the different models of rotationally driven
mixing. However, current results are too sparse to directly confirm
the expected relation between boron depletion and rotation, and leave
room to interpret boron depletion through other mixing processes. We
propose to observe boron in ten rather rapidly rotating early-B stars
in the 10 Myr old open cluster NGC 3293. Together with our previous
data on stars in this cluster, this increased sample with an expanded
range of V sin(i) values will provide a definitive test of rotational
mixing, and --- assuming that rotation actually drives the expected
mixing --- will allow for a tight calibration of its efficiency, which
is of critical importance for modeling the interior of massive stars,
with wide implications for their advanced evolutionary stages.

---------------------------------------------------------
Title: Before the Burst: The Properties of Rapidly Rotating,
Massive Supergiants
Authors: Bouret, Jean-Claude; Hillier, D. John; Depagne, Eric;
Fullerton, Alexander W.; Herve, Anthony; Lanz, Thierry; Marcolino,
Wagner; Martins, Fabrice; Walborn, Nolan R.
2016hst..prop14683B Altcode:
Rapidly rotating, evolved massive stars are thought to be the
progenitors of long gamma-ray bursts (LGRBs). Their rapid rotation
may be the result of (a) reduced angular-momentum loss by a single
star due to reduced mass loss at low metallicity; or (b) spin-up
via mass and angular momentum transfer from a companion or a stellar
merger. Independent of their formation mechanism, the properties of
their stellar winds are strongly affected by their rapid rotation. We
have recently obtained optical spectroscopy to study the properties
of a sample of ten fast rotating, evolved massive stars in the Large
Magellanic Cloud. Here, we propose to use HST FUV spectroscopy to
determine the wind properties of these objects. The information needed
to characterize the stellar winds of massive stars is encoded in
FUV spectra. Establishing the properties of these rotating outflows
is a critical issue, since the present uncertainties restrict
our understanding of the evolution of the angular momentum, the
initial-to-final mass relation, and the ultimate fate of a massive
star. By fitting line profiles from sophisticated model atmospheres
to the FUV spectra, we will measure photospheric CNO abundances, probe
the rotation law of the stellar wind, and study the influence of rapid
rotation on the profiles of wind lines, and hence on the determination
of mass-loss rates. We will investigate how rapid rotation affects the
onset of clumping, the presence of which reduces estimates of mass-loss
rates. This program will enable decisive steps toward determining the
nature of rapidly rotating, evolved, massive stars, as required to
evaluate their suitability as progenitors of LGRBs.

---------------------------------------------------------
Title: VLT/SPHERE deep insight of NGC 3603's core: Segregation
or confusion?
Authors: Khorrami, Z.; Lanz, T.; Vakili, F.; Lagadec, E.; Langlois,
M.; Brandner, W.; Chesneau, O.; Meyer, M. R.; Carbillet, M.; Abe, L.;
Mouillet, D.; Beuzit, J. L.; Boccaletti, A.; Perrot, C.; Thalmann,
C.; Schmid, H. -M.; Pavlov, A.; Costille, A.; Dohlen, K.; Le Mignant,
D.; Petit, C.; Sauvage, J. F.
2016A&A...588L...7K Altcode: 2016arXiv160300457K
We present new near-infrared photometric measurements of the core
of the young massive cluster NGC 3603 obtained with extreme adaptive
optics. The data were obtained with the SPHERE instrument mounted on
ESO's Very Large Telescope, and cover three fields in the core of this
cluster. We applied a correction for the effect of extinction to our
data obtained in the J and K broadband filters and estimated the mass
of detected sources inside the field of view of SPHERE/IRDIS, which
is 13.5”×13.5”. We derived the mass function (MF) slope for each
spectral band and field. The MF slope in the core is unusual compared to
previous results based on HST and VLT observations. The average slope in
the core is estimated as -1.06 ± 0.26 for the main sequence stars with
3.5 M⊙ &lt; M &lt; 120M⊙. Thanks to the SPHERE
extreme adaptive optics, 814 low-mass stars were detected to estimate
the MF slope for the pre-main sequence stars with 0.6 M⊙
&lt; M &lt; 3.5M⊙, Γ = -0.54 ± 0.11 in the K-band
images in two fields in the core of the cluster. For the first time,
we derive the MF of the very core of the NGC 3603 young cluster for
masses in the range 0.6-120 M⊙. Previous studies were
either limited by crowding, lack of dynamic range, or a combination of
both. Based on data collected at the European Southern Observatory,
Chile (guaranteed time observation 095.D-0309(A) and 095.D-0309(E))

---------------------------------------------------------
Title: The Massive Stars Nursery R136
Authors: Khorrami, Z.; Vakili, F.; Chesneau, O.; Lanz, T.
2015EAS....71..331K Altcode:
As most stars are born in a clustered mode, young massive star clusters
are the best places to find and study the formation and evolution
of massive stars. R136 is one of the most massive nearby clusters in
the LMC. It contains at least 72 known O and Wolf-Rayet stars. These
young stars are usually embedded in dust and gas so that correcting
the local extinction plays an important role to estimate their mass
from their luminosity. The extinction is derived for 26 O stars in
different HST filters using TLUSTY model atmospheres of O stars. We
derived the stellar masses, and hence the Mass Function (MF), using HST
multi-color photometry. In parallel, we simulated series of R136-like
clusters using the NBODY6 code to test the segregation scenario for
R136. We could check whether massive stars are preferentially formed
in the cluster core or formed homogeneously. By comparing the surface
brightness profiles (SBP) of simulated clusters mimicking R136 with
HST data, we could determine which scenario best represents R136. We
present here a method that we used to connect the results of the NBODY6
simulations to R136 HST imaging data. The results of these studies
bring a new insight to the understanding of R136 and similar clusters,
pending future VLT and E-ELT high-contrast imaging observations at
the diffraction limit at visible and IR wavelengths.

---------------------------------------------------------
Title: Foreword
Authors: Lagadec, E.; Millour, F.; Lanz, T.
2015EAS....71....1L Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Wind and Photosphere of the Unique DO White Dwarf RE
J0503-289
Authors: Barstow, Martin; Holberg, Jay B.; Hubeny, Ivan; Lanz, Thierry;
Sion, Edward M.
2015hst..prop.6628B Altcode:
We propose to obtain high S/N GHRS spectra of the key He-richwhite dwarf
RE J0503-289. These observations have twoprincipal aims. The first
is to investigate the phenomenon ofmass loss in this star. Excluding
planetary nebulae nuclei, REJ0503-289 is the only white dwarf in which
evidence of ongoingmass loss has been observed with IUE. However, the
reporteddetection is near the limit of the sensitivity of IUEpreventing
study of the wind during its less intense periods.Mass loss may well be
the missing ingredient in thetheoretical description of heavy element
abundances in whitedwarfs. RE J0503-289 is the only known white dwarf
where thiseffect can be studied as an on going phenomenon. The secondaim
is to search for Fe and Ni in the photosphere of thisstar. The presence
of these elements is required to explainthe large flux deficit observed
in the EUV. Detailed models,including those elements so far detected (C,
N, O and Si),substantially over predict EUV flux levels. RE J0503-289
isalso the only DO white dwarf for which a joint analysis of EUVand UV
data will be possible and for which we can conduct adefinitive study
of the composition and structure of thephotosphere.

---------------------------------------------------------
Title: Probing Supernovae Chemical Yields in Low Metallicity
Environments with UV Spectroscopy of Magellanic Cloud B-type Stars
Authors: Lanz, Thierry
2015hst..prop14081L Altcode:
Spectrum synthesis studies of the UV spectra of sharp-lined
main sequence B stars provide astronomers withsome of the best
determinations of the abundances of the light, Fe group, and neutron
capture elements. B stars are therefore best-suited to study the
chemical evolution of the Magellanic Clouds. But the HST archive
is virtually devoid of high resolution spectra of such objects. We
propose FUV and NUV observations with the COS G130M, G160M, G185M,
and G225M gratings. The four program stars have been observed with
the FUSE spacecraft, hence this project will produce continuous
high-resolution spectral coverage from 950 to 2400 A and provide a
permanent archive of fundamental spectra from which ground-breaking
studies of the Magellanic Clouds can be performed in the decades
to come. This limited program aims at producing an extragalactic,
low-metallicity counterpart to the bright star library of early B
stars that is currently being obtained as part of the HST Cycle 21
Treasury program Advanced Spectral Library II: Hot Stars (GO 13346,
PI T. Ayres).Spectral lines from most Fe group and s-process elements
are found only in the UV region in B stars and information on their
abundances is important for studying the chemical evolution of a
galaxy, computing opacities for stellar evolution calculations,
and assessing the validity of theoretical calculations of explosive
nucleosynthesis. Comparing the derived abundances of iron-peak and
heavier elements in galactic and Magellanic Cloud B main sequence
stars will provide an empirical probe of chemical yields ejected by
evolved stars and supernovae in different environments.

---------------------------------------------------------
Title: No breakdown of the radiatively driven wind theory in
low-metallicity environments
Authors: Bouret, J. -C.; Lanz, T.; Hillier, D. J.; Martins, F.;
Marcolino, W. L. F.; Depagne, E.
2015MNRAS.449.1545B Altcode: 2015arXiv150205641B
We present a spectroscopic analysis of Hubble Space Telescope/Cosmic
Origins Spectrograph observations of three massive stars in the
low metallicity dwarf galaxies IC 1613 and WLM. These stars, were
previously observed with Very Large Telescope (VLT)/X-shooter by
Tramper et al., who claimed that their mass-loss rates are higher than
expected from theoretical predictions for the underlying metallicity. A
comparison of the far ultraviolet (FUV) spectra with those of stars
of similar spectral types/luminosity classes in the Galaxy, and the
Magellanic Clouds provides a direct, model-independent check of the
mass-loss-metallicity relation. Then, a quantitative spectroscopic
analysis is carried out using the non-LTE (NLTE) stellar atmosphere
code CMFGEN. We derive the photospheric and wind characteristics,
benefiting from a much better sensitivity of the FUV lines to wind
properties than Hα. Iron and CNO abundances are measured, providing
an independent check of the stellar metallicity. The spectroscopic
analysis indicates that Z/Z⊙ = 1/5, similar to a Small
Magellanic Cloud-type environment, and higher than usually quoted for IC
1613 and WLM. The mass-loss rates are smaller than the empirical ones
by Tramper et al., and those predicted by the widely used theoretical
recipe by Vink et al. On the other hand, we show that the empirical,
FUV-based, mass-loss rates are in good agreement with those derived
from mass fluxes computed by Lucy. We do not concur with Tramper et
al. that there is a breakdown in the mass-loss-metallicity relation.

---------------------------------------------------------
Title: Non-LTE Abundances in OB stars: Preliminary Results for 5
Stars in the Outer Galactic Disk
Authors: Bragançca, G. A.; Lanz, T.; Daflon, S.; Cunha, K.; Garmany,
C. D.; Glaspey, J. W.; Borges Fernandes, M.; Oey, M. S.; Bensby, T.;
Hubeny, I.
2015IAUS..307...90B Altcode: 2014arXiv1408.5807B
The aim of this study is to analyse and determine elemental abundances
for a large sample of distant B stars in the outer Galactic disk in
order to constrain the chemical distribution of the Galactic disk and
models of chemical evolution of the Galaxy. Here, we present preliminary
results on a few stars along with the adopted methodology based on
securing simultaneous O and Si ionization equilibria with consistent
NLTE model atmospheres.

---------------------------------------------------------
Title: 3D Gray Radiative Properties of a Radiation Hydrodynamic
Model of a YSO Accretion Shock
Authors: Ibgui, L.; de Sá, L.; Stehlé, C.; Chièze, J. -P.; Orlando,
S.; Hubeny, I.; Lanz, T.; Matsakos, T.; González, M.; Bonito, R.
2014ASPC..488...83I Altcode:
We present preliminary results of radiative properties of a 1D gray
radiation hydrodynamic (RHD) model of an accretion shock on a young
stellar object (YSO). This model takes into account the transition
between the collisional equilibrium regime (local thermodynamic
equilibrium, LTE), and the coronal equilibrium regime. Based on the
1D planar structure, we built a 3D cylindrical one. Most notably,
the post-shock region obtained in our case is far less extended (by
a factor of 10 000) than the typical one obtained with models that
assume gray optically thin radiative losses. Moreover, we find that
the column is optically thin in its longitudinal dimension, and in the
transverse dimension, except over an extremely narrow region (≲ 700
m). Consequently, still under the gray assumption, the photons emitted
by the hot slab can propagate through the column and escape freely in
all directions, including towards the chromosphere. The radiation flux
has therefore components that are perpendicular to the accretion column,
which demonstrates that a multidimensional (2D or 3D) radiative model
is necessary for such a cylindrical structure. This study needs to
be taken forward and expanded, by improving the radiative treatment
of the RHD model, through relaxation of both the gray and the LTE
approximations for the calculation of opacities, in order to clarify
the structure of the post-shock region, which is a major source of
emission probed by observations.

---------------------------------------------------------
Title: An Overview of the MATISSE Instrument — Science, Concept
and Current Status
Authors: Lopez, B.; Lagarde, S.; Jaffe, W.; Petrov, R.; Schöller,
M.; Antonelli, P.; Beckmann, U.; Berio, P.; Bettonvil, F.;
Glindemann, A.; Gonzalez, J. -C.; Graser, U.; Hofmann, K. -H.;
Millour, F.; Robbe-Dubois, S.; Venema, L.; Wolf, S.; Henning, T.;
Lanz, T.; Weigelt, G.; Agocs, T.; Bailet, C.; Bresson, Y.; Bristow,
P.; Dugué, M.; Heininger, M.; Kroes, G.; Laun, W.; Lehmitz, M.;
Neumann, U.; Augereau, J. -C.; Avila, G.; Behrend, J.; van Belle, G.;
Berger, J. -P.; van Boekel, R.; Bonhomme, S.; Bourget, P.; Brast,
R.; Clausse, J. -M.; Connot, C.; Conzelmann, R.; Cruzalèbes, P.;
Csepany, G.; Danchi, W.; Delbo, M.; Delplancke, F.; Dominik, C.;
van Duin, A.; Elswijk, E.; Fantei, Y.; Finger, G.; Gabasch, A.; Gay,
J.; Girard, P.; Girault, V.; Gitton, P.; Glazenborg, A.; Gonté, F.;
Guitton, F.; Guniat, S.; De Haan, M.; Haguenauer, P.; Hanenburg, H.;
Hogerheijde, M.; ter Horst, R.; Hron, J.; Hugues, Y.; Hummel, C.;
Idserda, J.; Ives, D.; Jakob, G.; Jasko, A.; Jolley, P.; Kiraly,
S.; Köhler, R.; Kragt, J.; Kroener, T.; Kuindersma, S.; Labadie,
L.; Leinert, C.; Le Poole, R.; Lizon, J. -L.; Lucuix, C.; Marcotto,
A.; Martinache, F.; Martinot-Lagarde, G.; Mathar, R.; Matter, A.;
Mauclert, N.; Mehrgan, L.; Meilland, A.; Meisenheimer, K.; Meisner,
J.; Mellein, M.; Menardi, S.; Menut, J. -L.; Merand, A.; Morel, S.;
Mosoni, L.; Navarro, R.; Nussbaum, E.; Ottogalli, S.; Palsa, R.;
Panduro, J.; Pantin, E.; Parra, T.; Percheron, I.; Duc, T. P.; Pott,
J. -U.; Pozna, E.; Przygodda, F.; Rabbia, Y.; Richichi, A.; Rigal, F.;
Roelfsema, R.; Rupprecht, G.; Schertl, D.; Schmidt, C.; Schuhler, N.;
Schuil, M.; Spang, A.; Stegmeier, J.; Thiam, L.; Tromp, N.; Vakili,
F.; Vannier, M.; Wagner, K.; Woillez, J.
2014Msngr.157....5L Altcode:
MATISSE, a second generation Very Large Telescope Interferometer (VLTI)
instrument, is a combined imager and spectrograph for interferometry
in the 3-5 μm region (L- and M-bands) and the 8-13 μm window
(N-band). MATISSE builds on the experience gained with the VLTI's
first generation instruments. It employs multi-axial beam combination
while also providing wavelength differential visibility and phase,
and closure-phase aperture-synthesis imaging at a range of spectral
resolutions. MATISSE is designed for a broad range of science goals,
and its potential for studies of the discs around young stars and
active galactic nuclei are highlighted. The instrument concept and
operating modes are described; construction is in progress towards
installation at the VLTI in 2016.

---------------------------------------------------------
Title: 3D YSO accretion shock simulations: a study of the magnetic,
chromospheric and stochastic flow effects
Authors: Matsakos, T.; Chièze, J. -P.; Stehlé, C.; González, M.;
Ibgui, L.; de Sá, L.; Lanz, T.; Orlando, S.; Bonito, R.; Argiroffi,
C.; Reale, F.; Peres, G.
2014IAUS..302...66M Altcode:
The structure and dynamics of young stellar object (YSO) accretion
shocks depend strongly on the local magnetic field strength and
configuration, as well as on the radiative transfer effects responsible
for the energy losses. We present the first 3D YSO shock simulations
of the interior of the stream, assuming a uniform background magnetic
field, a clumpy infalling gas, and an acoustic energy flux flowing at
the base of the chromosphere. We study the dynamical evolution and
the post-shock structure as a function of the plasma-beta (thermal
pressure over magnetic pressure). We find that a strong magnetic field
(~hundreds of Gauss) leads to the formation of fibrils in the shocked
gas due to the plasma confinement within flux tubes. The corresponding
emission is smooth and fully distinguishable from the case of a weak
magnetic field (~tenths of Gauss) where the hot slab demonstrates
chaotic motion and oscillates periodically.

---------------------------------------------------------
Title: Type II Plateau supernovae as metallicity probes of the
Universe
Authors: Dessart, L.; Gutierrez, C. P.; Hamuy, M.; Hillier, D. J.;
Lanz, T.; Anderson, J. P.; Folatelli, G.; Freedman, W. L.; Ley,
F.; Morrell, N.; Persson, S. E.; Phillips, M. M.; Stritzinger, M.;
Suntzeff, N. B.
2014MNRAS.440.1856D Altcode: 2014arXiv1403.1167D; 2014MNRAS.tmp..580D
We explore a method for metallicity determinations based on quantitative
spectroscopy of Type II-Plateau supernovae (SNe II-P). For consistency,
we first evolve a set of 15 M⊙ main-sequence stars at
0.1, 0.4, 1, and 2 times the solar metallicity. At the onset of core
collapse, we trigger a piston-driven explosion and model the resulting
ejecta and radiation. Our theoretical models of such red supergiant
star explosions at different metallicity show that synthetic spectra
of SNe II-P possess optical signatures during the recombination phase
that are sensitive to metallicity variations. This sensitivity can be
quantified and the metallicity inferred from the strengths of metal-line
absorptions. Furthermore, these signatures are not limited to O,
but also include Na, Ca, Sc, Ti, or Fe. When compared to a sample
of SNe II-P from the Carnegie SN Project and previous SN followup
programmes, we find that most events lie at a metallicity between 0.4
and 2 times solar, with a marked scarcity of SN II-P events at small
magellanic cloud metallicity. This most likely reflects the paucity
of low-metallicity star-forming regions in the local Universe. SNe II-P have high-plateau luminosities that make them observable
spectroscopically at large distances. Because they exhibit signatures
of diverse metal species, in the future they may offer a means to
constrain the evolution of the composition (e.g. the O/Fe ratio)
in the Universe out to a redshift of 1 and beyond.

---------------------------------------------------------
Title: The Discovery of Pulsating Hot Subdwarfs in NGC 2808
Authors: Brown, T.; Landsman, W.; Randall, S.; Sweigart, A.; Lanz,
T.; Hubeny, I.
2014ASPC..481...29B Altcode:
We present preliminary results of a Hubble Space Telescope program
to search for pulsating hot subdwarfs in the core of NGC 2808. These
observations, obtained in March of 2013, were motivated by the recent
discovery of such stars in the outskirts of ω Cen. Both ω Cen and
NGC 2808 are massive globular clusters exhibiting complex stellar
populations and large numbers of extreme horizontal branch stars. Our
far-UV photometric monitoring of over 100 UV-bright stars has revealed
at least six pulsating subdwarfs with periods ranging from 100 to
150 seconds. In the UV color-magnitude diagram of NGC 2808, all six
of these stars lie immediately below the canonical horizontal branch,
a region populated by the subluminous “blue hook” stars. Three of
these six pulsators also have low-resolution far-UV spectroscopy that
is sufficient to broadly constrain their atmospheric abundances and
effective temperatures. Curiously, the spectroscopic and photometric
data do not exhibit the uniformity one might expect from a well-defined
instability strip.

---------------------------------------------------------
Title: 3D numerical modeling of YSO accretion shocks
Authors: Matsakos, T.; Chièze, J. -P.; Stehlé, C.; González, M.;
Ibgui, L.; de Sá, L.; Lanz, T.; Orlando, S.; Bonito, R.; Argiroffi,
C.; Reale, F.; Peres, G.
2014EPJWC..6404003M Altcode:
The dynamics of YSO accretion shocks is determined by radiative
processes as well as the strength and structure of the magnetic
field. A quasi-periodic emission signature is theoretically expected
to be observed, but observations do not confirm any such pattern. In
this work, we assume a uniform background field, in the regime of
optically thin energy losses, and we study the multi-dimensional shock
evolution in the presence of perturbations, i.e. clumps in the stream
and an acoustic energy flux flowing at the base of the chromosphere. We
perform 3D MHD simulations using the PLUTO code, modelling locally the
impact of the infalling gas onto the chromosphere. We find that the
structure and dynamics of the post-shock region is strongly dependent
on the plasma-beta (thermal over magnetic pressure), different values
of which may give distinguishable emission signatures, relevant for
observations. In particular, a strong magnetic field effectively
confines the plasma inside its flux tubes and leads to the formation
of quasi-independent fibrils. The fibrils may oscillate out of phase
and hence the sum of their contributions in the emission results in
a smooth overall profile. On the contrary, a weak magnetic field is
not found to have any significant effect on the shocked plasma and the
turbulent hot slab that forms is found to retain its periodic signature.

---------------------------------------------------------
Title: 3D Gray Radiative Properties of Accretion Shocks in Young
Stellar Objects
Authors: Ibgui, L.; Orlando, S.; Stehlé, C.; Chièze, J. -P.; Hubeny,
I.; Lanz, T.; de Sá, L.; Matsakos, T.; González, M.; Bonito, R.
2014EPJWC..6404005I Altcode:
We address the problem of the contribution of radiation to the structure
and dynamics of accretion shocks on Young Stellar Objects. Solving the
3D RTE (radiative transfer equation) under our "gray LTE approach",
i.e., using appropriate mean opacities computed in local thermodynamic
equilibrium, we post-process the 3D MHD (magnetohydrodynamic) structure
of an accretion stream impacting the stellar chromosphere. We find a
radiation flux of ten orders of magnitude larger than the accreting
energy rate, which is due to a large overestimation of the radiative
cooling. A gray LTE radiative transfer approximation is therefore
not consistent with the given MHD structure of the shock. Further
investigations are required to clarify the role of radiation, by
relaxing both the gray and LTE approximations in RHD (radiation
hydrodynamics) simulations. Post-processing the obtained structures
through the resolution of the non-LTE monochromatic RTE will provide
reference radiation quantities against which RHD approximate solutions
will be compared.

---------------------------------------------------------
Title: Accretion shock stability on a dynamically heated YSO
atmosphere with radiative transfer
Authors: de Sá, Lionel; Chièze, Jean-Pierre; Stehlé, Chantal;
Matsakos, Titos; Ibgui, Laurent; Lanz, Thierry; Hubeny, Ivan
2014EPJWC..6404002D Altcode:
Theory and simulations predict Quasi-Periodic Oscillations of
shocks which develop in magnetically driven accretion funnels
connecting the stellar disc to the photosphere of Young Stellar
Objects (YSO). X-ray observations however do not show evidence of
the expected periodicity. We examine here, in a first attempt, the
influence of radiative transfer on the evolution of material impinging
on a dynamically heated stellar atmosphere, using the 1D ALE-RHD code
ASTROLABE. The mechanical shock heating mechanism of the chromosphere
only slightly perturbs the flow. We also show that, since the impacting
flow, and especially the part which penetrates into the chromosphere,
is not treated as a purely radiating transparent medium, a sufficiently
efficient coupling between gas and radiation may affect or even suppress
the oscillations of the shocked column. This study shows the importance
of the description of the radiation effects in the hydrodynamics and
of the accuracy of the opacities for an adequate modeling.

---------------------------------------------------------
Title: The Discovery of Pulsating Hot Subdwarfs in NGC 2808
Authors: Brown, Thomas M.; Landsman, Wayne B.; Randall, Suzanna K.;
Sweigart, Allen V.; Lanz, Thierry
2013ApJ...777L..22B Altcode: 2013arXiv1310.0835B
We present the results of a Hubble Space Telescope program to
search for pulsating hot subdwarfs in the core of NGC 2808. These
observations were motivated by the recent discovery of such stars
in the outskirts of ω Cen. Both NGC 2808 and ω Cen are massive
globular clusters exhibiting complex stellar populations and large
numbers of extreme horizontal branch stars. Our far-UV photometric
monitoring of over 100 hot evolved stars has revealed six pulsating
subdwarfs with periods ranging from 85 to 149 s and UV amplitudes of
2.0%-6.8%. In the UV color-magnitude diagram of NGC 2808, all six of
these stars lie immediately below the canonical horizontal branch,
a region populated by the subluminous "blue-hook" stars. For three of
these six pulsators, we also have low-resolution far-UV spectroscopy
that is sufficient to broadly constrain their atmospheric abundances
and effective temperatures. Curiously, and in contrast to the ω Cen
pulsators, the NGC 2808 pulsators do not exhibit the spectroscopic or
photometric uniformity one might expect from a well-defined instability
strip, although they all fall within a narrow band (0.2 mag) of far-UV
luminosity. Based on observations made with the NASA/ESA Hubble
Space Telescope, obtained at STScI, which is operated by AURA, Inc.,
under NASA contract NAS 5-26555.

---------------------------------------------------------
Title: Radiative accretion shocks along nonuniform stellar magnetic
fields in classical T Tauri stars
Authors: Orlando, S.; Bonito, R.; Argiroffi, C.; Reale, F.; Peres,
G.; Miceli, M.; Matsakos, T.; Stehlé, C.; Ibgui, L.; de Sa, L.;
Chièze, J. P.; Lanz, T.
2013A&A...559A.127O Altcode: 2013arXiv1309.5038O
Context. According to the magnetospheric accretion model, hot
spots form on the surface of classical T Tauri stars (CTTSs) in
regions where accreting disk material impacts the stellar surface at
supersonic velocity, generating a shock. Aims: We investigate
the dynamics and stability of postshock plasma that streams along
nonuniform stellar magnetic fields at the impact region of accretion
columns. We study how the magnetic field configuration and strength
determine the structure, geometry, and location of the shock-heated
plasma. Methods: We model the impact of an accretion stream
onto the chromosphere of a CTTS by 2D axisymmetric magnetohydrodynamic
simulations. Our model considers the gravity, the radiative cooling,
and the magnetic-field-oriented thermal conduction (including the
effects of heat flux saturation). We explore different configurations
and strengths of the magnetic field. Results: The structure,
stability, and location of the shocked plasma strongly depend on
the configuration and strength of the magnetic field. In the case
of weak magnetic fields (plasma β ≳ 1 in the postshock region),
a large component of B may develop perpendicular to the stream at
the base of the accretion column, which limits the sinking of the
shocked plasma into the chromosphere and perturbs the overstable shock
oscillations induced by radiative cooling. An envelope of dense and cold
chromospheric material may also develop around the shocked column. For
strong magnetic fields (β &lt; 1 in the postshock region close to
the chromosphere), the field configuration determines the position of
the shock and its stand-off height. If the field is strongly tapered
close to the chromosphere, an oblique shock may form well above the
stellar surface at the height where the plasma β ≈ 1. In general,
we find that a nonuniform magnetic field makes the distribution of
emission measure vs. temperature of the postshock plasma at T &gt;
106 K lower than when there is uniform magnetic field. Conclusions: The initial magnetic field strength and configuration
in the region of impact of the stream are expected to influence
the chromospheric absorption and, therefore, the observability of
the shock-heated plasma in the X-ray band. In addition, the field
strength and configuration also influence the energy balance of the
shocked plasma with its emission measure at T &gt; 106 K,
which is lower than expected for a uniform field. The above effects
contribute to underestimating the mass accretion rates derived in
the X-ray band. Movies are available in electronic form at <A
href="http://www.aanda.org">http://www.aanda.org</A>

---------------------------------------------------------
Title: YSO accretion shocks: magnetic, chromospheric or stochastic
flow effects can suppress fluctuations of X-ray emission
Authors: Matsakos, T.; Chièze, J. -P.; Stehlé, C.; González, M.;
Ibgui, L.; de Sá, L.; Lanz, T.; Orlando, S.; Bonito, R.; Argiroffi,
C.; Reale, F.; Peres, G.
2013A&A...557A..69M Altcode: 2013arXiv1307.5389M
Context. Theoretical arguments and numerical simulations of radiative
shocks produced by the impact of the accreting gas onto young stars
predict quasi-periodic oscillations in the emitted radiation. However,
observational data do not show evidence of such periodicity. 
Aims: We investigate whether physically plausible perturbations in
the accretion column or in the chromosphere could disrupt the shock
structure influencing the observability of the oscillatory behavior. Methods: We performed local 2D magneto-hydrodynamical simulations
of an accretion shock impacting a chromosphere, taking optically thin
radiation losses and thermal conduction into account. We investigated
the effects of several perturbation types, such as clumps in the
accretion stream or chromospheric fluctuations, and also explored
a wide range of plasma-β values. Results: In the case of
a weak magnetic field, the post-shock region shows chaotic motion
and mixing, smoothing out the perturbations and retaining a global
periodic signature. On the other hand, a strong magnetic field confines
the plasma in flux tubes, which leads to the formation of fibrils
that oscillate independently. Realistic values for the amplitude,
length, and time scales of the perturbation are capable of bringing
the fibril oscillations out of phase, suppressing the periodicity of
the emission. Conclusions: The strength of a locally uniform
magnetic field in YSO accretion shocks determines the structure of the
post-shock region, namely, whether it will be somewhat homogeneous or
if it will split up to form a collection of fibrils. In the second
case, the size and shape of the fibrils is found to depend strongly
on the plasma-β value but not on the perturbation type. Therefore,
the actual value of the protostellar magnetic field is expected to
play a critical role in the time dependence of the observable emission.

---------------------------------------------------------
Title: Massive stars at low metallicity. Evolution and surface
abundances of O dwarfs in the SMC
Authors: Bouret, J. -C.; Lanz, T.; Martins, F.; Marcolino, W. L. F.;
Hillier, D. J.; Depagne, E.; Hubeny, I.
2013A&A...555A...1B Altcode: 2013arXiv1304.6923B
 Aims: We aim to study the properties of massive stars at
low metallicity, with an emphasis on their evolution, rotation, and
surface abundances. We focus on O-type dwarfs in the Small Magellanic
Cloud. These stars are expected to have weak winds that do not remove
significant amounts of their initial angular momentum. Methods:
We analyzed the UV and optical spectra of twenty-three objects using
the NLTE stellar atmosphere code cmfgen and derived photospheric and
wind properties. Results: The observed binary fraction of the
sample is ≈26%, which is consistent with more systematic studies if
one considers that the actual binary fraction is potentially larger
owing to low-luminosity companions and that the sample was biased
because it excluded obvious spectroscopic binaries. The location of
the fastest rotators in the Hertzsprung-Russell (H-R) diagram built
with fast-rotating evolutionary models and isochrones indicates that
these could be several Myr old. The offset in the position of these
fast rotators compared with the other stars confirms the predictions
of evolutionary models that fast-rotating stars tend to evolve more
vertically in the H-R diagram. Only one star of luminosity class
Vz, expected to best characterize extreme youth, is located on the
zero-age main sequence, the other two stars are more evolved. We
found that the distribution of O and B stars in the ɛ(N) - vsin i
diagram is the same, which suggests that the mechanisms responsible
for the chemical enrichment of slowly rotating massive stars depend
only weakly on the star's mass. We furthermore confirm that the group
of slowly rotating N-rich stars is not reproduced by the evolutionary
tracks. Even for more massive stars and faster rotators, our results
call for stronger mixing in the models to explain the range of
observed N abundances. All stars have an N/C ratio as a function of
stellar luminosity that match the predictions of the stellar evolution
models well. More massive stars have a higher N/C ratio than the less
massive stars. Faster rotators show on average a higher N/C ratio
than slower rotators, again consistent with the expected trend of
stronger mixing as rotation increases. When comparing the N/O versus
N/C ratios with those of stellar evolution models, the same global
qualitative agreement is reached. The only discrepant behavior is
observed for the youngest two stars of the sample, which both show
very strong signs of mixing, which is unexpected for their evolutionary
status. Based on observations made with the NASA-ESA Hubble Space
Telescope (program GO 11625), obtained at STScI, which is operated
by AURA, Inc., under NASA contract NAS 5-26555. Based on observations
collected at the European Southern Observatory Very Large Telescope,
program 079.D-0073.Appendix B is available in electronic form at <A
href="http://www.aanda.org">http://www.aanda.org</A>

---------------------------------------------------------
Title: Wind properties in hot massive stars at low metallicity
Authors: Lanz, Thierry; Bouret, J. C.; Hillier, D. J.; Martins, F.;
Marcolino, W. L. F.
2013msao.confE..41L Altcode:
We present our latest results on the wind properties of 40 O-type
stars in the SMC observed with STIS and COS aboard HST. We reexamine
the metallicity dependence of the wind parameters and the weak winds in
SMC O dwarfs. We discuss the presence and origin of discrete absorption
components in the FUV wind lines of some of these stars.

---------------------------------------------------------
Title: 3D Spectral Radiative Transfer with IRIS: Application to
the Simulation of Laboratory Models of Accretion Shocks in Young
Stellar Objects
Authors: Ibgui, L.; Hubeny, I.; Lanz, T.; Stehlé, C.; González,
M.; Chièze, J. P.
2013ASPC..474...66I Altcode:
IRIS is a new three-dimensional radiative transfer code that provides
spectroscopic diagnostics of hydrodynamics structures. We present its
major numerical features and capabilities. We investigate the dynamical,
radiative and spectral properties of laboratory generated radiative
shocks, with the aim of improving our understanding of the mechanisms of
accretion in young stellar objects. We employed IRIS to generate X-UV
theoretical spectra, while three-dimensional non-stationary radiation
hydrodynamics simulations were performed with the code HERACLES.

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Title: IRIS: a generic three-dimensional radiative transfer code
Authors: Ibgui, L.; Hubeny, I.; Lanz, T.; Stehlé, C.
2013A&A...549A.126I Altcode: 2012arXiv1211.4870I
Context. For most astronomical objects, radiation is the only probe of
their physical properties. Therefore, it is important to have the most
elaborate theoretical tool to interpret observed spectra or images,
thus providing invaluable information to build theoretical models of
the physical nature, the structure, and the evolution of the studied
objects. Aims: We present IRIS, a new generic three-dimensional
(3D) spectral radiative transfer code that generates synthetic spectra,
or images. It can be used as a diagnostic tool for comparison with
astrophysical observations or laboratory astrophysics experiments. Methods: We have developed a 3D short-characteristic solver that
works with a 3D nonuniform Cartesian grid. We have implemented a
piecewise cubic, locally monotonic, interpolation technique that
dramatically reduces the numerical diffusion effect. The code takes
into account the velocity gradient effect resulting in gradual Doppler
shifts of photon frequencies and subsequent alterations of spectral
line profiles. It can also handle periodic boundary conditions. This
first version of the code assumes local thermodynamic equilibrium
(LTE) and no scattering. The opacities and source functions are
specified by the user. In the near future, the capabilities of IRIS
will be extended to allow for non-LTE and scattering modeling. 
Results: IRIS has been validated through a number of tests. We provide
the results for the most relevant ones, in particular a searchlight
beam test, a comparison with a 1D plane-parallel model, and a test of
the velocity gradient effect. Conclusions: IRIS is a generic
code to address a wide variety of astrophysical issues applied to
different objects or structures, such as accretion shocks, jets in
young stellar objects, stellar atmospheres, exoplanet atmospheres,
accretion disks, rotating stellar winds, cosmological structures. It
can also be applied to model laboratory astrophysics experiments,
such as radiative shocks produced with high power lasers.

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Title: Hydrodynamic modeling of accretion shocks on a star with
radiative transport and a chromospheric model
Authors: de Sá, L.; Chièze, J. -P.; Stehlé, C.; Hubeny, I.;
Delahaye, F.; Lanz, T.
2012sf2a.conf..309D Altcode:
The aim of the project (ANR STARSHOCK) is to understand the dynamics
and the radiative properties of accretion columns, linking the
circumstellar disk to the surface photosphere of Young Stellar
Objects. The hydrodynamics is computed first, using a high resolution
hydrodynamic 1D ALE code (ASTROLABE) coupled to radiative transfer
and line cooling, along with a model for the acoustic heating of
the chromospheric plasma. Spectra are then post-processed with a 1D
radiative transfer code (SYNSPEC), using DFE solver and an extended
atomic database covering a wavelength range from X rays to visible.

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Title: 3D numerical simulations of laboratory models of accretion
shocks in young stellar objects
Authors: Ibgui, L.; Hubeny, I.; Lanz, T.; Stehlé, C.; González,
M.; Chièze, J. -P.
2012sf2a.conf..347I Altcode:
We show preliminary results of our numerical simulations of laboratory
experiments of radiative shocks. Such experiments aim at understanding
accretion shocks in young stellar objects. Three-dimensional
non-stationary radiation hydrodynamics calculations were performed
with the code HERACLES. X-UV spectra were then generated with the new
three-dimensional radiative transfer code IRIS.

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Title: Modeling periodic media with the three-dimensional radiative
transfer code IRIS
Authors: Ibgui, L.; Hubeny, I.; Lanz, T.; Stehlé, C.
2012sf2a.conf..343I Altcode:
We describe the implementation in our generic three-dimensional
radiative transfer code, IRIS, of an algorithm that allows the
modeling of periodic infinite media. We show how this algorithm has
been validated by comparison with well-established 1D plane-parallel
models. A particularly interesting astrophysical application will be
the calculation of synthetic spectra of the fully three-dimensional
solar atmosphere.

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Title: The influence of rotation on optical emission profiles of
O stars
Authors: Hillier, D. John; Bouret, Jean-Claude; Lanz, Thierry; Busche,
Joseph R.
2012MNRAS.426.1043H Altcode: 2012arXiv1207.0822H
We study the formation of photospheric emission lines in O stars and
show that the rectangular profiles, sometimes double peaked, that
are observed for some stars are a direct consequence of rotation,
and it is unnecessary to invoke an enhanced density structure in the
equatorial regions. Emission lines, such as N IV λ4058 and the N III
λλ4634-4640-4642 multiplet, exhibit non-standard 'limb-darkening'
laws. The lines can be in absorption for rays striking the centre
of the star and in emission for rays near the limb. Weak features in
the flux spectrum do not necessarily indicate an intrinsically weak
feature - instead the feature can be weak because of cancellation
between absorption in 'core' rays and emission from rays near the
limb. Rotation also modifies line profiles of wind diagnostics such
as He II λ4686 and Hα and should not be neglected when inferring
the actual stratification, level and nature of wind structures.

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Title: The Wind of Massive Stars in Low-Metallicity Galaxies
Authors: Lanz, Thierry
2012hst..prop12867L Altcode:
The ultimate fate of massive stars rests on the mass and angular
momentum that they loose during their evolution, hence depends on
the properties of their winds. Because of the radiation-driven nature
of these winds, we expect the mass loss rates to scale with stellar
metallicity. Such a relation has been established theoretically and
empirically with spectroscopic analyses of O stars in the Galaxy
{Z/Zsun=1} and in the LMC and SMC {Z/Zsun = 1/2 and 1/5}. Tramper et
al. {2011} derived mass loss rates of 6 O stars in galaxies with low
metallicities {Z/Zsun 1/7}. Based on the Halpha line, they derived
mass loss rates that are similar to those of LMC stars. This result, if
upheld, would have far-reaching consequences beyond our understanding of
radiatively-driven winds, and would imply for instance a lower number of
collapsars and of SN Ib and Ic in low metallicity environments, hence
in the high-z Universe.We propose to coroborate or refute Tramper et
al.'s result by obtaining FUV spectra of 3 O stars in the IC 1613 and
WLM galaxies with HST/COS. Because of numerous iron lines and lines
sensitive to wind properties, FUV spectroscopy is the adequate tool to
fully address and resolve this outstanding issue of the dependence of
hot, massive star mass loss rates with metallicity. A comparison of
these spectra with extant STIS or COS spectra of LMC and SMC stars
will provide a direct, model-independent check of the mass loss -
metallicity relation. A quantitative analysis will be carried out
using our state-of-the-art NLTE unified model atmospheres calculated
with the CMFGEN code to establish robustly the wind properties of
low-metallicity massive stars.

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Title: Properties of Galactic early-type O-supergiants. A combined
FUV-UV and optical analysis
Authors: Bouret, J. -C.; Hillier, D. J.; Lanz, T.; Fullerton, A. W.
2012A&A...544A..67B Altcode: 2012arXiv1205.3075B
 Aims: We aim to constrain the properties and evolutionary status
of early and mid-spectral type supergiants (from O4 to O7.5). These
posses the highest mass-loss rates among the O stars, and exhibit
conspicuous wind profiles. Methods: Using the non-LTE wind
code cmfgen we simultaneously analyzed the FUV-UV and optical spectral
range to determine the photospheric properties and wind parameters. We
derived effective temperatures, luminosities, surface gravities,
surface abundances, mass-loss rates, wind terminal velocities, and
clumping filling factors. Results: The supergiants define a very
clear evolutionary sequence, in terms of ages and masses, from younger
and more massive stars to older stars with lower initial masses. O4
supergiants cluster around the 3 Myr isochrone and are more massive than
60 M⊙, while the O5 to O7.5 stars have masses in the range
50-40 M⊙ and are 4 ± 0.3 Myr old. The surface chemical
composition is typical of evolved O supergiants (nitrogen-rich, carbon-
and oxygen-poor). While the observed ranges of carbon and nitrogen
mass-fractions are compatible with those expected from evolutionary
models for the measured stellar masses, the N/C ratios as a function
of age are inconsistent with the theoretical predictions for the
four earliest (O4 spectral type) stars of the sample. We question the
efficiency of rotational mixing as a function of age for these stars and
suggest that another mechanism may be needed to explain the observed
abundance patterns. Mass-loss rates derived with clumped-models range
within a factor of three of the theoretical mass-loss rates. The
corresponding volume-filling factors associated with small-scale
clumping are 0.05 ± 0.02. Clumping is found to start close to the
photosphere for all but three stars, two of which are fast rotators. Based on observations made with the NASA-CNES-CSA Far Ultraviolet
Spectroscopic Explorer (FUSE) and by the NASA-ESA-SERC International
Ultraviolet Explorer (IUE), and retrieved from the Multimission Archive
at the Space Telescope Science Institute (MAST). Based on observations
collected with the ELODIE spectrograph on the 1.93-m telescope
(Observatoire de Haute-Provence, France). Based on observations
collected with the FEROS instrument on the ESO 2.2 m telescope, program
074.D-0300 and 075.D-0061.Appendix A is available in electronic form
at <A href="http://www.aanda.org">http://www.aanda.org</A>

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Title: Flash Mixing on the White Dwarf Cooling Curve: Spectroscopic
Confirmation in NGC 2808
Authors: Brown, Thomas M.; Lanz, Thierry; Sweigart, Allen V.; Cracraft,
Misty; Hubeny, Ivan; Landsman, Wayne B.
2012ApJ...748...85B Altcode: 2012arXiv1201.4070B
We present new Hubble Space Telescope far-UV spectroscopy of two
dozen hot evolved stars in NGC 2808, a massive globular cluster with
a large population of "blue-hook" (BHk) stars. The BHk stars are
found in ultraviolet color-magnitude diagrams of the most massive
globular clusters, where they fall at luminosities immediately below
the hot end of the horizontal branch (HB), in a region of the H-R
diagram unexplained by canonical stellar evolution theory. Using
new theoretical evolutionary and atmospheric models, we have shown
that these subluminous HB stars are very likely the progeny of stars
that undergo extensive internal mixing during a late He-core flash
on the white dwarf cooling curve. This flash mixing leads to hotter
temperatures and an enormous enhancement of the surface He and C
abundances; these hotter temperatures, together with the decrease in H
opacity shortward of the Lyman limit, make the BHk stars brighter in the
extreme UV while appearing subluminous in the UV and optical. Our far-UV
spectroscopy demonstrates that, relative to normal HB stars at the
same color, the BHk stars of NGC 2808 are hotter and greatly enhanced
in He and C, thus providing unambiguous evidence of flash mixing in
the subluminous population. Although the C abundance in the BHk stars
is orders of magnitude larger than that in the normal HB stars, the
atmospheric C abundance in both the BHk and normal HB stars appears to
be affected by gravitational settling. The abundance variations seen in
Si and the Fe-peak elements also indicate that atmospheric diffusion is
at play in our sample, with all of our hot subdwarfs at 25,000-50,000
K exhibiting large enhancements of the iron-peak elements. The hottest
subdwarfs in our BHk sample may be pulsators, given that they fall in
the temperature range of newly discovered pulsating subdwarfs in ω
Cen. In addition to the normal hot HB and BHk stars, we also obtain
spectra of five blue HB stars, a post-HB star, and three unclassified
stars with unusually blue UV colors. Based on observations made
with the NASA/ESA Hubble Space Telescope, obtained at STScI, which is
operated by AURA, Inc., under NASA contract NAS 5-26555.

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Title: New Observational Evidence of Flash Mixing on the White Dwarf
Cooling Curve
Authors: Brown, T. M.; Lanz, T.; Sweigart, A. V.; Cracraft, M.;
Hubeny, I.; Landsman, W. B.
2012ASPC..452...23B Altcode: 2012arXiv1201.4204B
Blue hook stars are a class of subluminous extreme horizontal branch
stars that were discovered in UV images of the massive globular clusters
ω Cen and NGC 2808. These stars occupy a region of the HR diagram
that is unexplained by canonical stellar evolution theory. Using
new theoretical evolutionary and atmospheric models, we have shown
that the blue hook stars are very likely the progeny of stars that
undergo extensive internal mixing during a late helium-core flash
on the white dwarf cooling curve. This “flash mixing” produces
hotter-than-normal EHB stars with atmospheres significantly enhanced
in helium and carbon. The larger bolometric correction, combined with
the decrease in hydrogen opacity, makes these stars appear subluminous
in the optical and UV. Flash mixing is more likely to occur in stars
born with a high helium abundance, due to their lower mass at the
main sequence turnoff. For this reason, the phenomenon is more common
in those massive globular clusters that show evidence for secondary
populations enhanced in helium. However, a high helium abundance does
not, by itself, explain the presence of blue hook stars in massive
globular clusters. Here, we present new observational evidence for flash
mixing, using recent HST observations. These include UV color-magnitude
diagrams of six massive globular clusters and far-UV spectroscopy of
hot subdwarfs in one of these clusters (NGC 2808).

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Title: 3D modeling of accretion shocks in young stellar objects :
Simulation of laboratory experiments
Authors: Ibgui, L.; González, M.; Stehlé, C.; Hubeny, I.; Lanz, T.
2012EAS....58..149I Altcode:
In order to improve the understanding of the physics of accretion shocks
around young stellar objects, we have performed a three dimensional
simulation of a radiative shock generated in a laser installation. We
depict the 3D structure of such a shock. Radiation hydrodynamics is
modeled with the HERACLES code; then, radiative transfer post-processing
is performed with the IRIS code.

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Title: Chemical homogeneity in the Orion Association: Oxygen
abundances of B stars
Authors: Cunha, K.; Hubeny, I.; Lanz, T.
2012EPJWC..1908005C Altcode: 2011arXiv1110.5879C
We present non-LTE oxygen abundances for a sample of B stars in the
Orion association. The abundance calculations included non-LTE line
formation and used fully blanketed non-LTE model atmospheres. The
stellar parameters were the same as adopted in the previous study by
Cunha &amp; Lambert (1994). We find that the young Orion stars in
this sample of 10 stars are described by a single oxygen abundance
with an average value of A(O) = 8.78 and a small dispersion of ±0.05,
dex which is of the order of the uncertainties in the analysis. This
average oxygen abundance compares well with the average oxygen
abundance obtained previously in Cunha &amp; Lambert (1994): A(O) =
8.72 ± 0.13 although this earlier study, based upon non-blanketed
model atmospheres in LTE, displayed larger scatter. Small scatter
of chemical abundances in Orion B stars had also been found in our
previous studies for neon and argon; all based on the same effective
temperature scale. The derived oxygen abundance distribution for the
Orion association compares well with other results for the oxygen
abundance in the solar neighborhood.

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Title: SPEC3D: a three-dimensional radiative transfer code for
astrophysical and laboratory applications
Authors: Ibgui, L.; Hubeny, I.; Lanz, T.; Stehlé, C.
2011sf2a.conf..485I Altcode:
We have developed a generic three-dimensional radiative transfer code,
SPEC3D, aimed at post-processing 3D radiation magnetohydrodynamics
simulations. SPEC3D solves the monochromatic 3D radiative transfer
equation. The numerical approach and the major features of the
code are presented. The wide range of applications includes the
modeling of a number of astrophysical objects and structures,
such as accretion shocks around young stellar objects, stellar and
exoplanets atmospheres, cosmological structures, but also the modeling
of laboratory astrophysics experiments such as magnetohydrodynamics
jets and radiative shocks.

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Title: Synspec: General Spectrum Synthesis Program
Authors: Hubeny, Ivan; Lanz, Thierry
2011ascl.soft09022H Altcode:
Synspec is a user-oriented package written in FORTRAN for modeling
stellar atmospheres and for stellar spectroscopic diagnostics. It
assumes an existing model atmosphere, calculated previously with Tlusty
or taken from the literature (for instance, from the Kurucz grid of
models). The opacity sources (continua, atomic and molecular lines)
are fully specified by the user. An arbitrary stellar rotation and
instrumental profile can be applied to the synthetic spectrum.

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Title: TLUSTY: Stellar Atmospheres, Accretion Disks, and Spectroscopic
Diagnostics
Authors: Hubeny, Ivan; Lanz, Thierry
2011ascl.soft09021H Altcode:
TLUSTY is a user-oriented package written in FORTRAN77 for modeling
stellar atmospheres and accretion disks and wide range of spectroscopic
diagnostics. In the program's maximum configuration, the user may start
from scratch and calculate a model atmosphere of a chosen degree of
complexity, and end with a synthetic spectrum in a wavelength region of
interest for an arbitrary stellar rotation and an arbitrary instrumental
profile. The user may also model the vertical structure of annuli of
an accretion disk.

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Title: VizieR Online Data Catalog: Hot HB stars in ω Cen (Moehler+,
2011)
Authors: Moehler, S.; Dreizler, S.; Lanz, T.; Bono, G.; Sweigart,
A. V.; Calamida, A.; Nonino, M.
2011yCat..35260136M Altcode: 2011yCat..35269136M
Moderately high resolution spectra of hot HB stars in the globular
cluster omega Cen were analysed for radial velocity variations,
atmospheric parameters, and abundances using LTE and non-LTE model
atmospheres. The spectroscopic data were obtained in 2005 (4
observations) and in 2006 (5 observations) in Service Mode using the
MEDUSA mode of the multi-object fiber spectrograph FLAMES + GIRAFFE on
the UT2 Telescope of the VLT. We used the low spectroscopic resolution
mode with the spectral range 3964Å-4567Å (LR2, R=6400) and observed
spectra for a total of 109 blue hook and canonical blue HB/EHB star
candidates (see Table 1) and for 17 sky background positions. Each
observation had an exposure time of 2550s to keep the total execution
time of the observing block shorter than one hour. (3 data files).

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Title: The hot horizontal-branch stars in ω Centauri
Authors: Moehler, S.; Dreizler, S.; Lanz, T.; Bono, G.; Sweigart,
A. V.; Calamida, A.; Nonino, M.
2011A&A...526A.136M Altcode: 2010arXiv1009.3191M
Context. UV observations of some massive globular clusters have revealed
a significant population of stars hotter and fainter than the hot end of
the horizontal branch (HB), the so-called blue hook stars. This feature
might be explained either by the late hot flasher scenario where stars
experience the helium flash while on the white dwarf cooling curve
or by the progeny of the helium-enriched sub-population postulated to
exist in some clusters. Previous spectroscopic analyses of blue hook
stars in ω Cen and NGC 2808 support the late hot flasher scenario,
but the stars contain much less helium than expected and the predicted
C and N enrichment cannot be verified. Aims: We compare the
observed effective temperatures, surface gravities, helium abundances,
and carbon line strengths (where detectable) of our targets stars
with the predictions of the two aforementioned scenarios. 
Methods: Moderately high resolution spectra of hot HB stars in the
globular cluster ω Cen were analysed for radial velocity variations,
atmospheric parameters, and abundances using LTE and non-LTE model
atmospheres. Results: We find no evidence of close binaries
among our target stars. All stars below 30 000 K are helium-poor and
very similar to HB stars observed in that temperature range in other
globular clusters. In the temperature range 30 000 K to 50 000 K,
we find that 28% of our stars are helium-poor ({log{n_He}/{n_H}} &lt;
-1.6), while 72% have roughly solar or super-solar helium abundance
({log{n_He}/{n_H}} ≥ -1.5). We also find that carbon enrichment
is strongly correlated with helium enrichment, with a maximum carbon
enrichment of 3% by mass. Conclusions: A strong carbon enrichment
in tandem with helium enrichment is predicted by the late hot flasher
scenario, but not by the helium-enrichment scenario. We conclude that
the helium-rich HB stars in ω Cen cannot be explained solely by the
helium-enrichment scenario invoked to explain the blue main sequence. Based on observations with the ESO Very Large Telescope at Paranal
Observatory, Chile (proposal IDs 075.D-0280(A) and 077.D-0021(A)).

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Title: Hot, Massive Stars in I Zw 18
Authors: Heap, Sara R.; Hubeny, Ivan; Lanz, Thierry; Landsman, Wayne;
Lindler, Don; Malumuth, Eliot
2011BSRSL..80..149H Altcode:
We present the far-ultraviolet spectrum of the northwest component of I
Zw 18, a blue compact galaxy having a very low metallicity. The spectrum
is compatible with continuous star-formation over the past ∼ 15 Myr
(CSF age), and a very low metallicity, log Z/Z_⊙ ∼ -1.7, although
the stellar surface may be enhanced in carbon. Stellar winds are very
weak, and the edge velocity of wind lines is very low (∼ 250 km/s).

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Title: HST/WFC3 Photometry of NGC 2808 and Its Multiple Main Sequences
Authors: Jeffery, Elizabeth; Brown, T.; Dotter, A.; Hubeny, I.;
Landsman, W. B.; Lanz, T.; Sweigart, A. V.
2011AAS...21715212J Altcode: 2011BAAS...4315212J
Recent high precision photometric observations of several globular
clusters have revealed that these simple stellar populations may not
be as simple as we once believed. Some globular clusters have shown
interesting features in their color-magnitude diagrams indicating
the presence of multiple populations. Recent HST observations of the
massive globular cluster NGC 2808 have revealed the presence of three
main sequences. We present new observations of NGC 2808 using the
new WFC3 camera on HST. Observations are in five photometric bands
covering a large wavelength range from the UVIS to the IR channels
of WFC3. Moreover, these observations extend below the "turn down"
in the lower main sequence (as seen in the IR CMD) and therefore reach
deeper than past data sets on this cluster. We discuss various model
fits and compare with other observed CMDs of other globular clusters
to explore the origin of these multiple sequences.

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Title: Multiwavelength Observations of the Hot DB Star PG 0112+104
Authors: Dufour, P.; Desharnais, S.; Wesemael, F.; Chayer, P.; Lanz,
T.; Bergeron, P.; Fontaine, G.; Beauchamp, A.; Saffer, R. A.; Kruk,
J. W.; Limoges, M. -M.
2010ApJ...718..647D Altcode: 2010arXiv1006.0365D
We present a comprehensive multiwavelength analysis of the hot DB
white dwarf PG 0112+104. Our analysis relies on newly acquired FUSE
observations, on medium-resolution FOS and GHRS data, on archival
high-resolution GHRS observations, on optical spectrophotometry both in
the blue and around Hα, as well as on time-resolved photometry. From
the optical data, we derive a self-consistent effective temperature
of 31,300 ± 500 K, a surface gravity of log g = 7.8 ± 0.1 (M = 0.52
M sun), and a hydrogen abundance of log N(H)/N(He)&lt;
-4.0. The FUSE spectra reveal the presence of C II and C III lines
that complement the previous detection of C II transitions with the
GHRS. The improved carbon abundance in this hot object is log N(C)/N(He)
= -6.15 ± 0.23. No photospheric features associated with other heavy
elements are detected. We reconsider the role of PG 0112+104 in the
definition of the blue edge of the V777 Her instability strip in light
of our high-speed photometry and contrast our results with those of
previous observations carried out at the McDonald Observatory. Based on observations with the FUSE satellite, which is operated
by the Johns Hopkins University under NASA contract NAS 5-32985; with
the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope
Science Institute, which is operated by the Association of Universities
for Research in Astronomy, Inc., under the NASA contract NAS 5-26555;
and with the Kitt Peak National Observatory, National Optical Astronomy
Observatories, operated by the Association of Universities for Research
in Astronomy, Inc., under cooperative agreement with the National
Science Foundation.

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Title: The Blue Hook Populations of Massive Globular Clusters
Authors: Brown, Thomas M.; Sweigart, Allen V.; Lanz, Thierry; Smith,
Ed; Landsman, Wayne B.; Hubeny, Ivan
2010ApJ...718.1332B Altcode: 2010arXiv1006.1591B
We present new Hubble Space Telescope ultraviolet color-magnitude
diagrams of five massive Galactic globular clusters: NGC 2419, NGC 6273,
NGC 6715, NGC 6388, and NGC 6441. These observations were obtained to
investigate the "blue hook" (BH) phenomenon previously observed in UV
images of the globular clusters ω Cen and NGC 2808. Blue hook stars are
a class of hot (approximately 35,000 K) subluminous horizontal branch
stars that occupy a region of the HR diagram that is unexplained by
canonical stellar evolution theory. By coupling new stellar evolution
models to appropriate non-LTE synthetic spectra, we investigate various
theoretical explanations for these stars. Specifically, we compare our
photometry to canonical models at standard cluster abundances, canonical
models with enhanced helium (consistent with cluster self-enrichment
at early times), and flash-mixed models formed via a late helium-core
flash on the white dwarf cooling curve. We find that flash-mixed models
are required to explain the faint luminosity of the BH stars, although
neither the canonical models nor the flash-mixed models can explain the
range of color observed in such stars, especially those in the most
metal-rich clusters. Aside from the variation in the color range, no
clear trends emerge in the morphology of the BH population with respect
to metallicity. Based on observations made with the NASA/ESA Hubble
Space Telescope, obtained at STScI, and associated with proposal 10815.

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Title: Pulsed Thermal Emission from the Accreting Pulsar XMMU
J054134.7-682550
Authors: Manousakis, A.; Walter, R.; Audar, M.; Lanz, T.
2010ASPC..424..138M Altcode:
Soft X-ray excesses have been detected in several Be/X-ray binaries and
interpreted as the signature of hard X-ray reprocessing in the inner
accretion disk. The system XMMU J054134.7- 682550, located in the LMC,
featured a giant Type II outburst in August 2007. The geometry of
this system can be understood by studying the response of the soft
excess emission to the hard X-ray pulses. We have analyzed series
of simultaneous observations obtained with XMM-Newton/EPIC-MOS and
RXTE/PCA in order to derive spectral and temporal characteristics of
the system, before, during and after the giant outburst. Spectral fits
were performed and a timing analysis has been carried out. Spectral
variability, spin period evolution and energy dependent pulse shapes
are analysed. The outburst (LX ≍ 3×1038 erg/s
∼ LEDD) spectrum could be modeled successfully using a
cutoff powerlaw, a cold disk emission, a hot blackbody, and a cyclotron
absorption line. The magnetic field and magnetospheric radius could be
constrained. The thickness of the inner accretion disk is broadened
to a width of 75 km. The hot blackbody component features sinusoidal
modulations indicating that the bulk of the hard X-ray emission is
emitted preferentially along the magnetic equator. The spin period
of the pulsar decreased very significantly during the outburst. This
is consistent with a variety of neutron star equations of state and
indicates a very high accretion rate.

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Title: Experimental study of radiative shocks at PALS facility
Authors: Stehlé, Chantal; González, Matthias; Kozlova, Michaela; Rus,
Bedrich; Mocek, Tomas; Acef, Ouali; Colombier, Jean Philippe; Lanz,
Thierry; Champion, Norbert; Jakubczak, Krzysztof; Polan, Jiri; Barroso,
Patrice; Bauduin, Daniel; Audit, Edouard; Dostal, Jan; Stupka, Michal
2010LPB....28..253S Altcode: 2010arXiv1003.2739S
We report on the investigation of strong radiative shocks generated
with the high energy, sub-nanosecond iodine laser at PALS. These shock
waves are characterized by a developed radiative precursor and their
dynamics is analyzed over long time scales (~50 ns), approaching a
quasi-stationary limit. We present the first preliminary results on
the rear side XUV spectroscopy. These studies are relevant to the
understanding of the spectroscopic signatures of accretion shocks in
Classical T Tauri Stars.

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Title: The Determination of the Abundances of the Fe Group Elements
in Early B Stars from High Resolution FUV Spectra
Authors: Peters, Geraldine J.; Adelman, Saul J.; Hubeny, Ivan;
Lanz, Thierry
2010IAUS..265..213P Altcode:
We present selected results from an investigation that is currently
underway to determine the abundances of the Fe group elements in early
B stars and assess the extent to which contemporary NLTE and LTE models
represent their atmospheres. High resolution UV and optical spectra of
B stars that display ultrasharp lines are compared with computations
from TLUSTY/SYNSPEC and SYNTHE. Some results from our analysis of the
abundance standard ι Her (B3V) are presented here.

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Title: The Blue Hook Populations of Massive Globular Clusters
Authors: Brown, Thomas M.; Sweigart, A. V.; Lanz, T.; Smith, E.;
Landsman, W. B.; Hubeny, I.
2010AAS...21542516B Altcode: 2010BAAS...42..338B
We present new HST ultraviolet color-magnitude diagrams of 5 massive
Galactic globular clusters: NGC 2419, NGC 6273, NGC 6715, NGC 6388,
and NGC 6441. &amp;nbspThese observations were obtained in order
to investigate the "blue hook" phenomenon previously observed in
HST ultraviolet images of the globular clusters omega Cen and NGC
2808. &amp;nbspBlue hook stars are a class of hot (approximately
35,000 K) subluminous horizontal branch stars that occupy a
region of the HR diagram that is unexplained by canonical stellar
evolution theory. &amp;nbspBy coupling new stellar evolution models
to appropriate non-LTE synthetic spectra, we investigate various
theoretical explanations for these stars. &amp;nbspSpecifically, we
compare the color-magnitude diagrams to stellar models that assume
normal evolution at cluster abundances, models with enhanced helium
resulting from cluster self-enrichment, and flash-mixed models formed
via late helium core flash on the white dwarf cooling curve.

---------------------------------------------------------
Title: Quantitative Analysis of the Spectra of Early B Stars with
Ultrasharp Lines
Authors: Peters, Geraldine J.; Adelman, S.; Hubeny, I.; Lanz, T.
2009AAS...21460512P Altcode:
The extent to which contemporary NLTE and LTE models represent
the atmospheres of the early B stars with the sharpest lines is
discussed. Due to the low rotational broadening in these stars, line
blending presents minimal problems and the intrinsic line profiles are
revealed. Spectral data from the HST and FUSE spacecraft and the KPNO
Coude Feed Telescope are compared with computations from the Hubeny/Lanz
codes TLUSTY/SYNSPEC and the Kurucz code SYNTHE. The B stars include
ι Her (B3V), HR 1886 (B1V), HR 1887 (B0.5V), and AV 304 (B0.5V) in the
SMC, all of which are considered to be abundance standards. Contemporary
NLTE model atmospheres represent the photospheres of early B stars
remarkably well. Some lines are fit much better in NLTE even for the B3
stars, but widespread discrepancies seem to emerge for stars hotter than
B0.5, which also show dynamic atmospheres. Future work need to be done
on the following: 1) The determination and meaning of microturbulence
and its effect on the atmosphere, 2) Further attention to the strong
Si II lines, especially Multiplet 4, 3) Treatment of Ti, V, Cr, Mn,
Co, and Ni in NLTE, 4) Wind models for B0-B0.5 stars. The authors
appreciate support from NASA grants NAG5-11802, NAG5-12239, NAG5-13212,
and STScI grants GO-09848 &amp; GO-06709.

---------------------------------------------------------
Title: On The Importance Of The Interclump Medium For Superionization:
O VI Formation In The Wind Of ζ Pup.
Authors: Zsargo, Janos; Hillier, J. D.; Bouret, J. C.; Lanz, T.;
Leutenegger, M. A.; Cohen, D. H.
2009AAS...21340815Z Altcode:
We have studied superionization and X-ray line formation in the
spectra of ζ Pup using our new stellar atmosphere code (XCMFGEN)
that can be used to simultaneously analyze optical, UV, and X-ray
observations. Here, we present results on the formation of the O VI
λλ1032, 1038 doublet. Our simulations, supported by simple theoretical
calculations, show that clumped wind models that assume void in the
interclump space cannot reproduce the observed O VI profiles. However,
enough O VI can be produced if the voids are filled by a low density
gas. The recombination of O VI is very efficient in the dense material
but in the tenuous interclump region an observable amount of O VI can be
maintained. We also find that different UV resonance lines are sensitive
to different density regimes in ζ Pup : C IV is almost exclusively
formed within the densest regions, while the majority of O VI resides
between clumps. N V is an intermediate case, with contributions
from both the tenuous gas and clumps. Acknowledgments: This
research was supported by STScI grant HST-AR-10693.02 and by SAO grant
TM6-7003X. M. A. L. acknowledges support from a fellowship administered
by Oak Ridge Associated Universities under the NASA Postdoctoral
Program. J.-C. B. acknowledges financial support from the French
National Research Agency (ANR) through program number ANR-06-BLAN-0105.

---------------------------------------------------------
Title: Scaling stellar jets to the laboratory: The power of
simulations
Authors: Stehlé, C.; Ciardi, A.; Colombier, J. -P.; González, M.;
Lanz, T.; Marocchino, A.; Kozlova, M.; Rus, B.
2009LPB....27..709S Altcode: 2009arXiv0912.4279S
Advances in laser and Z-pinch technology, coupled with the development
of plasma diagnostics and the availability of high-performance
computers, have recently stimulated the growth of high-energy density
laboratory astrophysics. In particular a number of experiments have
been designed to study radiative shocks and jets with the aim of
shedding new light on physical processes linked to the ejection and
accretion of mass by newly born stars. Although general scaling laws
are a powerful tools to link laboratory experiments with astrophysical
plasmas, the phenomena modelled are often too complicated for simple
scaling to remain relevant. Nevertheless, the experiments can still
give important insights into the physics of astrophysical systems and
can be used to provide the basic experimental validation of numerical
simulations in regimes of interest to astrophysics. We will illustrate
the possible links between laboratory experiments, numerical simulations
and astrophysics in the context of stellar jets. First we will discuss
the propagation of stellar jets in a cross-moving interstellar medium
and the scaling to Z-pinch produced jets. Our second example focuses
on slab-jets produced at the PALS (Prague Asterix Laser System) laser
installation and their practical applications to astrophysics. Finally,
we illustrate the limitations of scaling for radiative shocks, which
are found at the head of the most rapid stellar jets.

---------------------------------------------------------
Title: Quantitative Analysis of the Spectra of Early B Stars with
Ultrasharp Lines
Authors: Peters, Geraldine J.; Adelman, Saul J.; Hubeny, Ivan;
Lanz, Thierry
2009AIPC.1171..345P Altcode:
We present selected results from an investigation that is currently
underway to determine the abundances of light and Fe group elements
in early B stars and assess the extent to which contemporary NLTE and
LTE models represent their atmospheres. Spectral data of B stars that
display ultrasharp lines, obtained with HST, FUSE, and the KPNO Coudé
Feed Telescope, are compared with computations from TLUSTY/SYNSPEC
and SYNTHE. The B stars include the abundance standards ι Her (B3V),
HR 1886 (B1V), and HR 1887 (B0.5V).

---------------------------------------------------------
Title: Pulsed Thermal Emission from the Accreting Pulsar XMMU
J054134.7-682550
Authors: Manousakis, Antonis; Walter, Roland; Audard, Marc; Lanz,
Thierry
2009AIPC.1126..325M Altcode: 2009arXiv0903.0580M
XMMU J054134.7-682550, located in the LMC, featured a type II outburst
in August 2007. We analyzed XMM-Newton (EPIC-MOS) and RXTE (PCA)
data in order to derive the spectral and temporal characteristics of
the system throughout the outburst. Spectral variability, spin period
evolution, energy dependent pulse shape are discussed. The outburst
(LX~3×1038 erg/s~LEDD) spectrum
can be modeled using, cutoff power law, soft X-ray blackbody, disk
emission, and cyclotron absorption line. The blackbody component shows a
sinusoidal behavior, expected from hard X-ray reprocessing on the inner
edge of the accretion disk. The thickness of the inner accretion disk
(width of ~75 km) can be constrained. The spin-up of the pulsar during
the outburst is the signature of a (huge) accretion rate. Simbol-X
will provide similar capabilities as XMM-Newton and RXTE together,
for such bright events.

---------------------------------------------------------
Title: Analysis of Galactic late-type O dwarfs: more constraints on
the weak wind problem
Authors: Marcolino, W. L. F.; Bouret, J. -C.; Martins, F.; Hillier,
D. J.; Lanz, T.; Escolano, C.
2009A&A...498..837M Altcode: 2009arXiv0902.1833M
Aims: We investigate the stellar and wind properties of a sample of
late-type O dwarfs. Previous analyses of such stars have found very low
mass-loss rates; rates much lower than predicted by theory (the weak
wind problem). Methods: Far-UV to optical spectra of five Galactic
O stars were analyzed: HD 216898 (O9IV/O8.5V), HD 326329 (O9V), HD 66788
(O8V/O9V), ζ Oph (O9.5Vnn), and HD 216532 (O8.5V((n))). We used a grid
of TLUSTY models to obtain effective temperatures, gravities, rotational
velocities, and to identify wind lines. Wind parameters for each object
were obtained using expanding atmosphere models calculated with the
CMFGEN code. Results: The spectra of our sample have primarily
a photospheric origin. A weak wind signature is seen in C iv λλ1548,
1551, from which mass-loss rates consistent with previous CMFGEN results
for O8-O9V stars were derived (∼10-10-10-9 M_⊙
yr-1). A discrepancy of roughly two orders of magnitude
is found between these mass-loss rates and the values predicted
by theory (dot{M}_Vink), confirming a breakdown or a steepening of
the modified wind momentum-luminosity relation at log L_star/L_⊙
⪉ 5.2. We have estimated the carbon abundance for the stars of our
sample and concluded that its value cannot be reduced to sufficiently
small values to solve the weak wind problem. Upper limits on dot{M}
were established for all objects using lines of different ions: P v
λλ1118, 1128, C iii λ 1176, N v λλ1239, 1243, Si iv λλ1394,
1403, and N iv λ1718. All the values obtained are in disagreement
with theoretical predictions, bringing support to the reality of weak
winds. Together with C iv λλ1548, 1551, the use of N v λλ1239, 1243
results in the lowest mass-loss rates: the upper limits indicate that
dot{M} must be less than about -1.0 dex dot{M}_Vink. Upper mass-loss
rate limits obtained for other transitions are also low: they indicate
that dot{M} must be less than about (-0.5 ± 0.2) dex dot{M}_Vink. We
studied the behavior of the Hα line with different mass-loss
rates. For two stars, only models with very low dot{M}'s provide the
best fit to the UV and optical spectra. We also explored ways to fit
the observed spectra with the theoretical mass-loss rates. By using
large amounts of X-rays, we could reduce the predicted wind emission
to the observed levels. However, unrealistic X-ray luminosities
had to be used (log L_X/L_Bol ⪆ -3.5). The validity of the models
used in our analyses is discussed. Based on observations made
with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer and by
the NASA-ESA-SERC International Ultraviolet Explorer , and retrieved
from the Multimission Archive at the Space Telescope Science Institute
(MAST). Based on observations collected with the ELODIE spectrograph on
the 1.93-m telescope (Observatoire de Haute-Provence, France). Based
on observations collected with the FEROS instrument on the ESO 2.2 m
telescope, program 074.D-0300 and 075.D-0061. Appendices A and B are
only available in electronic form at http://www.aanda.org

---------------------------------------------------------
Title: Effective area calibration of the reflection grating
spectrometers of XMM-Newton. II. X-ray spectroscopy of DA white dwarfs
Authors: Kaastra, J. S.; Lanz, T.; Hubeny, I.; Paerels, F. B. S.
2009A&A...497..311K Altcode: 2009arXiv0902.2282K
Context: White dwarf spectra have been widely used as a calibration
source for X-ray and EUV instruments. The in-flight effective area
calibration of the reflection grating spectrometers (RGS) of XMM-Newton
depend upon the availability of reliable calibration sources. Aims: We investigate how well these white dwarf spectra can be used
as standard candles at the lowest X-ray energies in order to gauge the
absolute effective area scale of X-ray instruments. Methods:
We calculate a grid of model atmospheres for Sirius B and HZ 43A,
and adjust the parameters using several constraints until the ratio of
the spectra of both stars agrees with the ratio as observed by the low
energy transmission grating spectrometer (LETGS) of Chandra. This ratio
is independent of any errors in the effective area of the LETGS. Results: We find that we can constrain the absolute X-ray spectrum
of both stars with better than 5% accuracy. The best-fit model for
both stars is close to a pure hydrogen atmosphere, and we put tight
limits to the amount of helium or the thickness of a hydrogen layer
in both stars. Our upper limit to the helium abundance in Sirius B
is 4 times below the previous detection based on EUVE data. We also
find that our results are sensitive to the adopted cut-off in the Lyman
pseudo-continuum opacity in Sirius B. We get best agreement with a long
wavelength cut-off. Conclusions: White dwarf model atmospheres
can be used to derive the effective area of X-ray spectrometers in the
lowest energy band. An accuracy of 3-4% in the absolute effective area
can be achieved.

---------------------------------------------------------
Title: Pulsed thermal emission from the accreting pulsar XMMU
J054134.7-682550
Authors: Manousakis, A.; Walter, R.; Audard, M.; Lanz, T.
2009A&A...498..217M Altcode:
Aims: Soft X-ray excesses have been detected in several Be/X-ray
binaries and interpreted as the signature of hard X-ray reprocessing
in the inner accretion disk. The system XMMU J054134.7-682550, located
in the LMC, featured a giant Type II outburst in August 2007. The
geometry of this system can be understood by studying the response
of the soft excess emission to the hard X-ray pulses. Methods:
We have analyzed series of simultaneous observations obtained with
XMM-Newton/EPIC-MOS and RXTE/PCA in order to derive spectral and
temporal characteristics of the system, before, during and after the
giant outburst. Spectral fits were performed and a timing analysis
has been carried out. Spectral variability, spin period evolution
and energy dependent pulse shapes are analysed. Results: The
outburst ({L}_X= 3× 1038 erg/s≈ {L}_EDD) spectrum could
be modeled successfully using a cutoff powerlaw, a cold disk emission,
a hot blackbody, and a cyclotron absorption line. The magnetic field
and magnetospheric radius could be constrained. The thickness of the
inner accretion disk is broadened to a width of 75 km. The hot blackbody
component features sinusoidal modulations indicating that the bulk of
the hard X-ray emission is emitted preferentially along the magnetic
equator. The spin period of the pulsar decreased very significantly
during the outburst. This is consistent with a variety of neutron star
equations of state and indicates a very high accretion rate. Based
on observations obtained with XMM-Newton, an ESA science mission with
instruments and contributions directly funded by ESA Member States
and NASA.

---------------------------------------------------------
Title: Recent Progress in Modeling Stellar Atmospheres
Authors: Hubeny, I.; Lanz, T.
2009ASSP....7..211H Altcode: 2009nqsa.conf..211H
We briefly review recent progress in modeling stellar atmospheres. We
first summarize various levels of approximations being used in
constructing the models, and present a brief overview of existing
model atmosphere codes. We then concentrate on a description of our
universal computer program Tlusty, and our recent grids of NLTE metal
line blanketed model photospheres of O-stars - OSTAR2002 - and early
B-stars - BSTAR2006.

---------------------------------------------------------
Title: Radiative Shocks in the Context of Young Stellar Objects:
A Combined Analysis from Experiments and Simulations
Authors: Stehlé, Chantal; González, Matthias; Audit, Edouard;
Lanz, Thierry
2009ASSP...13..359S Altcode: 2009pjc..book..359S
Hypersonic flows occurring during stellar formation are structured
by radiation. When radiation is reabsorbed in different locations of
the accretion flows or in the bow shocks of highly supersonic jets,
the coupling between hydrodynamics and radiation becomes an important
feature that significantly affects the hydrodynamical structure as well
as spectroscopic signatures of these shocks. Radiative shocks can now
be studied in the laboratory using high-energy lasers. Using recent
laboratory experiment and state-of-the-art multi-dimensional radiative
hydrodynamics simulations, we present an up to date description of the
physical and hydrodynamical properties of radiative shocks, with an
emphasis on the aspects that are important for stellar hypersonic flows.

---------------------------------------------------------
Title: The magnetic field of the supergiant star ζ Ori A
Authors: Escolano, C.; Bouret, J. -C.; Donati, J. -F.; Martins, F.;
Lanz, T.; Marcolino, W.; Howarth, I.
2008sf2a.conf..465E Altcode:
We present the results obtained on the O9.7 supergiant ζ Ori with
the spectropolarimeter NARVAL at the 2M Telescope Bernard Lyot atop
Pic du Midi (France). We detected the presence of a weak magnetic
field of about 50-100G, making ζ Ori the third O star known to
host a magnetic field and the first magnetic O star with a 'normal'
rotationnal velocity. The magnetic field of Zeta Ori is the weakest
magnetic field ever detected on a massive star and is lower than
the thermal equipartition limit (about 100 G). By fitting synthetic
spectra (obtained from NLTE stellar atmosphere models), we derived
the physical properties of ζ Ori. This lattest is a 40 M_{⊙} star,
with a radius of 25 R_{⊙} and appears quite evolved with an age
of 5-6Myr. Despite its evolutionnary status, ζ Ori does not show
signs of nitrogen surface enrichment. Concerning the wind of ζ Ori,
we estimated a mass loss rate of about 2×10^{-6} M_{⊙}.yr^{-1}. The
magnetic topology of ζ Ori is apparently more complex than a simple
dipole and involves two main magnetic polarities located on both sides
of the northern hemisphere. Our data also suggest that ζ Ori rotates
in about 7.0 days and is about 40 degrees away from pole-on to an
Earth-based observer. Despite its weakness, the detected field appears
sufficient to affect significantly the wind structure: the corresponding
Alfvén radius is however very close to the surface of the star, thus
generating a rotational modulation in wind lines different than that
reported on the two other known magnetic O stars. Finally, the
rapid rotation of ζ Ori with respect to θ^{1} Ori C is surprising
since both stars have similar unsigned magnetic fluxes (once rescaled
to the same radius). This may indicate that the field of ζ Ori is not
a fossil remnant (as opposed to that of θ^{1} Ori C and HD191612)
but rather the result of exotic dynamo processes produced through
MHD instabilities.

---------------------------------------------------------
Title: The Gaia satellite: a tool for Emission Line Stars and
Hot Stars
Authors: Martayan, C.; Frémat, Y.; Blomme, R.; Jonckheere, A.; Borges,
M.; de Batz, B.; Leroy, B.; Sordo, R.; Bouret, J. -C.; Martins, F.;
Zorec, J.; Neiner, C.; Nazé, Y.; Alecian, E.; Floquet, M.; Hubert,
A. -M.; Briot, D.; Miroshnichenko, A.; Kolka, I.; Stee, P.; Lanz,
T.; Meynet, G.
2008sf2a.conf..499M Altcode: 2008arXiv0809.4417M
The Gaia satellite will be launched at the end of 2011. It will
observe at least 1 billion stars, and among them several million
emission line stars and hot stars. Gaia will provide parallaxes for
each star and spectra for stars till V magnitude equal to 17. After a
general description of Gaia, we present the codes and methods, which
are currently developed by our team. They will provide automatically
the astrophysical parameters and spectral classification for the hot
and emission line stars in the Milky Way and other close local group
galaxies such as the Magellanic Clouds.

---------------------------------------------------------
Title: On the Importance of the Interclump Medium for Superionization:
O VI Formation in the Wind of ζ Puppis
Authors: Zsargó, J.; Hillier, D. J.; Bouret, J. -C.; Lanz, T.;
Leutenegger, M. A.; Cohen, D. H.
2008ApJ...685L.149Z Altcode: 2008arXiv0808.2233Z
We have studied superionization and X-ray line formation in the
spectra of ζ Pup using our new stellar atmosphere code (XCMFGEN)
that can be used to simultaneously analyze optical, UV, and X-ray
observations. Here, we present results on the formation of the O VI
λλ1032, 1038 doublet. Our simulations, supported by simple theoretical
calculations, show that clumped wind models that assume void in the
interclump space cannot reproduce the observed O VI profiles. However,
enough O VI can be produced if the voids are filled by a low-density
gas. The recombination of O VI is very efficient in the dense material,
but in the tenuous interclump region an observable amount of O VI
can be maintained. We also find that different UV resonance lines
are sensitive to different density regimes in ζ Pup: C IV is almost
exclusively formed within the densest regions, while the majority
of O VI resides between clumps. N V is an intermediate case, with
contributions from both the tenuous gas and clumps.

---------------------------------------------------------
Title: The weak magnetic field of the O9.7 supergiant ζOrionisA
Authors: Bouret, J. -C.; Donati, J. -F.; Martins, F.; Escolano, C.;
Marcolino, W.; Lanz, T.; Howarth, I. D.
2008MNRAS.389...75B Altcode: 2008arXiv0806.2162B; 2008MNRAS.tmp..842B
We report here the detection of a weak magnetic field of 50-100G on
the O9.7 supergiant ζOrionisA (ζOriA), using spectropolarimetric
observations obtained with NARVAL at the 2-m Télescope Bernard
Lyot atop Pic du Midi (France). ζOriA is the third O star known
to host a magnetic field (along with θ1OriC and
HD191612), and the first detection on a `normal' rapidly rotating
O star. The magnetic field of ζOriA is the weakest magnetic field
ever detected on a massive star. The measured field is lower than
the thermal equipartition limit (about 100G). By fitting non-local
thermodynamic equilibrium (NLTE) model atmospheres to our spectra,
we determined that ζOriA is a 40Msolar star with a
radius of 25Rsolar and an age of about 5-6Myr, showing no
surface nitrogen enhancement and losing mass at a rate of about 2 ×
10-6Msolaryr-1. The magnetic
topology of ζOriA is apparently more complex than a dipole and
involves two main magnetic polarities located on both sides of the same
hemisphere; our data also suggest that ζOriA rotates in about 7.0d and
is about 40° away from pole-on to an Earth-based observer. Despite its
weakness, the detected magnetic field significantly affects the wind
structure; the corresponding Alfvén radius is however very close to
the surface, thus generating a different rotational modulation in wind
lines than that reported on the two other known magnetic O stars. The rapid rotation of ζOriA with respect to θ1OriC
appears as a surprise, both stars having similar unsigned magnetic
fluxes (once rescaled to the same radius); it may suggest that the
subequipartition field detected on ζOriA is not a fossil remnant (as
opposed to that of θ1 OriC and HD191612), but the result
of an exotic dynamo action produced through magnetohydrodynamics (MHD)
instabilities. Based on observations obtained at the Télescope
Bernard Lyot (TBL), operated by the Institut National des Science
de l'Univers of the Centre National de la Recherche Scientifique of
France. E-mail: jean-claude.bouret@oamp.fr

---------------------------------------------------------
Title: CNO abundance determination in massive fast rotating stars
Authors: Frémat, Y.; Zorec, J.; Martayan, C.; Lanz, T.
2008RMxAC..33...53F Altcode:
Be stars are early-type Main Sequence objects which have shown at least
once some emission in the hydrogen line series. Their most important
characteristic is fast rotation, which on average reach ∼ 90% of the
critical rate, i.e. Ω/Ω_{c} %∼ 0.90. Since a number of them have
masses M &gt; 15 M_⊙, they are ideal targets to study the effects
of rotational mixing on the atmospheric CNO abundance and to perform
comparisons with the predictions obtained from recent evolutionary
models with rotation.

---------------------------------------------------------
Title: The spectra of massive stars with Gaia
Authors: Bouret, J. -C.; Lanz, T.; Frémat, Y.; Martins, F.; Lefever,
K.; Blomme, R.; Martayan, C.; Neiner, C.; Quinet, P.; Zorec, J.
2008RMxAC..33...50B Altcode:
ESA's cornerstone mission Gaia is scheduled to be launched in late
2011. It will determine the positions and distances of more than 24
million stars within 3 kpc of the Sun with a 1% relative accuracy. About
150,000 O and B-type stars are expected in the whole sample, as well
as additional peculiar stars such as Be, B[e], and WR stars. Gaia will
also provide spectrophotometry over the spectral range 330-1150 nm
(RP/BP bands) and medium-resolution spectroscopy in the domain ranging
from 847 to 874 nm (RVS spectral range). A large effort is now under
way to develop the necessary software aiming at automatically derive
the astrophysical parameters and radial velocities. One component of
this development is the computation of grids of synthetic spectra for
different types of stars and, in particular, for massive stars.

---------------------------------------------------------
Title: Pulsed thermal emission in the accreting pulsar HMXB XMMU
J054134.7-682550 reveals accretion geometry
Authors: Manousakis, Antonia; Walter, Roland; Audard, Marc; Lanz,
Thierry
2008xru..confE..51M Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Argon Abundances in the Solar Neighborhood: Non-LTE Analysis
of Orion Association B-Type Stars
Authors: Lanz, Thierry; Cunha, Katia; Holtzman, Jon; Hubeny, Ivan
2008ApJ...678.1342L Altcode: 2007arXiv0709.2147L
Argon abundances have been derived for a sample of B main-sequence
stars in the Orion association. The abundance calculations are based on
non-LTE (NLTE) metal line-blanketed model atmospheres calculated with
the NLTE code TLUSTY and an updated and complete argon model atom. We
derive an average argon abundance for this young population of A(Ar)
= 6.66 +/- 0.06. While our result is in excellent agreement with a
recent analysis of the Orion Nebula, it is significantly higher than
the currently recommended solar value, which is based on abundance
measurements in the solar corona. Moreover, the derived argon abundances
in the Orion B stars agree very well with a measurement from a solar
impulsive flare during which unmodified solar photospheric material
was brought to flare conditions. We therefore argue that the argon
abundances obtained independently for both the Orion B stars and the
Orion Nebula are representative of the disk abundance value in the
solar neighborhood. The lower coronal abundance may reflect a depletion
related to the first ionization potential (FIP) effect. We propose a new
reference value for the abundance of argon in the solar neighborhood,
A(Ar) = 6.63 +/- 0.10, corresponding to Ar/O = 0.009 +/- 0.002. Based on observations obtained with the Apache Point Observatory
3.5 m telescope, which is owned and operated by the Astrophysical
Research Consortium.

---------------------------------------------------------
Title: Clumping in O-type Supergiants
Authors: Bouret, J. -C.; Lanz, T.; Hillier, D. J.; Foellmi, C.
2008cihw.conf...31B Altcode:
We have analyzed the spectra of seven Galactic O4 supergiants, with the
NLTE wind code CMFGEN. For all stars, we have found that clumped wind
models match well lines from different species spanning a wavelength
range from FUV to optical, and remain consistent with Hα data. We have
achieved an excellent match of the P V λλ1118, 1128 resonance doublet
and N IV λ1718, as well as He II λ4686 suggesting that our physical
description of clumping is adequate. We find very small volume filling
factors and that clumping starts deep in the wind, near the sonic
point. The most crucial consequence of our analysis is that the mass
loss rates of O stars need to be revised downward significantly, by a
factor of 3 and more compared to those obtained from smooth-wind models.

---------------------------------------------------------
Title: Orbital Parameters and Chemical Composition of Four White
Dwarfs in Post-Common-Envelope Binaries
Authors: Kawka, Adela; Vennes, Stéphane; Dupuis, Jean; Chayer,
Pierre; Lanz, Thierry
2008ApJ...675.1518K Altcode: 2007arXiv0711.1526K
We present FUSE observations of the hot white dwarfs in the
post-common-envelope binaries Feige 24, EUVE J0720-317, BPM 6502, and
EUVE J2013+400. The spectra show numerous photospheric absorption lines,
which trace the white dwarf orbital motion. We report the detection of
C III, O VI, P V, and Si IV in the spectra of Feige 24, EUVE J0720-317,
and EUVE J2013+400 and the detection of C III, N II, Si III, Si IV,
and Fe III in the spectra of BPM 6502. Abundance measurements support
the possibility that white dwarfs in post-common-envelope binaries
accrete material from the secondary star wind. The FUSE observations
of BPM 6502 and EUVE J2013+400 cover a complete binary orbit. We
used the FUSE spectra to measure the radial velocities traced by the
white dwarf in the four binaries, where the zero-point velocities
were fixed using the ISM velocities in the line of sight of the
stellar systems. For BPM 6502 we determined a white dwarf velocity
semiamplitude of KWD = 18.6 +/- 0.5 km s-1,
and with the velocity semiamplitude of the red dwarf companion
(KRD = 75.2 +/- 3.1 km s-1), we estimate the
mass ratio to be q = 0.25 +/- 0.01. Adopting a spectroscopic mass
determination for the white dwarf, we infer a low secondary mass of
MRD = 0.14 +/- 0.01 M⊙. For EUVE J2013+400
we determine a white dwarf velocity semiamplitude of KWD =
36.7 +/- 0.7 km s-1. The FUSE observations of EUVE J0720-317
cover approximately 30% of the binary period and, combined with the HST
GHRS measurements, we update the binary properties. FUSE observations
of Feige 24 cover approximately 60% of the orbit, and we combine this
data set with HST STIS data to update the binary properties.

---------------------------------------------------------
Title: White Dwarf Spectra and Calibration of X-ray Grating
Spectrometers
Authors: Kaastra, Jelle S.; Lanz, T.; Hubeny, I.; Paerels, F.
2008HEAD...10.0406K Altcode:
White dwarf spectra have been widely used as calibration sources for
X-ray and EUV instruments. The in-flight effective area calibration
of the RGS of XMM-Newton and of the LETGS of Chandra depend upon the
availability of reliable calibration sources. We calculate a grid of
model atmospheres for Sirius B and HZ 43A, and adjust the parameters
using several constraints until the ratio of the spectra of both stars
agrees with the ratio observed with the LETGS of Chandra. This ratio
is independent of any errors in the effective area of the LETGS. We
determine how accurately the effective area of the LETGS is determined
using our method, and find interesting constraints on the parameters
for both stars. We discuss the role of the Lyman pseudo-continuum
in the calculation of the spectrum of Sirius B. The treatment of
that pseudo-continuum appears to play a fundamental role in the
ultimate accuracy that can be reached. With the proper treatment of
the pseudo-continuum, the soft X-ray flux of both stars and thereby
the absolute effective area of the LETGS can be determined with an
uncertainty of less than 5 %.

---------------------------------------------------------
Title: CAL 83 detected again in X-rays, dimming in optical
Authors: Lanz, T.; Audard, M.; Walter, F.
2008ATel.1438....1L Altcode:
We report a detection of CAL 83 with the Swift X-ray telescope during
a short exposure (490 sec) started on 2008 March 20 at 10:27 UT, with
a count rate of about 0.024 ct/s, corresponding to about 15% of the
normal X-ray flux of CAL 83. The latest X-ray off-state of CAL 83 was
discovered with Swift on 2008 Jan 2 (Greiner et al. 2008, ATel #1357),
following an increase of brightness in the optical a week earlier.

---------------------------------------------------------
Title: Testing the Evolution of Young Stars with FU Orionis Objects
Authors: Audard, Marc; Baldovin Saavedra, Carla; Carmona, Andres;
Lanz, Thierry
2008sptz.prop50654A Altcode:
FU Orionis objects (FUors) play a central role in the study of the
evolution of young stars. They undergo accretion outbursts that can
bring up to about 0.01 solar masses per outburst, therefore providing
a significant amount of mass in the pre-main sequence life of a
star. Spitzer IRS observations of FUors have shown either silicate
features in absorption and ice bands or silicates in emission with no
ice bands. This led some authors to propose an evolutionary paradigm
in which the former FUors are younger than the latter FUors. The FUor
phase might be a common but rarely observed phase (about 20 FUors
known) of most young low-mass stars and might be the link between
Class I and Class II sources. So far, Spitzer has obtained IRS spectra
of about half of the FUor catalog, focusing on its most famous class
members. With this short Spitzer proposal, we aim to observe FUors that
have not yet been observed with the IRS or had no high-resolution module
spectra. We will study the composition and evolution of dust grains and
of ices in FUors surrounded by envelopes, and we aim at detecting faint
molecular hydrogen and atomic lines with the high-resolution module
spectra. The IRS spectra will probe the stellar environment, and thus
test the above evolutionary paradigm and the different theories for the
origin of FUor outburst. The Spitzer IRS observations of our sample of
FUors will therefore provide additional insights about a significant
phase in the life of young stars with the aim to better describe the
evolutionary sequence from Class I star to Class II sources.

---------------------------------------------------------
Title: New Views of the Horizontal Branch in ω Centauri
Authors: Moehler, S.; Dreizler, S.; Lanz, T.; Bono, G.; Sweigart,
A. V.; Calamida, A.; Monelli, M.; Nonino, M.
2008ASPC..392...47M Altcode:
UV observations of some massive globular clusters uncovered a
significant population of very hot stars below the hot end of the
horizontal branch, the so-called blue hook stars. This feature might
be explained either by the late hot flasher scenario where stars
experience the helium flash while on the white dwarf cooling curve
or by the helium-rich sub-population recently postulated to exist in
some clusters. Previous spectroscopic analyses of blue hook stars in
ω Cen and NGC 2808 support the late hot flasher scenario, but the
stars were found to contain much less helium than expected and the
predicted carbon and nitrogen enrichment could not be verified. New
moderately high resolution spectra of stars at the hot end of the blue
horizontal branch in ω Cen were analysed for atmospheric parameters
( Teff, log g, and log{nHe}/{nH})
and abundances using LTE and non-LTE model atmospheres. In the
temperature range 30,000 K to 50,000 K we find that 35% of our
stars are helium-poor (log{nHe/{nH} &lt;
-2), 51% have solar helium abundance within a factor of 3 (-1.5 ≤
log{nHe}/{nH} ≤ -0.5) and 14% are helium-rich
(log{nHe/{nH} &gt; -0.4). We also find carbon
enrichment along with helium enrichment, with a maximum carbon abundance
of 3% by mass. At least 14% of the hottest horizontal branch stars
in ωCen show helium abundances well above the highest predictions
from the helium enrichment scenario (Y≈ 0.42, corresponding to
log{nHe}/{nH} ≈ -0.74). In addition, the most
helium-rich stars show high carbon abundances as predicted by the late
hot flasher scenario. We conclude that the helium-rich horizontal branch
stars in ω Cen cannot be explained solely by the helium-enrichment
scenario invoked to explain the blue main sequence.

---------------------------------------------------------
Title: The hottest horizontal-branch stars in ω Centauri. Late hot
flasher vs. helium enrichment
Authors: Moehler, S.; Dreizler, S.; Lanz, T.; Bono, G.; Sweigart,
A. V.; Calamida, A.; Monelli, M.; Nonino, M.
2007A&A...475L...5M Altcode: 2007arXiv0709.2839M
Context: UV observations of some massive globular clusters have revealed
a significant population of very hot stars below the hot end of the
horizontal branch (HB), the so-called blue hook stars. This feature
might be explained either as a result of the late hot flasher scenario
where stars experience the helium flash while on the white dwarf cooling
curve or by the progeny of the helium-enriched sub-population recently
postulated to exist in some clusters. Previous spectroscopic analyses
of blue hook stars in ω Cen and NGC 2808 support the late hot flasher
scenario, but the stars contain much less helium than expected and the
predicted C, N enrichment could not be verified. Aims: We compare
observed effective temperatures, surface gravities, and abundances of
He, C, and N of blue hook and canonical extreme horizontal branch (EHB)
star candidates to the predictions of the two scenarios. Methods:
Moderately high resolution spectra of stars at the hot end of the
blue HB in the globular cluster ω Cen were analysed for atmospheric
parameters and abundances using LTE and non-LTE model atmospheres. Results: In the temperature range 30 000 K to 50 000 K we find that
35% of our stars are helium-poor (log{{n_He}/{n_H}} &lt; -2), 51% have
solar helium abundance within a factor of 3 (-1.5 ⪉ log{{n_He}/{n_H}}
⪉ -0.5) and 14% are helium-rich (log{{n_He}/{n_H}}} &gt; -0.4). We
also find carbon enrichment strongly correlated with helium enrichment,
with a maximum carbon enrichment of 3% by mass. Conclusions: At
least 14% of the hottest HB stars in ω Cen show helium abundances well
above the highest predictions from the helium enrichment scenario (Y =
0.42 corresponding to log{{n_He}/{n_H}} ≈ -0.74). In addition, the
most helium-rich stars show strong carbon enrichment, as predicted by
the late hot flasher scenario. We conclude that the helium-rich HB stars
in ω Cen cannot be explained solely by the helium-enrichment scenario
invoked to explain the blue main sequence. Based on observations
with the ESO Very Large Telescope at Paranal Observatory, Chile
(proposal IDs 075.D-0280(A) and 077.D-0021(A)).

---------------------------------------------------------
Title: VizieR Online Data Catalog: VLT-FLAMES survey of massive stars
(Hunter+, 2007)
Authors: Hunter, I.; Dufton, P. L.; Smartt, S. J.; Ryans, R. S. I.;
Evans, C. J.; Lennon, D. J.; Trundle, C.; Hubeny, I.; Lanz, T.
2007yCat..34660277H Altcode:
Files table3.dat, table4.dat and table5.dat contain the equivalent
widths and abundances estimates of the observed metal lines for
B-type stars in NGC 6611, N11 and NGC 346 respectively. The equivalent
widths of the spectral lines have been measured using the line fitting
program ELF in the spectral analysis package DIPSO (Howarth et al.,
1994, Starlink User Note, No. 50.15). The nitrogen equivalent widths
and abundances are upper limits for objects N11-037, N11-047, N11-101,
NGC 346-029, NGC 346-039, NGC 346-040, NGC 346-043, NGC 346-044 and
NGC 346-075. table6.dat contains details of the adopted atomic
data for the absorption lines considered in this analysis. Model atoms
are discussed in Lanz &amp; Hubeny (2003ApJS..146..417L) and Allende
Prieto et al. (2003ApJS..147..363A), see Dufton et al. (2005, Cat. )
for further details. Note, the loggf values of the Si II lines at 4128
and 4131 angstroms have been updated to include improved atomic data
(A. Korn, private communication). (5 data files).

---------------------------------------------------------
Title: The VLT-FLAMES survey of massive stars: surface chemical
compositions of B-type stars in the Magellanic Clouds
Authors: Hunter, I.; Dufton, P. L.; Smartt, S. J.; Ryans, R. S. I.;
Evans, C. J.; Lennon, D. J.; Trundle, C.; Hubeny, I.; Lanz, T.
2007A&A...466..277H Altcode: 2006astro.ph..9710H
We present an analysis of high-resolution FLAMES spectra of
approximately 50 early B-type stars in three young clusters at different
metallicities, NGC 6611 in the Galaxy, N 11 in the Large Magellanic
Cloud (LMC) and NGC 346 in the Small Magellanic Cloud (SMC). Using
the tlusty non-LTE model atmospheres code, atmospheric parameters and
photospheric abundances (C, N, O, Mg and Si) of each star have been
determined. These results represent a significant improvement on the
number of Magellanic Cloud B-type stars with detailed and homogeneous
estimates of their atmospheric parameters and chemical compositions. The
relationships between effective temperature and spectral type are
discussed for all three metallicity regimes, with the effective
temperature for a given spectral type increasing as one moves to a
lower metallicity regime. Additionally the difficulties in estimating
the microturbulent velocity and the anomalous values obtained,
particularly in the lowest metallicity regime, are discussed. Our
chemical composition estimates are compared with previous studies,
both stellar and interstellar with, in general, encouraging agreement
being found. Abundances in the Magellanic Clouds relative to the
Galaxy are discussed and we also present our best estimates of the
base-line chemical composition of the LMC and SMC as derived from B-type
stars. Additionally we discuss the use of nitrogen as a probe of the
evolutionary history of stars, investigating the roles of rotational
mixing, mass-loss, blue loops and binarity on the observed nitrogen
abundances and making comparisons with stellar evolutionary models
where possible. Based on observations at the European Southern
Observatory in programmes 171.0237 and 073.0234. Tables 3-6 and
Figs. 7-31 are only available in electronic form at http://www.aanda.org

---------------------------------------------------------
Title: A Grid of NLTE Line-blanketed Model Atmospheres of Early
B-Type Stars
Authors: Lanz, Thierry; Hubeny, Ivan
2007ApJS..169...83L Altcode: 2006astro.ph.11891L
We have constructed a comprehensive grid of 1540 metal line-blanketed,
NLTE, plane-parallel, hydrostatic model atmospheres for the basic
parameters appropriate to early B-type stars. The BSTAR2006
grid considers 16 values of effective temperatures, 15,000
K&lt;=Teff&lt;=30,000 K with 1000 K steps, 13 surface
gravities, 1.75&lt;=logg&lt;=4.75 with 0.25 dex steps, six chemical
compositions, and a microturbulent velocity of 2 km s-1. The
lower limit of logg for a given effective temperature is set by an
approximate location of the Eddington limit. The selected chemical
compositions range from twice to one-tenth of the solar metallicity
and metal-free. Additional model atmospheres for B supergiants
(logg&lt;=3.0) have been calculated with a higher microturbulent
velocity (10 km s-1) and a surface composition that is
enriched in helium and nitrogen and depleted in carbon. This new grid
complements our earlier OSTAR2002 grid of O-type stars (our Paper
I). The paper contains a description of the BSTAR2006 grid and some
illustrative examples and comparisons. NLTE ionization fractions,
bolometric corrections, radiative accelerations, and effective
gravities are obtained over the parameter range covered by the grid. By
extrapolating radiative accelerations, we have determined an improved
estimate of the Eddington limit in absence of rotation between 55,000
and 15,000 K. The complete BSTAR2006 grid is available at the TLUSTY
Web site.

---------------------------------------------------------
Title: A Mid-Infrared Spitzer Study of the Herbig Be Star R Mon and
the Associated HH 39 Herbig-Haro Object
Authors: Audard, M.; Skinner, S.; Guedel, M.; Lanz, T.; Paerels, F.;
Arce, H.
2007astro.ph..1906A Altcode:
We report on initial results of our Spitzer Cycle 2 program to observe
the young massive star R Mon and its associated HH 39 Herbig-Haro object
in the mid-infrared. Our program used all instruments on-board Spitzer
to obtain deep images with IRAC of the HH 39 complex and of R Mon and
its surroundings, a deep image of HH 39 at 24 and 70 $\mu$m with MIPS,
and mid-infrared spectra with the SH, LH, and LL modules of IRS. The
aim of this program is to study the physical links in a young massive
star between accretion disk, outflows and jets, and sh ocks in the
associated HH object. Our preliminary analysis reveals that several
knots of HH 39 are clearly detected in most IRAC bands. In IRAC4 (8
$\mu$m), diffuse emission, probably from PAHs, appears as foreground
emission covering the HH 39 emission. The HH 39 knots are detected at
24 microns, despite the fact that dust continuum emission covers the
knots and shows the same structure as observed with IRAC4. The IRS
spectra of HH 39 show weak evidence of [Ne II] 12.8 $\mu$m and 0--0
S(1) H$_2$ 17.0 $\mu$m lines. A more detailed analysis is, however,
required due to the faintness of the Herbig-Haro knots. Finally,
we obtained the SH and MIPS SED spectra of R Mon. A PAH emission
feature at 11.3 $\mu$m is detected on top of the strong continuum;
although no strong emission or absorption lines are observed, we will
seek to detect faint lines. The combined IRAC, IRS, and MIPS data of
the R Mon/HH 39 system will help us to understand circumstellar disk
processing, and the connection between jets, outflows, and HH objects.

---------------------------------------------------------
Title: Iron in Hot DA White Dwarfs
Authors: Vennes, Stéphane; Chayer, Pierre; Dupuis, Jean; Lanz, Thierry
2006ApJ...652.1554V Altcode: 2006astro.ph..8416V
We present a study of the iron abundance pattern in hot,
hydrogen-rich (DA) white dwarfs. The study is based on new and
archival far-ultraviolet spectroscopy of a sample of white dwarfs in the
temperature range 30,000 K&lt;~Teff&lt;~64,000 K. The spectra
obtained with the Far Ultraviolet Spectroscopic Explorer, along with
spectra obtained with the Hubble Space Telescope Imaging Spectrograph
and the International Ultraviolet Explorer, sample Fe III-Fe VI
absorption lines, enabling a detailed iron abundance analysis over a
wider range of effective temperatures than previously afforded. The
measurements reveal abundance variations in excess of 2 orders of
magnitude between the highest and the lowest temperatures probed,
but also show considerable variations (over 1 order of magnitude)
between objects with similar temperatures and surface gravities. Such
variations in cooler objects may be imputed to accretion from unseen
companions or so-called circumstellar debris, although the effect of
residual mass loss and selective radiation pressure in the hottest
objects in the sample remain dominant.

---------------------------------------------------------
Title: Modeling multidimensional effects in the propagation of
radiative shocks
Authors: Leygnac, Sébastien; Boireau, Laurent; Michaut, Claire;
Lanz, Thierry; Stehlé, Chantal; Clique, Christine; Bouquet, Serge
2006PhPl...13k3301L Altcode: 2006astro.ph.10876L
Radiative shocks (also called supercritical shocks) are high Mach
number shock waves that photoionize the medium ahead of the shock
front and give rise to a radiative precursor. They are generated
in the laboratory using high-energy or high-power lasers and are
frequently present in a wide range of astronomical objects. Their
modelization in one dimension has been the subject of numerous studies,
but generalization to three dimensions is not straightforward. We
calculate analytically the absorption of radiation in a gray uniform
cylinder and show how it decreases with χR, the product of the
opacity χ and of the cylinder radius R. Simple formulas, whose
validity range increases when χR diminishes, are derived for the
radiation field on the axis of symmetry. Numerical calculations in
three dimensions of the radiative energy density, flux, and pressure
created by a stationary shock wave show how the radiation decreases
with R. Finally, the bidimensional structures of both the precursor
and the radiation field are calculated with time-dependent radiation
hydrodynamics numerical simulations and the influence of two-dimensional
effects on the electron density, the temperature, the shock velocity,
and the shock geometry are exhibited. These simulations show how the
radiative precursor shortens, cools, and slows down when R is decreased.

---------------------------------------------------------
Title: The Origin of CAL 83 X-Ray Off-States
Authors: Lanz, Thierry
2006xmm..prop...26L Altcode:
The close binary supersoft X-ray source CAL 83 is a most promising
candidate of SN Ia progenitors. Our XMM-Newton and Chandra
observations show a massive white dwarf, exhibit large amplitude
short-term variations of the X-ray flux, and establish definitively
its recurrent nature. We propose now to conduct a campaign with EPIC
and OM on XMM-Newton to substantiate the correlation between X-ray
and optical variations on a short, medium, and long-term basis. This
campaign will trace the physical relation between the accretion disk
and the white dwarf, and will discriminate between several models
explaining the origin of CAL 83 off-states. XMM-Newton data will offer
unique and timely insights into accretion and unstable nuclear burning
during the last evolution stages of SN Ia progenitors.

---------------------------------------------------------
Title: On the sensitivity of He I singlet lines to the Fe IV model
atom in O stars
Authors: Najarro, F.; Hillier, D. J.; Puls, J.; Lanz, T.; Martins, F.
2006A&A...456..659N Altcode: 2006astro.ph..5211N
Aims.Recent calculations and analyses of O star spectra have revealed
discrepancies between theory and observations, and between different
theoretical calculations, for the strength of optical He I singlet
transitions. We investigate the source of these discrepancies. 
Methods: .Using a non-LTE radiative transfer code we have undertaken
detailed test calculations for a range of O star properties. Our
principal test model has parameters similar to those of the O9V star,
10 Lac. Results: .We show that the discrepancies arise from
uncertainties in the radiation field in the He I 1s2
^1S-1s 2p ^1P° transition near 584 Å. The radiation field at 584
Å is influenced by model assumptions, such as the treatment of
line-blanketing and the adopted turbulent velocity, and by the Fe
IV atomic data. It is shown that two Fe IV transitions near 584 Å
can have a substantial influence on the strength of the He I singlet
transitions. Conclusions: .Because of the difficulty of modeling
the He I singlet lines, particularly in stars with solar metalicity,
the He I triplet lines should be preferred in spectral analyses. These
lines are much less sensitive to model assumptions.

---------------------------------------------------------
Title: Supersoft Sources at Low Metallicity: SMC 13
Authors: Lanz, Thierry
2006cxo..prop.2120L Altcode:
The identification of SN Ia progenitors remains elusive. A promising
channel leading to a SN Ia event involves accreting white dwarfs (WD)
that sustain steady nuclear burning. They are observed as close binary
X-ray sources with a supersoft spectrum (CBSS). We propose to obtain
the first Chandra LETGS spectrum of SMC 13, a CBSS in the SMC. From
absorption line spectroscopy, we will achieve significant advances in
determing the properties of SMC 13 and provide unique data to constrain
new evolutionary calculations of accreting hot white dwarfs. At
low metallicities, the population of SN Ia progenitors shifts toward
systems with higher initial WD masses and lower mass donor stars. This
difference might turn out to be crucial to understanding the physics
of local vs. distant SN Ia.

---------------------------------------------------------
Title: High Resolution Fe Lyman and Balmer Band Spectroscopy of the
Bursts in EXO 0748-676
Authors: Cottam, Jean; Paerels, F.; Telis, G.; Audard, M.; Mendez, M.;
Boirin, L.; Lanz, T.; Lewin, W.; Marshall, H.; Bildsten, L.; Chang, P.
2006HEAD....9.0205C Altcode: 2006BAAS...38..345C
We present the results of high resolution spectroscopy of the x-ray
bursts in the LMXB EXO 0748-676. Early observations with XMM/RGS showed
evidence for gravitationally redshifted absorption lines, which can
be identified with the n=2-3 transitions of H- and He-like Fe arising
in the photosphere of the neutron star at z=0.35. We will discuss
the results from a 600 ks follow-up observation with the XMM/RGS,
and from a 300 ks observation with the Chandra/HETGS.

---------------------------------------------------------
Title: Neon Abundances in B Stars of the Orion Association: Solving
the Solar Model Problem?
Authors: Cunha, Katia; Hubeny, Ivan; Lanz, Thierry
2006ApJ...647L.143C Altcode: 2006astro.ph..6738C
We report on non-LTE Ne abundances for a sample of B-type stellar
members of the Orion association. The abundances were derived by means
of non-LTE fully metal-blanketed model atmospheres and extensive model
atoms with updated atomic data. We find that these young stars have
a very homogeneous abundance of A(Ne)=8.11+/-0.04. This abundance
is higher by ~0.3 dex than the currently adopted solar value,
A(Ne)=7.84, which is derived from lines produced in the corona and
active regions. The general agreement between the abundances of C, N,
and O derived for B stars with the solar abundances of these elements
derived from three-dimensional hydrodynamical models atmospheres
strongly suggests that the abundance patterns of the light elements in
the Sun and B stars are broadly similar. If this hypothesis is true,
then the Ne abundance derived here will help to reconcile solar models
with helioseismological observations.

---------------------------------------------------------
Title: VizieR Online Data Catalog: SMC NGC 346-11 and AV 304 spectra
(Hunter+, 2005)
Authors: Hunter, I.; Dufton, P. L.; Ryans, R. S. I.; Lennon, D. J.;
Rolleston, W. R. J.; Hubeny, I.; Lanz, T.
2006yCat..34360687H Altcode:
High-resolution spectra have been obtained for both NGC 346-11 and AV
304 using the Ultraviolet and Visual Echelle Spectrograph, UVES, on
the UT2 (Kueyen) telescope at the European Southern Observatory. NGC
346-11 was observed during a three night run in November 2001,
whilst the observations of AV 304, taken during a two night run
in January 2001, have been previously discussed by Rolleston et
al. (2003A&amp;A...400...21R). (1 data file).

---------------------------------------------------------
Title: Hydrogen Column Density Measurements toward White Dwarfs for
D/H Studies
Authors: Vennes, S.; Chayer, P.; Dupuis, J.; Moos, H. W.; Lanz, T.
2006ASPC..348..460V Altcode:
Hot white dwarfs provide sources of extreme ultraviolet (EUV) radiation
against which the photoionization continua of interstellar H I, He I,
and He II are measured. The column density measurements depend on a
detailed knowledge of the intrinsic EUV luminosity of the sources. We
discuss recent progress in the study of low- and high-metallicity
white dwarfs enabling a study of interstellar hydrogen and helium
continuum absorptions toward many lines of sight. We compare the
EUV-based measurements to recent hydrogen column density measurements
based on Lyα line profiles and discuss the implications for D/H and
O/H ratios in the local interstellar medium.

---------------------------------------------------------
Title: FUSE Spectroscopy of Hybrid White Dwarfs
Authors: Lanz, T.; Holberg, J. B.; Hubeny, I.
2006ASPC..348..197L Altcode:
We have obtained FUSE spectra of three white dwarfs showing hybrid
helium and hydrogen spectra. We present initial results of our
NLTE model atmosphere analysis, and discuss their implication for
understanding the origin of DAB white dwarfs and the physical processes
in white dwarf atmospheres.

---------------------------------------------------------
Title: High Resolution Fe Lyman and Balmer Band Spectroscopy of the
Bursts in EXO 0748-676
Authors: Cottam, J.; Paerels, F. B. S.; Telis, G. A.; Mendez, M.;
Boirin, L.; Audard, M.; Lanz, T.; Lewin, W. H. G.; Marshall, H. L.;
Bildsten, L.; Chang, P.
2006AAS...207.3208C Altcode:
We present the results of high resolution spectroscopy of the X-ray
bursts in the LMXB EXO 0749-676. Early observations with XMM/RGS
showed evidence for gravitationally redshifted absorption lines,
which can be identified with the n=2-3 transitions in H- and He-like
Fe arising in the stellar photosphere at z=0.35. We will discuss
the results of spectroscopy of the Fe Lyman band with Chandra/HETGS,
and a reobservation in the Fe Balmer band with XMM/RGS.

---------------------------------------------------------
Title: The Density Structure of O Star Winds
Authors: Lanz, Thierry; Audard, Marc; Bouret, Jean-Claude; Hillier,
John
2006sptz.prop30030L Altcode:
Because of their dense hypersonic winds, O stars have a great
influence on their environment by the deposition of mechanical
energy and momentum, as well as by their copious emission of
ionizing photons. There is a mounting evidence that O stars have
highly-structured winds, ranging from X-ray wind emission, line
profile variability, and from detailed NLTE analyses of UV wind line
profiles. Substantially reduced mass loss rates are derived from simple
clumped wind models compared to the classical analyses performed with
smooth, homogeneous wind models. Mid-infrared recombination lines
and free-free continuum are particularly sensitive to the density at
the base of flow and thus allow us to investigate the region where
clumping is believed to be initiated. Moreover, the IR lines are not
as sensitive to the wind ionization as UV lines. We propose therefore
to obtain IRS high resolution spectroscopy and MIPS photometry of a
sample of O dwarfs and supergiants to empirically characterize the
density structure of O star winds, and derive robust estimates of
the mass loss rates. Because the ionizing spectrum of O stars is very
sensitive to the wind properties, the Spitzer data will be an essential
step towards a reliable prediction of ionizing fluxes of massive stars.

---------------------------------------------------------
Title: Fundamental Properties of O-Type Stars
Authors: Heap, Sara R.; Lanz, Thierry; Hubeny, Ivan
2006ApJ...638..409H Altcode: 2004astro.ph.12345H
We present a comprehensive analysis of high-resolution, far-ultraviolet
HST STIS, FUSE, and optical spectra of 18 O stars in the Small
Magellanic Cloud. Our analysis is based on the OSTAR2002 grid of
NLTE metal-line-blanketed model atmospheres calculated with our
code TLUSTY. We systematically explore and present the sensitivity
of various UV and optical lines to different stellar parameters. We
have obtained consistent fits of the UV and the optical spectrum to
derive the effective temperature, surface gravity, surface composition,
and microturbulent velocity of each star. Stellar radii, masses, and
luminosities follow directly. For stars of the same spectral subtype,
we find a general good agreement between effective temperature
determinations obtained with TLUSTY, CMFGEN, and FASTWIND models,
which are all lower than the standard Teff calibration
of O stars. We propose a new calibration between the spectral type
and effective temperature based on our results from UV metal lines,
as well as optical hydrogen and helium lines. The lower effective
temperatures translate into ionizing luminosities that are smaller
by a factor of 3 compared to luminosities inferred from previous
standard calibrations. The chemical composition analysis reveals
that the surface of about 80% of the program stars is moderately to
strongly enriched in nitrogen, while showing the original helium,
carbon, and oxygen abundances. Our results support the new stellar
evolution models that predict that the surface of fast rotating
stars becomes nitrogen-rich during the main-sequence phase because of
rotationally induced mixing. Enrichment factors are, however, larger
than predicted by stellar evolution models. Most stars exhibit the
“mass discrepancy” problem, which we interpret as a result of fast
rotation that lowers the measured effective gravity. Nitrogen enrichment
and low spectroscopic masses are therefore two manifestations of fast
rotation. Our study thus emphasizes the importance of rotation in
our understanding of the properties of massive stars and provides a
framework for investigating populations of low-metallicity massive
stars at low and high redshifts. Based on observations with
the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope
Science Institute, which is operated by the Association of Universities
for Research in Astronomy, Inc., under NASA contract NAS5-2655. Also
based on observations made with the NASA-CNES-CSA Far Ultraviolet
Spectroscopic Explorer, which is operated for NASA by Johns Hopkins
University under NASA contract NAS5-32985.

---------------------------------------------------------
Title: Lower Mass Loss Rates in O Stars
Authors: Bouret, J. -C.; Lanz, T.; Hillier, D. J.
2005AAS...20718223B Altcode: 2005BAAS...37.1467B
We have analyzed the far-UV and optical spectra of 5 Galactic O4
supergiants with the NLTE wind code CMFGEN. Stellar and wind parameters,
and the surface composition of the stars have been derived. The surface
composition for all 5 stars is typical of an evolved O supergiant
(nitrogen-rich, carbon and oxygen-poor), and reflects the appearance
of CNO processed material at the stellar surface. For all stars only
highly-clumped wind models match key lines of different species
in the far-UV and optical spectrum, in particular the \ion{P}{5}
resonance doublet, \ion{N}{4}1718, and Hα . Compared to homogeneous
wind models, models with clumping exhibit increased recombination in
the clumps and provide a better description of the wind ionization
structure. We also achieve a good fit to \ion{He}{2}4686 (especially
the electron scattering wings) with clumping. The sharp transition from
absorption to emission in the profile of UV wind lines indicate that
clumping starts deep in the wind, just above the sonic point. The most
crucial consequence of wind clumping is that the mass loss rates of O
stars need to be revised downward significantly, by a factor of 3 and
more. Similarly, we also recently derived lower mass loss rates for a
few O-type dwarfs. Our study therefore calls for a fundamental revision
in our understanding of mass loss and of O-type star stellar winds. Support from the NASA Astrophysics Data Program (grant NNG04GC81G)
is gratefully acknowledged.

---------------------------------------------------------
Title: Non-LTE Line-Blanketed Model Atmospheres of B-type Stars
Authors: Lanz, T.; Hubeny, I.
2005AAS...20718221L Altcode: 2005BAAS...37.1467L
We present an extension of our OSTAR2002 grid of NLTE model atmospheres
to B-type stars. We have calculated over 1,300 metal line-blanketed,
NLTE, plane-parallel, hydrostatic model atmospheres for the basic
parameters appropriate to B stars. The grid covers 16 effective
temperatures from 15,000 to 30,000 K, with 1000 K steps, 13 surface
gravities, log g≤ 4.75 down to the Eddington limit, and 5 compositions
(2, 1, 0.5, 0.2, and 0.1 times solar). We have adopted a microturbulent
velocity of 2 km/s for all models. In the lower surface gravity range
(log g≤ 3.0), we supplemented the main grid with additional model
atmospheres accounting for higher microtutbulent velocity (10 km/s)
and for alterated surface composition (He and N-rich, C-deficient),
as observed in B supergiants. The models incorporate basically all
known atomic levels of 46 ions of H, He, C, N, O, Ne, Mg, Al, Si, S,
and Fe, which are grouped into 1127 superlevels. Models and spectra
will be available at our Web site, http://nova.astro.umd.edu.

---------------------------------------------------------
Title: A Spectroscopic Analysis of Blue Stragglers, Horizontal Branch
Stars, and Turnoff Stars in Four Globular Clusters
Authors: De Marco, Orsola; Shara, Michael M.; Zurek, D.; Ouellette,
John A.; Lanz, Thierry; Saffer, Rex A.; Sepinsky, Jeremy F.
2005ApJ...632..894D Altcode: 2005astro.ph.11497D
We present a spectroscopic analysis of HST STIS and FOS low- and
intermediate-resolution spectroscopy of 55 stars in four globular
clusters (47 Tucanae, M3, NGC 6752, and NGC 6397). Stars hotter
than Teff=5750 K and with a signal-to-noise ratio larger
than 15 were analyzed with non-local thermodynamic equilibrium model
atmospheres, and values for their effective temperatures and gravities
were obtained. Using photometric fluxes, we also obtained radii,
luminosities, and spectroscopic masses. Twenty-four stars in
our sample are blue stragglers (BSs). Their photometric colors and
magnitudes place these BSs above and redward of the clusters' zero-age
main sequence: this is consistent with the gravities we find for
these stars, which are lower than zero-age main-sequence gravities. A
comparison with stellar evolutionary tracks shows that almost all of our
BSs are in the Hertzsprung gap. This is contrary to theory, because of
the short timescale expected for stars in this evolutionary phase. The mean BS mass is 1.04 Msolar for 14 nonvariable stars,
or 1.07 Msolar counting all 24 BSs in our sample. For the
nonvariable stars the mean BS masses for individual clusters are 1.73,
1.01, 0.95, and 0.72 Msolar for NGC 6397, NGC 6752, 47 Tuc,
and M3, respectively. Adding the variable stars (which improves the
statistics but increases the uncertainty), the mean masses become 1.27,
1.05, 0.99, and 0.99 Msolar, respectively. Although there is
considerable scatter, the BS spectroscopic masses correlate with both
effective temperature and brightness of the stars, as expected. The mean nonvariable turnoff star mass (0.58 Msolar) is
significantly below the values determined for the BSs and below the
main-sequence turnoff mass. The mean nonvariable horizontal-branch
(HB) star mass is higher than expected (0.79 Msolar). In
particular, several HB stars have masses well above the main-sequence
turnoff mass. Some of these HB stars are suspected of actually being
BSs, since most of them reside at ambiguous locations on the CMD,
making them prone to misclassification. Values and limits to the
stellar rotation rates (vsini) are imposed by fitting weak metal lines,
the Ca II K line wings, or the helium lines for the hotter stars. Five
BSs with reasonably constrained rotations show average and median
vsini values of 109 and 100 km s-1, respectively, suggesting
v~160 km s-1. At least some GC BSs are very rapid rotators,
but this information cannot yet constrain their origin as stellar
collision or binary mergers because of the lack of clear theoretical
predictions. Six extreme HB stars have rotation rates vsini between 50
and 200 km s-1, which are high for these stars and might
indicate a binary origin. De Marco et al. found that four BSs
and two HB stars in our sample have Balmer jumps that are too large
for the effective temperatures implied by the slopes of their Paschen
continua. Two additional HB stars are now identified in the current
study as having the same feature. For these stars, the presence of a
disk of partly ionized material is suspected, although high stellar
rotation rates could also partly explain the data.

---------------------------------------------------------
Title: The Supersoft X-Ray Source CAL 83: A Massive White Dwarf
Authors: Lanz, Thierry; Audard, Marc; Paerels, Frits; Telis, Gisela A.
2005AIPC..797..313L Altcode:
We have obtained Chandra HRC-S/LETG spectroscopy of the prototypical
supersoft source CAL 83 in the Large Magellanic Cloud. The data
reveal a very rich absorption line spectrum from the hot white dwarf
photosphere. We have recently completed the analysis of the Chandra
spectrum and of an earlier XMM-Newton RGS spectrum of CAL 83 with new
non-LTE line-blanketed model atmospheres that explicitly include 74
ions of the 11 most abundant species. We have successfully matched the
Chandra and XMM-Newton spectra, and have thus derived the basic stellar
parameters of the hot white dwarf. In particular, we have obtained the
first direct spectroscopic evidence that the white dwarf is massive
(M &gt; 1Msolar). We also found no spectral signatures of a wind from
the white dwarf. These results provide direct support for supersoft
sources as likely progenitors of SN Ia.

---------------------------------------------------------
Title: Characterizing Type Ia Supernova Progenitor Candidates
Authors: Lanz, Thierry
2005cxo..prop.4038L Altcode:
The identification of SN Ia progenitors still remains elusive. The most
promising channel to form SN Ia involves accreting white dwarfs that
sustain steady nuclear burning. They are observed as ultrasoft X-ray
sources. Our NLTE analysis of the LETGS spectrum of the supersoft source
CAL 83 gives the first direct spectroscopic evidence of a massive WD. We
need now to show that massive WDs are common in supersoft sources. We
propose to perform a similar NLTE model atmosphere analysis of the
5 other supersoft sources that have been observed with Chandra. We
will derive the properties of the WDs and of the binary systems, thus
constraining the evolution of Close Binary Supersoft X-ray Sources. We
will explore the role of nova-like winds in CBSS for understanding
the WD evolution.

---------------------------------------------------------
Title: Lyman Line Spectroscopy of Hot Hydrogen-Rich White Dwarfs
Revisited
Authors: Vennes, S.; Chayer, P.; Dupuis, J.; Lanz, T.
2005ASPC..334..185V Altcode:
We present a study of 40 hot DA white dwarfs observed with the Far
Ultraviolet Spectroscopic Explorer (FUSE). We measured the effective
temperatures and surface gravities using a grid of pure-hydrogen
model atmospheres in local thermodynamic equilibrium (LTE). Systematic
differences with similar measurements based on Balmer line spectroscopy
are in part due to non-LTE departure in the hydrogen atom, but for
the most part they are due to the presence of heavy-elements in many
white dwarf atmospheres throughout the range of effective temperatures
covered in our sample.

---------------------------------------------------------
Title: Lower mass loss rates in O-type stars: Spectral signatures
of dense clumps in the wind of two Galactic O4 stars
Authors: Bouret, J. -C.; Lanz, T.; Hillier, D. J.
2005A&A...438..301B Altcode: 2004astro.ph.12346B
We have analyzed the far-ultraviolet spectrum of two Galactic O4 stars,
the O4If+ supergiant HD 190429A and the O4V((f)) dwarf HD 96715,
using archival FUSE and IUE data. We have conducted a quantitative
analysis using the two NLTE model atmosphere and wind codes, tlusty
and cmfgen, which incorporate a detailed treatment of NLTE metal line
blanketing. From the far-UV spectrum, we have derived the stellar and
wind parameters and the surface composition of the two stars. The
surface of HD 190429A has a composition typical of an evolved O
supergiant (nitrogen-rich, carbon and oxygen-poor), while HD 96715
exhibits surface nitrogen enhancement similar to the enrichment found
in SMC O dwarfs which has been attributed to rotationally-induced
mixing. Following studies of Magellanic Cloud O stars, we find
that homogeneous wind models could not match the observed profile
of O vλ1371 and require very low phosphorus abundance to fit the P
vλλ1118-1128 resonance lines. We show, on the other hand, that we
are able to match the O v and P v lines using clumped wind models. In
addition to these lines, we find that N ivλ1718 is also sensitive to
wind clumping. For both stars, we have calculated clumped wind models
that match well all these lines from different species and that remain
consistent with Hα data. In particular, we have achieved an excellent
match of the P v resonance doublet, indicating that our physical
description of clumping is adequate. These fits therefore provide a
coherent and thus much stronger evidence of wind clumping in O stars
than earlier claims. We show that the success of the clumped wind
models in matching these lines results from increased recombination
in the clumps, hence from a better description of the wind ionization
structure. We find that the wind of these two stars is highly clumped,
as expressed by very small volume filling factors, namely f_∞ =
0.04 for HD 190429A and f_∞ = 0.02 for HD 96715. In agreement with
our analysis of SMC stars, clumping starts deep in the wind, just
above the sonic point. The most crucial consequence of our analysis
is that the mass loss rates of O stars need to be revised downward
significantly, by a factor of 3 and more. These lower mass loss rates
will affect substantially the evolution of massive stars. Accounting
for wind clumping is essential when determining the wind properties
of O stars. Our study therefore calls for a fundamental revision in
our understanding of mass loss and of O-type star stellar winds.

---------------------------------------------------------
Title: Searching for the Donor Star in Close Binary Supersoft Sources
Authors: Lanz, Thierry; Audard, Marc; Hubeny, Ivan; Paerels, Frits
2005sptz.prop20130L Altcode:
The identification of Type Ia Supernova progenitors still remains
elusive. The most promising channel to form SNe Ia involves accreting
white dwarfs (WDs) in close binary systems. The hot WDs sustain
steady nuclear burning and are observed as ultrasoft X-ray sources. The
companions have remained until now undetected because the UV and optical
spectrum is dominated by the emission from the accretion disk around
the WDs. At present, these close binary supersoft sources (CBSS) have
not been observed in the infrared, with the single exception of near-IR
photometry of the Galactic CBSS QR And that provides the first hint
of the secondary. Because of the general lack of a direct detection,
the nature of the donor stars in these systems is still debated. By
combining ultraviolet and optical SEDs with Spitzer observations of 4
CBSS in the mid-infrared, we will be able to definitively identify the
nature of the donor stars in CBSS systems, as well as determining the
size of the accretion disks. We propose to observe eclipsing systems
at different phases to disentangle the contribution of the companions
from the emission of the disks. Spitzer observations of CBSS will
therefore represent a significant step forward in characterizing the
binary systems that are best candidates for becoming SNe Ia.

---------------------------------------------------------
Title: Shocks and Outflows in Young Massive Stars: The R Mon /
HH 39 System
Authors: Audard, Marc; Guedel, Manuel; Lanz, Thierry; Paerels,
Frederik; Skinner, Stephen
2005sptz.prop20034A Altcode:
We propose to use Spitzer's sensitive imaging and spectroscopic
capabilities to undertake a comprehensive infrared study of the massive
young star R Mon and its associated Herbig-Haro object HH 39. We
will reach unprecendeted sensitivities with IRAC to search for mid-IR
emission from R Mon's circumstellar disk, and from its powerful jets
and outflow. We will obtain the first deep mid-IR image of the HH 39
complex located 7' North of R Mon with IRAC and MIPS. We will search
for mid and far-IR emission from shocked material, similar to that
recently detected by IRAC in HH 46/47. With IRS, we will obtain the
first mid-IR spectra of R Mon's bipolar jet and of several emission
knots in HH 39. We will also derive the low-resolution SED of R
Mon with MIPS. Despite its spectacular nature, R Mon has received
little attention in the mid-IR and was never observed by ISO. This
short Spitzer proposal aims at studying in young massive stars the
physical links between accretion disks, outflows and jets, and shocks
in the associated HH objects. The latter can help us understand the
importance of the mechanical energy input of young stars for the origin
of turbulence in molecular clouds. The R Mon/HH 39 system is ideal to
fulfill our goals.

---------------------------------------------------------
Title: A non-LTE analysis of the spectra of two narrow lined main
sequence stars in the SMC
Authors: Hunter, I.; Dufton, P. L.; Ryans, R. S. I.; Lennon, D. J.;
Rolleston, W. R. J.; Hubeny, I.; Lanz, T.
2005A&A...436..687H Altcode: 2005astro.ph..3662H
An analysis of high-resolution VLT/UVES spectra of two B-type main
sequence stars, NGC 346-11 and AV 304, in the Small Magellanic Cloud
(SMC), has been undertaken, using the non-LTE tlusty model atmospheres
to derive the stellar parameters and chemical compositions of each
star. The chemical compositions of the two stars are in reasonable
agreement. Moreover, our stellar analysis agrees well with earlier
analyses of H II regions. The results derived here should be
representative of the current base-line chemical composition of the
SMC interstellar medium as derived from B-type stars.

---------------------------------------------------------
Title: B-type supergiants in the SMC: Chemical compositions and
comparison of static and unified models
Authors: Dufton, P. L.; Ryans, R. S. I.; Trundle, C.; Lennon, D. J.;
Hubeny, I.; Lanz, T.; Allende Prieto, C.
2005A&A...434.1125D Altcode: 2004astro.ph.12367D
High-resolution UCLES/AAT spectra are presented for nine B-type
supergiants in the SMC, chosen on the basis that they may show
varying amounts of nuclear-synthetically processed material mixed to
their surface. These spectra have been analysed using a new grid of
approximately 12 000 non-LTE line blanketed tlusty model atmospheres
to estimate atmospheric parameters and chemical composition. The
abundance estimates for O, Mg and Si are in excellent agreement with
those deduced from other studies, whilst the low estimate for C may
reflect the use of the C II doublet at 4267 Å. The N estimates
are approximately an order of magnitude greater than those found
in unevolved B-type stars or H II regions but are consistent with
the other estimates in AB-type supergiants. These results have been
combined with results from a unified model atmosphere analysis of
UVES/VLT spectra of B-type supergiants (Trundle et al. 2004, A&amp;A,
417, 217) to discuss the evolutionary status of these objects. For
two stars that are in common with those discussed by Trundle et al.,
we have undertaken a careful comparison in order to try to understand
the relative importance of the different uncertainties present in
such analyses, including observational errors and the use of static
or unified models. We find that even for these relatively luminous
supergiants, tlusty models yield atmospheric parameters and chemical
compositions similar to those deduced from the unified code fastwind.

---------------------------------------------------------
Title: VizieR Online Data Catalog: B-type Supergiants in the SMC
(Dufton+, 2005)
Authors: Dufton, P. L.; Ryans, R. S. I.; Trundle, C.; Lennon, D. J.;
Hubeny, I.; Lanz, T.; Allende Prieto, C.
2005yCat..34341125D Altcode:
Table 4 contains the adopted atomic data, equivalent widths and
abundance estimates for all the metal lines observed in the SMC
supergiants. It also contains data for the SMC near main sequence
star AzV 304, which has been used in a differential analysis. (2
data files).

---------------------------------------------------------
Title: Constraining Galaxy Formation Epoch
Authors: Yi, S.; Brown, T.; Heap, S.; Hubeny, I.; Landsman, W.; Lanz,
T.; Sweigart, A.
2005IAUS..201..536Y Altcode: 2000IAUS..201E.108Y
The galaxy formation epoch (zf) is an important and unique
product of cosmological models, and thus much effort has been made to
constrain it. The most direct way of constraining zf is to
estimate the ages of bright elliptical galaxies at high redshifts
arrucately, but it has not been an easy task. We have finally
reached the stage where the stellar ages can be determined within
15% uncertainty. Using this advantage, the recent spectral analysis
of LBDS 53W091 by Spinrad and his collaborators (1997) has suggested
that this red galaxy at z=1.552 is at least 3.5 Gyr old. This provides
a significant constraint to cosmology, i.e., zf &gt; 6.5,
as LBDS 53W091 is the most distant passiviely evolving galaxy whose UV
spectrum is available. The results, based on our population synthesis
models, however, suggest a much smaller age for this galaxy, i.e.,
1.5 - 2 Gyr. The large discrepancy between their age estimate and ours
originates from the difference in the model integrated spectra: their
model spectra are significantly bluer than ours. Such a large difference
in the models for an apparently simple population is unreasonable. A
simple test seems to favor our models. If we adopt our age estimate,
the continuum of this galaxy suggests zf ≈ 2 - 3. This
anaysis, when performed to a larger sample, would efficiently constrain
the global galaxy formation epoch.

---------------------------------------------------------
Title: Non-LTE Model Atmosphere Analysis of the Large Magellanic
Cloud Supersoft X-Ray Source CAL 83
Authors: Lanz, Thierry; Telis, Gisela A.; Audard, Marc; Paerels,
Frits; Rasmussen, Andrew P.; Hubeny, Ivan
2005ApJ...619..517L Altcode: 2004astro.ph.10093L
We present a non-LTE (NLTE) model atmosphere analysis of Chandra High
Resolution Camera (HRC-S) and Low Energy Transmission Grating (LETG)
and XMM-Newton Reflection Grating Spectrometer (RGS) spectroscopy
of the prototypical supersoft source CAL 83 in the Large Magellanic
Cloud. Taken with a 16 month interval, the Chandra and XMM-Newton
spectra are very similar. They reveal a very rich absorption-line
spectrum from the hot white dwarf photosphere but no spectral signatures
of a wind. We also report a third X-ray off-state during a later Chandra
observation, demonstrating the recurrent nature of CAL 83. Moreover,
we found evidence of short-timescale variability in the soft X-ray
spectrum. We completed the analysis of the LETG and RGS spectra of
CAL 83 with new NLTE line-blanketed model atmospheres that explicitly
include 74 ions of the 11 most abundant species. We successfully matched
the Chandra and XMM-Newton spectra assuming a model composition with
LMC metallicity. We derived the basic stellar parameters of the hot
white dwarf, but the current state of atomic data in the soft X-ray
domain precludes a detailed chemical analysis. We have obtained the
first direct spectroscopic evidence that the white dwarf is massive
(MWD&gt;~1 Msolar). The short timescale of
the X-ray off-states is consistent with a high white dwarf mass. Our
analysis thus provides direct support for supersoft sources as likely
progenitors of Type Ia supernovae (SNe Ia).

---------------------------------------------------------
Title: Mid-Ultraviolet Spectral Diagnostics of Galaxy Evolution
Authors: Heap, Sara R.; Lanz, Thierry
2005mmgf.conf..210H Altcode:
We describe how optical spectra of z=1-2 galaxies can help test the
hypothesis of hierarchical galaxy formation.

---------------------------------------------------------
Title: Blue Straggler Stars in 4 Globular Clusters: Masses, Rotation
Rates and Disks
Authors: De Marco, O.; Zurek, D.; Ouellette, J. A.; Lanz, T.; Shara,
M. M.; Saffer, R. A.; Sepinsky, J. F.
2004AAS...20515905D Altcode: 2004BAAS...36.1611D
We present an analysis of optical HST/STIS and HST/FOS spectroscopy of
blue stragglers found in the globular clusters 47Tuc, M3, NGC6752 and
NGC6397. Spectroscopic masses, and rotation rates are presented which
impose constraints on their evolutionary history. 6 blue stragglers
in our sample cannot be fit with state of the art non-LTE stellar
atmosphere models. The 6 misfits possess Balmer jumps which are
too large for the effective temperatures implied by their Paschen
continua. We find that our data for these stars are consistent with
models only if we account for extra absorption of stellar Balmer
photons by an ionized circumstellar disk. Column densities of HI and
CaII are derived as are the the disks' thicknesses. This is the first
time that a circumstellar disk is detected around blue stragglers. The
presence of magnetically-locked disks attached to the stars has been
suggested as a mechanism to lose the large angular momentum imparted
by the collision event at the birth of these stars. The disks implied
by our study might not be massive enough to constitute such an angular
momentum sink, but they could be the leftovers of once larger disks.

---------------------------------------------------------
Title: NLTE Analysis of Chandra and XMM-Newton Spectroscopy of CAL 83
Authors: Lanz, T.; Telis, G. A.; Audard, M.; Paerels, F.; Hubeny, I.
2004HEAD....8.1703L Altcode: 2004BAAS...36..936L
We present a new NLTE model atmosphere analysis of Chandra HRC-S/LETG
and XMM-Newton RGS spectroscopy of the supersoft source CAL 83 in
the LMC. The two spectra are very similar, and reveal a very rich
absorption line spectrum from the hot white dwarf photosphere. We also
report a third X-ray off-state during a later Chandra observation,
showing the recurrent nature of CAL 83. Moreover, we found evidence
of short-timescale variability in the soft X-ray spectrum. The Chandra
and XMM-Newton spectra have been successfully matched with our new NLTE
line-blanketed model atmospheres that explicitly include 74 ions of the
11 most abundant species. We derived the basic stellar parameters of the
hot white dwarf, obtaining the first direct spectroscopic evidence that
the white dwarf is massive. The short timescale of the X-ray off-states
is consistent with a high WD mass. Our analysis, therefore, provides
direct support for supersoft sources as likely progenitors of SN Ia. This work was supported by a grant from the NASA Astrophysics Theory
Program. The Columbia group acknowledges support from NASA to Columbia
University for XMM-Newton mission support and data analysis

---------------------------------------------------------
Title: Chandra High Resolution Spectroscopy of the Burst Spectrum
of EXO 0748-67
Authors: Telis, G.; Paerels, F.; Audard, M.; Lanz, T.; Cottam, J.;
Mendez, M.; Bildsten, L.; Chang, P.; Marshall, H.
2004HEAD....8.2503T Altcode: 2004BAAS...36..954T
We have observed EXO0748-67 for approximately 300 ksec with the High
Energy Transmission Grating Spectrometer on Chandra. A total of 35
Type I X-ray bursts occurred during our observation, and from these
we obtained a composite burst spectrum with high sensitivity in the
Fe K band. Along with the spectrum of GS1826-238, this is the highest
sensitivity short-wavelength photospheric spectrum of a neutron star
ever obtained. The total summed spectrum does not appear to exhibit a
clear, narrow Fe Ly alpha absorption line; a provisional upper limit
on the equivalent width of an unresolved feature is approximately
5 mA (11 eV). We present the spectroscopy and briefly discuss the
physical implications of our analysis in the context of attempts to
measure the fundamental properties of neutron stars by photospheric
spectroscopic techniques. Analysis of the broad-band characteristics of
the short-wavelength spectrum suggests that the majority of the bursts
shows significant photospheric radius expansion. We will present the
results of a spectroscopic analysis performed on subsets of the data
selected by the apparent state of the photosphere, to allow for the fact
that any narrow Fe absorption spectrum strongly depends on the physical
parameters of the atmosphere. This research was supported by NASA,
through Chandra grant GO4-5045A.

---------------------------------------------------------
Title: NGC 1535 : UV Observations and Models
Authors: Koesterke, L.; Werner, K.; Kruk, J. W.; Lanz, T.
2004ASPC..313..123K Altcode: 2004apnw.conf..123K; 2004astro.ph..2049K
We re-investigate the UV spectrum of NGC 1535 by means of recently
developed fully line-blanketed non-LTE models. These new models account
for the wind in spherical geometry while handling the atomic data in
a very similar way to the Tlusty code. This approach ensures at the
same time realistic predictions of the photospheric absorption lines
and of the emission lines formed in the wind. Our analysis confirms
the results of previous studies. We derive T*=70 kK, dot
M =10-7.8 M⊙/yr}, and v∞=2000 km/s.

---------------------------------------------------------
Title: Stellar activity and magnetism studied by optical
interferometry
Authors: Rousselet-Perraut, K.; Stehlé, C.; Lanz, T.; Le Bouquin,
J. B.; Boudoyen, T.; Kilbinger, M.; Kochukhov, O.; Jankov, S.
2004A&A...422..193R Altcode:
By means of numerical simulations, we investigate the ability of
optical interferometry, via the fringe phase observable, to address
stellar activity and magnetism. To derive abundance maps and stellar
rotation axes, we use color differential interferometry which couples
high angular resolution to high spectral resolution. To constrain
magnetic field topologies, we add to this spectro-interferometer a
polarimetric mode. Two cases of well-known Chemically Peculiar (CP)
stars (βCrB and α2CVn) are simulated to derive instrumental
requirements to obtain 2D-maps of abundance inhomogeneities and magnetic
fields. We conclude that the near-infrared instrument AMBER of the VLTI
will allow us to locate abundance inhomogeneities of CP stars larger
than a fraction of milliarcsecond whereas the polarimetric mode of
the French GI2T/REGAIN interferometer would permit one to disentangle
various magnetic field topologies on CP stars. We emphasize the crucial
need for developing and validating inversion algorithms so that future
instruments on optical aperture synthesis arrays can be optimally used.

---------------------------------------------------------
Title: Effects of Rotation in Low-Metallicity Stars
Authors: Heap, S. R.; Lanz, T.
2004IAUS..215..220H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Evidence for Flash Mixing in He-rich sdB Stars
Authors: Sweigart, Allen V.; Lanz, Thierry; Brown, Thomas M.; Hubeny,
Ivan; Landsman, Wayne B.
2004Ap&SS.291..367S Altcode: 2003astro.ph..8517S
We present FUSE spectra of three He-rich sdB stars. Two of these stars,
PG1544+488 and JL87, reveal extremely strong C III lines, suggesting
that they have mixed triple-α carbon from the deep interior out to
their surfaces. Using TLUSTY NLTE line-blanketed model atmospheres, we
find that PG1544+488 has a surface composition of 96% He, 2% C, and 1%
N. JL87 shows a similar surface enrichment of C and N but still retains
a significant amount of hydrogen. In contrast, the third star, LB1766,
is devoid of hydrogen and strongly depleted of carbon, indicating that
its surface material has undergone CN-cycle processing.

---------------------------------------------------------
Title: First Evidence of Circumstellar Disks around Blue Straggler
Stars
Authors: De Marco, Orsola; Lanz, Thierry; Ouellette, John A.; Zurek,
David; Shara, Michael M.
2004ApJ...606L.151D Altcode: 2004astro.ph..4065D
We present an analysis of optical Hubble Space Telescope Space Telescope
Imaging Spectrograph and Faint Object Spectrograph spectroscopy of
six blue stragglers found in the globular clusters M3, NGC 6752, and
NGC 6397. These stars are a subsample of a set of ~50 blue stragglers
and stars above the main-sequence turnoff in four globular clusters
that will be presented in a forthcoming paper. All but the six stars
presented here can be well fitted with non-LTE model atmospheres. The
six misfits, on the other hand, possess Balmer jumps that are too large
for the effective temperatures implied by their Paschen continua. We
find that our data for these stars are consistent with models only
if we account for extra absorption of stellar Balmer photons by an
ionized circumstellar disk. Column densities of H I and Ca II are
derived as are the the disks' thicknesses. This is the first time
that a circumstellar disk is detected around blue stragglers. The
presence of magnetically locked disks attached to the stars has been
suggested as a mechanism to lose the large angular momentum imparted
by the collision event at the birth of these stars. The disks implied
by our study might not be massive enough to constitute such an angular
momentum sink, but they could be the leftovers of once larger disks. Based on observations with the NASA/ESA Hubble Space Telescope,
obtained at the Space Telescope Science Institute, which is operated
by the Association of Universities for Research in Astronomy, Inc.,
under NASA contract NAS5-2655.

---------------------------------------------------------
Title: Flash Mixing on the White Dwarf Cooling Curve: Far Ultraviolet
Spectroscopic Explorer Observations of Three He-rich sdB Stars
Authors: Lanz, Thierry; Brown, Thomas M.; Sweigart, Allen V.; Hubeny,
Ivan; Landsman, Wayne B.
2004ApJ...602..342L Altcode: 2003astro.ph..8440L
We present Far-Ultraviolet Spectroscopic Explorer (FUSE) spectra of
three He-rich sdB stars. Two of these stars, PG 1544+488 and JL 87,
reveal extremely strong C III lines at 977 and 1176 Å, while the
carbon lines are quite weak in the third star, LB 1766. We have
analyzed the FUSE data using TLUSTY non-LTE line-blanketed model
atmospheres and find that PG 1544+488 has a surface composition of
96% He, 2% C, and 1% N. JL 87 shows a similar surface enrichment of
carbon and nitrogen, but some significant fraction of hydrogen still
remains in its atmosphere. Finally, LB 1766 has a surface composition
devoid of hydrogen and strongly depleted of carbon, indicating that
its surface material has undergone CN-cycle processing. We interpret
these observations with new evolutionary calculations which suggest
that He-rich sdB stars with C-rich compositions are the progeny of
stars which underwent a delayed helium-core flash on the white-dwarf
cooling curve. During such a flash the interior convection zone will
penetrate into the hydrogen envelope, thereby mixing the envelope
with the He- and C-rich core. Such “flash-mixed” stars will arrive
on the extreme horizontal branch (EHB) with He- and C-rich surface
compositions and will be hotter than the hottest canonical (i.e.,
unmixed) EHB stars. Two types of flash mixing are possible: “deep” and
“shallow,” depending on whether the hydrogen envelope is mixed deeply
into the site of the helium flash or only with the outer layers of the
core. Based on both their stellar parameters and surface compositions,
we suggest that PG 1544+488 and JL 87 are examples of “deep” and
“shallow” flash mixing, respectively. Flash mixing may therefore
represent a new evolutionary channel for producing the hottest EHB
stars. However, flash mixing cannot explain the abundance pattern in
LB 1766, which remains a challenge to current evolutionary models. Based on observations made with the NASA-CNES-CSA Far Ultraviolet
Spectroscopic Explorer. FUSE is operated for NASA by the Johns Hopkins
University under NASA contract NAS5-32985.

---------------------------------------------------------
Title: STIS Coronagraphic Observations of β Pictoris
Authors: Heap, S. R.; Lindler, D. J.; Lanz, T. M.
2004IAUS..202..338H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Quantitative Spectroscopy of O Stars at Low Metallicity:
O Dwarfs in NGC 346
Authors: Bouret, J. -C.; Lanz, T.; Hillier, D. J.; Heap, S. R.;
Hubeny, I.; Lennon, D. J.; Smith, L. J.; Evans, C. J.
2003ApJ...595.1182B Altcode: 2003astro.ph..1454B
We present the results of a detailed analysis of the properties of dwarf
O-type stars in a metal-poor environment. High-resolution, high-quality
ultraviolet and optical spectra of six O-type stars in the H II region
NGC 346 have been obtained from a spectroscopic survey of O stars in the
SMC. Stellar parameters and chemical abundances have been determined
using non-LTE (NLTE) line-blanketed photospheric models calculated
with TLUSTY. Additionally, we have modeled the spectra with the NLTE
line-blanketed wind code CMFGEN to derive wind parameters. Stellar
parameters, chemical abundances, and in particular iron abundances
obtained with the two NLTE codes compare quite favorably. This
consistency demonstrates that basic photospheric parameters of
main-sequence O stars can be reliably determined using NLTE static
model atmospheres. With the two NLTE codes, we need to introduce a
microturbulent velocity to match the observed spectra. Our results
hint at a decrease of the required microturbulent velocity from a value
close to the sonic velocity in early O stars to a low value in late O
stars. As in several recent studies of Galactic, LMC, and SMC stars, we
derive effective temperatures lower than predicted from the widely used
relation between spectral type and Teff, resulting in lower
stellar luminosities and lower ionizing fluxes. From evolutionary tracks
in the H-R diagram, we find the age 3×106 yr for NGC 346. A
majority of the stars in our sample reveal CNO cycle-processed material
at their surface during the main-sequence stage, thus indicating fast
stellar rotation and/or very efficient mixing processes. We obtain an
overall metallicity Z=0.2Zsolar, in good agreement with other
recent analyses of SMC stars. We study the dependence of the mass-loss
rate on the stellar metallicity and find a satisfactory agreement with
recent theoretical predictions for the three most luminous stars of
the sample. The wind momentum-luminosity relation for our sample stars
derived for these stars agrees with previous studies. However, the three
other stars of our sample reveal very weak signatures of mass loss. We
obtain mass-loss rates that are significantly lower than 10-8
Msolar yr-1, below the predictions of radiative
line-driven wind theory by an order of magnitude or more. Furthermore,
evidence of clumping in the wind of main-sequence O stars is provided
by O V λ1371. As in previous studies of O star winds, we are unable to
reproduce this line with homogeneous-wind models, but we have achieved
very good fits with clumped models. Clumped-wind models systematically
yield lower mass-loss rates than theoretical predictions. Based
on observations with the NASA/ESA Hubble Space Telescope, obtained
at the Space Telescope Science Institute, which is operated by the
Association of Universities for Research in Astronomy, Inc., under
NASA contract NAS 5-2655. Also based on observations obtained at the
European Southern Observatory (La Silla) and at the Anglo-Australian
Observatory (Siding Spring).

---------------------------------------------------------
Title: Non-LTE Model Atmospheres for Late-Type Stars. I. A Collection
of Data for Light Neutral and Singly Ionized Atoms
Authors: Allende Prieto, Carlos; Lambert, David L.; Hubeny, Ivan;
Lanz, Thierry
2003ApJS..147..363A Altcode: 2003astro.ph..3559A; 2003astro.ph..3559P
With the goal of producing a reliable set of model atoms and singly
ionized ions for use in building NLTE model atmospheres, we have
combined measured energy levels, critically compiled line transition
probabilities, and resonance-averaged calculations of photoionization
cross sections. A majority of the elements from Li to Ca are
considered, covering most of the important species in late-type
atmospheres. These include elements that contribute free electrons
and/or continuous opacity in the ultraviolet (e.g., Mg and Si), as well
as trace elements whose abundance determinations rely on ultraviolet
lines (e.g., B from B I lines). The new data complement and, for the
species in common, supersede a previous collection of model atoms
originally designed for use in studies of early-type stars.

---------------------------------------------------------
Title: Erratum: “A Grid of Non-LTE Line-blanketed Model Atmospheres
of O-Type Stars” (<A href="bib_query?2003ApJS...146..417L">ApJS,
146, 417 [2003]</A>)
Authors: Lanz, Thierry; Hubeny, Ivan
2003ApJS..147..225L Altcode:
We have constructed a comprehensive grid of 680 metal line-blanketed,
non-LTE, plane-parallel, hydrostatic model atmospheres for the basic
parameters appropriate to O-type stars. The OSTAR2002 grid considers 12
values of effective temperatures, 27,500K&lt;=Teff&lt;=55,000
K with 2500 K steps, eight surface gravities, 3.0&lt;=logg&lt;=4.75
with 0.25 dex steps, and 10 chemical compositions, from metal-rich
relative to the Sun to metal-free. The lower limit of logg for a
given effective temperature is set by an approximate location of the
Eddington limit. The selected chemical compositions have been chosen
to cover a number of typical environments of massive stars: the
Galactic center, the Magellanic Clouds, blue compact dwarf galaxies
like I Zw 18, and galaxies at high redshifts. The paper contains a
description of the OSTAR2002 grid and some illustrative examples and
comparisons. The complete OSTAR2002 grid is available at our Web site
at <A href="/abs/2003ApJ..S.146..417L">ApJS, 146, 417 [2003]</A>. Laboratory for Astronomy and Solar Physics, NASA Goddard Space Flight
Center, Code 681, Greenbelt, MD 20771.

---------------------------------------------------------
Title: A Grid of Non-LTE Line-blanketed Model Atmospheres of O-Type
Stars
Authors: Lanz, Thierry; Hubeny, Ivan
2003ApJS..146..417L Altcode: 2002astro.ph.10157L
We have constructed a comprehensive grid of 680 metal line-blanketed,
non-LTE, plane-parallel, hydrostatic model atmospheres for the basic
parameters appropriate to O-type stars. The OSTAR2002 grid considers 12
values of effective temperatures, 27,500K&lt;=Teff&lt;=55,000
K with 2500 K steps, eight surface gravities, 3.0&lt;=logg&lt;=4.75
with 0.25 dex steps, and 10 chemical compositions, from metal-rich
relative to the Sun to metal-free. The lower limit of logg for a given
effective temperature is set by an approximate location of the Eddington
limit. The selected chemical compositions have been chosen to cover a
number of typical environments of massive stars: the Galactic center,
the Magellanic Clouds, blue compact dwarf galaxies like I Zw 18, and
galaxies at high redshifts. The paper contains a description of the
OSTAR2002 grid and some illustrative examples and comparisons. The
complete OSTAR2002 grid is available on-line.

---------------------------------------------------------
Title: Discovery of a Little Homunculus within the Homunculus Nebula
of η Carinae
Authors: Ishibashi, Kazunori; Gull, Theodore R.; Davidson, Kris;
Smith, Nathan; Lanz, Thierry; Lindler, Don; Feggans, Keith; Verner,
Ekaterina; Woodgate, Bruce E.; Kimble, Randy A.; Bowers, Charles W.;
Kraemer, Steven; Heap, Sarah R.; Danks, Anthony C.; Maran, Stephen
P.; Joseph, Charles L.; Kaiser, Mary Elizabeth; Linsky, Jeffrey L.;
Roesler, Fred; Weistrop, Donna
2003AJ....125.3222I Altcode:
We report long-slit spectroscopic mapping of the η Carinae nebula
obtained using the Space Telescope Imaging Spectrograph. The
observations reveal the presence of a previously unknown bipolar
emission nebula (roughly +/-2" along its major axis) embedded within
the well-known and larger Homunculus Nebula. A preliminary analysis
suggests that this embedded nebula may have originated from a minor
eruption event circa 1890, 50 years after the formation of the larger
Homunculus. Based on observations made with the NASA/ESA Hubble
Space Telescope, and supported by GO grants 7302 and 8327 from the
Space Telescope Science Institute and STIS GTO funding. This paper is a
merger of the HST GO programs (PI: K. Davidson) and STIS IDT Key Project
8483, which used orbits allocated in the shared Guaranteed Telescope
Observations. The STScI is operated by the Association of Universities
for Research in Astronomy, Inc., under NASA contract NAS 5-26555.

---------------------------------------------------------
Title: A Tale of Two Stars: The Extreme O7 Iaf+ Supergiant AV 83
and the OC7.5 III((f)) star AV 69
Authors: Hillier, D. John; Lanz, T.; Heap, S. R.; Hubeny, I.; Smith,
L. J.; Evans, C. J.; Lennon, D. J.; Bouret, J. C.
2003ApJ...588.1039H Altcode:
We present a detailed study of AV 83, an O7 Iaf+ supergiant,
and AV 69 [OC7.5 III((f))] in the SMC. The stars have similar
effective temperatures and luminosities but show very different wind
signatures. For our study we have used the non-LTE line-blanketed
atmosphere code developed by Hillier and Miller, which explicitly
allows for line blanketing by C, N, O, S, Ar, Ne, Fe, and other
elements. Our study finds that AV 83 has an effective temperature of
approximately 33,000 K and logg~3.25. It has an extended photosphere
as a result of a “low” effective surface gravity and a much denser
wind than main-sequence O stars. We can match the spectrum only by
using a slow velocity law with β~2, a value that is much larger
than the values of around 1 predicted by standard radiation wind
theory. Further, we show that the Hα emission profile in AV 83 is
sensitive to the adopted surface gravity. To fit the spectrum of AV 83,
we have considered conventional models in which the wind is smooth
and alternate models in which the winds are highly clumped. Both
types of winds yield a satisfactory fit to the majority of lines
in the observed spectrum; however, strong UV photospheric lines
and the P V resonance transitions favor a clumped wind. If clumping
is important, it must begin at relatively low velocities (i.e., 30
km s-1, not 300 km s-1). In the smooth wind,
the line force is too small to drive the wind. In the clumped wind,
the line force is generally sufficient to drive the wind, although
there are still some discrepancies around the sonic point. In AV
83, the N abundance is substantially enhanced relative to normal
SMC abundances, while both C and O are SMC-like, consistent with
the presence of internally processed CNO material at the stellar
surface. The N III λ4640 multiplet, which is known to be produced by
dielectronic recombination, is well reproduced by the models. These
lines, and the adjacent C III λ4649 multiplet, show a significant
sensitivity to surface gravity, as well as the usual sensitivity to
abundance and effective temperature. Incoherent electron scattering,
occurring within the photosphere, can explain the broad wings seen
on these lines. We have modeled the Fe spectrum (Fe IV-Fe VI) in the
UV in both AV 83 and AV 69. For stars with an effective temperature
around 33,000 K, the Fe IV-to-Fe V line ratios form a useful effective
temperature diagnostic and give results consistent with those found
from optical and UV line diagnostics. The derived iron abundance, which
is sensitive to the adopted microturbulent velocity, is 0.2-0.4 times
the solar iron abundance in AV 83, while 0.2 solar gives a good fit
for AV 69. The wind of AV 69 is substantially less dense than that of
AV 83. Because of the lack of suitable diagnostics, it is impossible
to constrain the mass-loss rate and velocity law independently. Its
spectrum indicates that it has a similar effective temperature to
AV 83 (Teff~34,000 K), a substantially higher gravity
(logg=3.5) than AV 83, and a CNO abundance pattern that has not been
influenced by internal CNO processing. We show that the N/C abundance
ratio is substantially below solar, in agreement with SMC nebular and
stellar abundance studies. The differences between the spectra of AV
83 and AV 69, and between the derived masses and surface abundances,
are striking. We have examined possible causes, and only one seems
consistent with the observations and our current understanding of
massive star evolution. AV 83 was most likely a fast rotator that
experienced rotationally enhanced mass loss. The presence of enhanced
N but almost normal C and O abundances is a direct indication of
rotationally induced mixing. On the other hand, AV 69 is a slow
rotator. As part of our analyses, we have systematically examined
the influence of the H/He abundance ratio, the mass-loss rate, the
velocity law, the Fe abundance, microturbulence, and clumping on
the theoretical spectrum. We illustrate which lines provide useful
diagnostics and highlight some of the difficulties associated with
spectroscopic analyses of O stars. The spectrum of AV 83 shows the
presence of photospheric absorption lines, the presence of lines formed
at the base of the wind, and numerous wind lines. Since these lines
sample the photosphere and the entire wind, extreme O If supergiants,
such as AV 83, are ideal candidates to probe conditions in stellar
winds and hence further our knowledge of O star winds. Based on
observations with the NASA/ESA Hubble Space Telescope, obtained at the
Space Telescope Science Institute, which is operated by the Association
of Universities for Research in Astronomy, Inc., under NASA contract
NAS5-26555. Based in part, on observations with the NASA-CNES-CSA Far
Ultraviolet Spectroscopic Explorer. FUSE is operated by Johns Hopkins
University under NASA contract NAS5-32985.

---------------------------------------------------------
Title: UV Spectral Templates for High-Redshift Galaxies
Authors: Heap, Sara; Lindler, Don; Lanz, Thierry
2003AAS...202.1107H Altcode: 2003BAAS...35R.715H
New instrumentation such as DEIMOS on Keck-II now enable deep spectral
surveys, and thereby samples of galaxies at younger ages. At a redshift,
z=1, all galaxies are less than 6 Gyr old and hence, have not yet formed
horizontal-branch stars. Also, at z=1, the restframe-UV comes into view,
and with it, a new set of spectral diagnostics. UV spectral features
are especially important because most of the UV flux comes from stars
at the main-sequence turnoff (MSTO). Hence, UV spectral diagnostics
enable the ages of z=1 galaxies to be estimated directly from MSTO
stars. In preparation for these high-redshift spectral surveys,
we are developing UV spectral templates for stellar populations younger
than 6 Gyr using UV-optical spectra of stars observed by HST/STIS. We
are also planning to supplement these observations with theoretical
spectral grids of stars of various metallicities. In this paper, we
present a progress report on the observation-based spectral templates
and spectral diagnostics.

---------------------------------------------------------
Title: An analysis of the optical spectra of the post-asymptotic
giant branch stars LSIV -12 111 and HD 341617
Authors: Ryans, R. S. I.; Dufton, P. L.; Mooney, C. J.; Rolleston,
W. R. J.; Keenan, F. P.; Hubeny, I.; Lanz, T.
2003A&A...401.1119R Altcode:
High spectral resolution and signal-to-noise observations of the
absorption and emission line spectra in two post-asymptotic-giant-branch
(PAGB) stellar candidates, LSIV -12 111 and HD 314617 are discussed. The
absorption line spectra have been analysed using non-LTE model
atmosphere techniques to determine stellar atmospheric parameters
and chemical compositions, both in absolute terms and relative to
a standard star, HD 13841. The atmospheric parameters differ from
previous estimates based on LTE model atmospheres, probably due to
non-LTE effects. In turn these imply stellar masses that are generally
larger than have been previously estimated. Both PAGB candidates have
relative uniform underabundances of metals with mean values of -0.35
dex for LSIV -12 111 and -0.50 dex for HD 314617. Furthermore,
their abundance patterns are remarkably similar to that observed
for optically bright, F-type PAGBs. From the emission spectra, the
plasma parameters and angular size of the circumstellar material are
constrained, and these are consistent with previous estimates and with
a PAGB evolutionary stage.

---------------------------------------------------------
Title: Mapping abundance inhomogeneities and magnetic fields of
chemically peculiar (CP) stars with optical aperture synthesis arrays
Authors: Rousselet-Perraut, Karine; Stehle, Chantal; Lanz, Thierry;
Boudoyen, Thomas; Jankov, Slobodan; Vakili, Farrokh; Kilbinger,
Martin; Lebouquin, Jean-Baptiste; Kochukhov, Oleg
2003SPIE.4838.1396R Altcode:
Simultaneously mapping the abundance inhomogeneities and the
magnetic fields of chemically peculiar (CP) stars is essential
to improve our understanding of stellar magnetism and its key
role in structuring stellar atmospheres, in particular relative
to ion migration and chemical stratification. However, magnetic
fields and chemical inhomogenities tend to have similar effects on
classical observables. Magnetic and abundance maps have therefore
to be reconstructed most often either independently or in making a
priori assumptions. To overcome these difficulties, we propose to
take benefit of optical aperture synthesis arrays to resolve local
magnetic structures and patchy stellar surfaces. This requires ability
to resolve polarimetrically magnetically-sensitive spectral lines,
and thus to add a polarimetric device at the combined focus of an
interferometric array. Within this instrumental context, it becomes
possible to map magnetic fields with visibility and phase measurements
in circularly polarized light and to map the chemical inhomogeneities
thanks to "classical" interferometric measurements (i.e. without the
polarimeter). In this paper, we show that the interference fringe phase
is the suitable observable for polarimetric measurements and for mapping
patchy surfaces (see also Jankov et al. in these proceedings). We
present some illustrative cases of different magnetic topologies and
abundance distributions. We focus on two well-known CP stars, βCrB
and α2CVn, and we show observational predictions with
different instruments currently in operation (GI2T, VLTI).

---------------------------------------------------------
Title: A New Formation Mechanism for the Hottest Horizontal-Branch
Stars
Authors: Sweigart, A. V.; Brown, T. M.; Lanz, T.; Landsman, W. B.;
Hubeny, I.
2003ASPC..296..313S Altcode: 2003nhgc.conf..313S; 2002astro.ph..7343S
Stars with very large mass loss on the red-giant branch can undergo
the helium flash while descending the white-dwarf cooling curve. Under
these conditions the flash convection zone will mix the hydrogen
envelope with the hot helium- burning core. Such “flash-mixed”
stars will arrive on the extreme horizontal branch (EHB) with helium-
and carbon-rich envelopes and will lie at higher temperatures than the
hottest canonical (i.e., unmixed) EHB stars. Flash mixing provides a
new evolutionary channel for populating the hot end of the EHB and may
explain the origin of the high gravity, helium-rich sdO and sdB stars.

---------------------------------------------------------
Title: Multidimensional ALI Radiative Transfer in Cartesian,
Cylindrical and Spherical Coordinate Systems
Authors: van Noort, M.; Hubeny, I.; Lanz, T.
2003ASPC..288..445V Altcode: 2003sam..conf..445V
A new Radiative Transfer code that can calculate the non-LTE line
transfer problem in a two-level atom formulation in Cartesian,
cylindrical and spherical coordinate systems is presented. The transfer
equation is solved using the ALI and the short characteristics methods,
while allowing for an arbitrary 3-dimensional velocity field. The
code is modularised so that changing geometry can be accomplished by
simply setting a switch, and parallelised for use on a networked PC
cluster to increase computational speed. The spatial parallelization
method is employed. It is found to be robust and efficient, while not
relying heavily on fast communication. The internal accuracy of
the code is tested extensively in all three geometries and is shown
to be in good agreement with appropriate 1-D solutions.

---------------------------------------------------------
Title: Quantitative analysis of O-type stars properties, at low
metallicity
Authors: Bouret, Jean-Claude; Lanz, Thierry M.; Heap, Sara R.; Hubeny,
Ivan; Hillier, D. John; Lennon, Daniel J.; Evans, Christopher J.;
Smith, Linda J.
2003IAUS..212..156B Altcode:
We have investigated the properties of main-sequence O-type stars in
the SMC. Mass-loss rates, luminosities and Teff are much
smaller for these stars than for Galactic ones, resulting in a steeper
wind-momentum relation.

---------------------------------------------------------
Title: Using Optical Interferometry for Studying Stellar Activity
and Magnetism
Authors: Le Bouquin, J. B.; Rousselet-Perraut, K.; Stehlé, C.; Lanz,
T.; Jankov, S.
2003ASPC..305..215L Altcode: 2003mfob.conf..215L
No abstract at ADS

---------------------------------------------------------
Title: NLTE in a Hot Hydrogen Star: Auer &amp; Mihalas Revisited
Authors: Wiersma, J.; Rutten, R. J.; Lanz, T.
2003ASPC..288..130W Altcode: 2003sam..conf..130W
We pay tribute to two landmark papers published by Auer &amp; Mihalas
in 1969. They modeled hot-star NLTE-RE hydrogen-only atmospheres,
using two simplified hydrogen atoms: ApJ 156, 157: H I levels 1,
2 and c, Lyman α the only line ApJ 156, 681: H I levels 1, 2, 3 and
c, Balmer α the only line and computed LTE and NLTE models with the
single line turned on and off. The results were extensively analyzed
in the two papers. Any student of stellar line formation should
take these beautiful papers to heart. The final exercise in Rutten's
lecture notes “Radiative Transfer in Stellar Atmospheres” asks the
student to work through five pages of questions concerning diagrams from
the first paper alone! That exercise led to the present work in which
we recompute the Auer-Mihalas hot-hydrogen-star models with TLUSTY,
adding results from a complete hydrogen atom for comparison. Our
motivation for this Auer-Mihalas re-visitation is twofold: 1. to
add diagnostic diagrams to the ones published by Auer &amp; Mihalas,
in particular Bν, Jν, Sν graphs to
illustrate the role of the radiation field, and radiative heating &amp;
cooling graphs to illustrate the radiative energy budget, 2. to
see the effect of adding the rest of the hydrogen atom.

---------------------------------------------------------
Title: Nitrogen Enrichment in Massive Stars
Authors: Heap, S. R.; Lanz, T.
2003ASPC..304...41H Altcode: 2003cnou.conf...41H
No abstract at ADS

---------------------------------------------------------
Title: A Grid of Non-LTE Line-Blanketed Model Atmospheres of O Stars
Authors: Lanz, T.; Hubeny, I.
2003ASPC..288..157L Altcode: 2003sam..conf..157L
We have constructed a grid of over 300 NLTE fully-blanketed model
atmospheres covering the parameter range of O-type stars at various
metallicities. We have assumed a plane-parallel geometry, hydrostatic
and radiative equilibria. The models incorporate about 100,000 NLTE
atomic levels of over 40 ions of H, He, C, N, O, Ne, Si, P, S, Fe,
and Ni, which are grouped into about 900 superlevels. The models will
be made publicly available in the coming months.

---------------------------------------------------------
Title: Quantitative Spectroscopy of O stars at low metallicity. O
Dwarfs in NGC 346
Authors: Bouret, J. -C.; Lanz, T.; Heap, S.; Hubeny, I.; Hillier,
D.; Lennon, D.; Evans, C. J.; Smith, L.
2003sf2a.conf..499B Altcode: 2003sf2a.confE.214B
Dwarf O-type stars have been observed in NGC 346, the largest HII
region in the SMC. UV and optical spectra have been analysed with
NLTE line-blanketed models. These stars reveal CNO- cycle processed
material at their surface, indicating fast stellar rotation and/or
very efficient mixing processes. We obtain an overall metallicity
Z = 0.2 Zodot. The dependence of the mass loss rate with
the stellar metallicity is in good agreement with recent theoretical
predictions for three most luminous stars of the sample. However, three
other stars reveal mass loss rates that are significantly lower than
10-8 msol/yr, which is below the predictions of radiative
line-driven wind theory by an order of magnitude or more. Clumping in
the wind is evidenced by OV at 1371. Clumped wind models systematically
yield lower mass loss rates than theoretical predictions.

---------------------------------------------------------
Title: NLTE Line Formation in Late-Type Stellar Atmospheres
Authors: Allende Prieto, C.; Hubeny, I.; Lambert, D. L.; Lanz, T.
2003IAUS..210P.A24A Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Observing stellar activity with optical interferometers
Authors: Lebouquin, J. -B.; Rousselet-Perraut, K.; Stehlé, C.; Lanz,
T.; Jankov, S.
2003sf2a.conf..539L Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Model Photospheres with Accelerated Lambda Iteration
Authors: Hubeny, I.; Lanz, T.
2003ASPC..288...51H Altcode: 2003sam..conf...51H
No abstract at ADS

---------------------------------------------------------
Title: Multidimensional radiative effects in supercritical shocks
Authors: Leygnac, S.; Lanz, T.; Stehlé, C.; Michaut, C.; Korĉáková,
D.
2003sf2a.conf..595L Altcode:
Recent radiative shocks experiments performed on the LULI laser at
Ecole Polytechnique in France (Fleury et al., Lasers and Particle
Beams 20, 263, 2002) put in evidence a supercritical shock wave in
a xenon gas cell. The structure of these shocks is quite similar to
those of accretion shock wave in the case of stellar formation, as
indicated in Stehlé and Chieze (SF2A - Paris proceedings, 2002). Some points require further studies like the contribution of
the gas excitation/ionization energy to the compression ratio and
the understanding of the discrepancy, which was noted between the
velocity of the radiative precursor in the experiment and in the 1D
simulation. Thus, to understand the physics of the radiative
shock waves, the academic case of the stationary shock is particularly
interesting. We have thus studied the structure of a radiative shock
wave which propagates in an ionized gas. We study the extended Rankine
Hugoniot equations in various media with inclusion of radiation pressure
and energy and study also the extension of the radiative precursor
in the diffusion approximation. We also study the equations of
multidimensional radiative transfer for a snapshot of the experimental
shock in xenon in order to quantify the radiative losses in the finite
experimental cell. This academic approach will help to improve
the knowledge of the physical processes which take place in radiative
shocks of astrophysical interest, like in the birth and death of stars,
and prepare ourselves to define appropriate experiments on future high
power lasers like LIL and LMJ in Bordeaux.

---------------------------------------------------------
Title: Atomic Data in Non-LTE Model Stellar Atmospheres
Authors: Lanz, T.; Hubeny, I.
2003ASPC..288..117L Altcode: 2003sam..conf..117L
Extensive sources of atomic data are required to calculate NLTE
line-blanketed model atmospheres. I will discuss their implementation in
our NLTE model atmosphere code, TLUSTY, and in our spectrum synthesis
code, SYNSPEC, with a particular attention to the statistical methods
required to incorporate the opacity of iron-peak elements. A few
typical results and comparisons to other codes will be shown.

---------------------------------------------------------
Title: NLTE Line-Blanketed Model Stellar Atmospheres
Authors: Lanz, T.; Hubeny, I.; Heap, S. R.
2003IAUS..210...67L Altcode:
No abstract at ADS

---------------------------------------------------------
Title: FUSE Observations of He-rich sdB Stars
Authors: Lanz, T.; Brown, T. M.; Sweigart, A. V.; Hubeny, I.; Landsman,
W. B.
2002AAS...20111308L Altcode: 2002BAAS...34.1285L
Subdwarf B stars all show significant abundance anomalies. Most are
extremely deficient in helium and selected light elements, but a
minority are helium-rich. Deficiencies in helium and heavier elements
have been attributed to gravitational settling, but the helium-enriched
members of the class present a puzzling exception, because radiative
levitation should be too weak to prevent gravitational settling. New
evolutionary calculations suggest that these helium-rich sdB stars are
the result of a delayed helium-core flash on the white dwarf cooling
curve. The convective zone produced by this flash will penetrate
the hydrogen envelope, mixing hydrogen into the hot helium-burning
interior, where it is rapidly consumed. The resulting star should show
greatly enhanced helium and carbon with respect to the other heavy
elements. This phenomenon is analogous to the born again scenario
for producing hydrogen-deficient R CrB stars following a very late
helium-shell flash. We have recently obtained FUSE spectra of two
helium-rich sdB stars, revealing huge C III lines at 977 and 1176
Å. Our preliminary analysis yields a surface composition of 97% He
and 3% C, in agreement with the new evolutionary scenario. This work
is supported in part by NASA grant NAG5-12383.

---------------------------------------------------------
Title: Erratum: “Isolating Clusters with Wolf-Rayet Stars in I Zw
18” (<A href="/abs/2002ApJ...579L..75B">ApJ, 579, L75 [2002]</A>)
Authors: Brown, Thomas M.; Heap, Sara R.; Hubeny, Ivan; Lanz, Thierry;
Lindler, Don
2002ApJ...581L.129B Altcode:
During publication, the software used to process Figure 1 corrupted
the Y-axis label. The label should read “Flux (10-16
ergs s-1 cm-2 Å-1)” instead of the
published “Flux (1016 ergs s1 cm2
Å1).” The Press sincerely regrets this error.

---------------------------------------------------------
Title: Abundance of Trace Elements in the Atmospheres of Three Hot
White Dwarfs in Post-CE Binaries
Authors: Dupuis, J.; Vennes, S.; Lanz, T.; Chayer, P.
2002AAS...20111908D Altcode: 2002BAAS...34.1301D
We present FUSE observations of the hot white dwarfs in the post-CE
binaries BPM 6502, EUVEJ2013+400, and EUVEJ0720-317. The abundance
pattern observed in these stars is possibly affected by accretion from
the cool star companion. In order to better understand the effect of
accretion on the atmospheric composition of these stars, we measure the
abundances of C, N, O, Si, P, and Fe from lines detected in the FUSE
spectra. We report the detection of CIII, OVI, PV, and SiIV in the
spectra of EUVEJ2013+400 (Teff = 48,000K) and EUVEJ0720-317 (Teff =
53,000K). BPM 6502, a cooler DA (Teff = 21,000K), shows photospheric
absorption lines of CIII, NII, SiIII, SiIV, and FeIII. We measure the
following abundances in EUVEJ0720-317 and EUVEJ2013+400 respectively:
log (C/H) = -6.0, -5.8; log (O/H) = -6.1, -5.6; log (Si/H) = -6.1,
-6.5; log (P/H) = -7.0, -7.7. For BPM 6502, we measure log (C/H) =
-6.2, log (N/H) = -6.6, log (Si/H) = -6.1, and log (Fe/H) = -7.5. It
is remarkable that although the carbon abundance is quite similar
in the three stars, we see a decrease in the silicon and phosphorus
abundances with decreasing effective temperature (as predicted
by the radiative levitation theory) and an increase in the oxygen
abundance. The presence of iron in BPM 6502 is surprising since it
should not be supported by radiative levitation and is a possible
evidence of ongoing accretion. This research is funded by NASA grants
NAG5-11717 and NAG5-6551.

---------------------------------------------------------
Title: The Helium and Nitrogen-Rich UV-Bright Star UIT-644 in
Omega Cen
Authors: Landsman, W. B.; Crotts, A. P. S.; Heap, S.; Lanz, T.;
Sweigart, A.
2002AAS...201.0715L Altcode: 2002BAAS...34.1103L; 2002AAS...201..715L
We report on STIS medium-resolution echelle (E230M) observations of the
UV-Bright star UIT-644, located near the center of the globular cluster
Omega Centauri. The observations cover the wavelength region 1625 --
2364 Å with a resolution R=30,000. Earlier low-resolution optical
spectra with the CTIO-4m have been used to derive the temperature
(20,000 K), gravity (log g = 3.5) and bolometric luminosity (log
L/LSun = 2.9), while revealing the star to be helium-rich
(Y=0.68). The STIS spectra are used to show that the star is also
extremely nitrogen-rich ([N/H] ~ 10), while having a typical cluster
iron abundance ([Fe/H] = -1.5). The large nitrogen abundance probably
requires that the products of helium burning be mixed to the surface,
with the carbon subsequently converted to nitrogen. We speculate on a
scenario in which UIT-644 ejected its envelope subsequent to a helium
flash on the AGB.

---------------------------------------------------------
Title: The Role of Rotation in the Evolution of Massive Stars
Authors: Heap, S. R.; Lanz, T. M.
2002AAS...20111309H Altcode: 2002BAAS...34.1285H
Recent evolutionary models of massive stars (Maeder &amp; Meynet 2000,
2001; Heger &amp; Langer 2000) predict important effects of rotation
including: increasing the rate of mass-loss; lowering the effective
gravity; altering the evolutionary track on the HRD; extending the
main-sequence phase (both on the HR diagram and in time); and mixing of
CNO-processed elements up to the stellar surface. Observations suggest
that rotation is a more important factor at lower metallicities because
of higher initial rotational velocities and weaker winds. This makes
the SMC, a low-metallicity galaxy (Z=0.2Zsun), an excellent
environment for discerning the role of rotation in massive stars. We
report on a FUSE+STIS+optical spectral analysis of 17 O-type stars in
the SMC, where we found an enormous range in N abundances. Three stars
in the sample have the same (low) CN abundances as the nebular material
out of which they formed, namely C=0.085 Csun and N=0.034
Nsun. However, more than half show N ~Nsun,
an enrichment factor of 30X! Such unexpectedly high levels of N have
ramifications for the evolution of massive stars including precursors
to supernovae. They also raise questions about the sources of nitrogen
in the early universe. This study was supported in part by grants from
NASA's ADP, HST GO-7437, and FUSE B134.

---------------------------------------------------------
Title: Laboratory Astrophysics: Study of Radiative Shocks
Authors: Leygnac, S.; Lanz, T.; Stehlé, C.; Michaut, C.
2002AAS...201.5415L Altcode: 2002BAAS...34.1201L
Radiative shocks are high Mach number shocks with a strong coupling
between radiation and hydrodynamics which leads to a structure
governed by a radiative precursor. They might be encountered in
various astrophysical systems: stellar accretion shocks, pulsating
stars, interaction of supernovae with the intestellar medium etc. A
numerical one dimensional (1D) stationary study of the coupling between
hydrodynamics and radiative transfer is being performed. An estimate of
the error made by the 1D approach in the radiative transfer treatment
is done by an approximate short characteristics approach. It shows,
for exemple, how much of the radiation escapes from the medium in the
configuration of the experiment. The experimental study of these shocks
has been performed with the high energy density laser of the LULI,
at the École Polytechnique (France). We have observed several shocks
identified as radiative shocks. The shock waves propagate at about 50
km/s in a tiny 10 mm3 shock tube filled with gaz. From the
measurements, it is possible to infer several features of the shock
such as the speed and the electronic density.

---------------------------------------------------------
Title: Isolating Clusters with Wolf-Rayet Stars in I Zw 18
Authors: Brown, Thomas M.; Heap, Sara R.; Hubeny, Ivan; Lanz, Thierry;
Lindler, Don
2002ApJ...579L..75B Altcode: 2002astro.ph.10089B
We present UV images and spectra of the starburst galaxy I Zw 18,
taken with the Space Telescope Imaging Spectrograph. The high
spatial resolution of these data allows us to isolate clusters
containing Wolf-Rayet stars of the subtype WC. Our far-UV spectra
clearly show C IV λλ1548, 1551 and He II λ1640 emission of WC
stars in two clusters: one within the bright (northwest) half of I
Zw 18 and one on the outskirts of this region. The latter spectrum
is unusual because the C IV is seen only in emission, indicating a
spectrum dominated by WC stars. These data also demonstrate that the
H I column in I Zw 18 is strongly peaked in the fainter (southeast)
half of I Zw 18, with a column depth far larger than that reported in
previous analyses. Based on observations made with the NASA/ESA Hubble
Space Telescope, obtained at the Space Telescope Science Institute,
which is operated by the Association of Universities for Research in
Astronomy, Inc., under NASA contract NAS 5-26555. These observations
are associated with proposal 9054.

---------------------------------------------------------
Title: A chromospheric scenario for the activity of beta Pictoris,
as revealed by FUSE
Authors: Bouret, J. -C.; Deleuil, M.; Lanz, T.; Roberge, A.; Lecavelier
des Etangs, A.; Vidal-Madjar, A.
2002A&A...390.1049B Altcode:
We investigate a chromospheric scenario to explain the characteristics
of the far-UV emission lines of beta Pictoris, revealed by FUSE
spectra of this famous circumstellar disk system. The model assumes
a thin region heated up to a few 105 K located close to
the stellar photosphere. The resonance lines of C III at 977 Å
and O VI at 1032-1037 Å, seen in emission, are produced in this
chromosphere-transition region complex. Using complementary data in
the mid and near UV (specifically, a resonance doublet of C IV at
1548-1550 Å and Mg II h &amp; k), we show that the whole dataset is
remarquably well reproduced by the model. In addition, we investigate
the properties of the C III* multiplet at 1176 Å and conclude that
this line likely forms in a weak warm wind, originating from the
prominent circumstellar disk of beta Pictoris. Finally, radiative
losses have been calculated and have provided estimates of the amount
of non radiative energy dissipated in the external atmosphere of beta
Pictoris, which is a measure of the star's activity level. Such behavior
for an A5 V star presents a challenge for both evolution and activity
models since the former predict that main-sequence A stars should not
be active, while the latter are unable to quantitatively account for
the characteristics of the chromospheric heating of beta Pictoris.

---------------------------------------------------------
Title: Young, Massive Stars at Low Metallicity
Authors: Heap, S. R.; Lanz, T.
2002AAS...200.0713H Altcode: 2002BAAS...34..652H
We report on an extensive analysis of O-type stars in the Small
Magellanic Cloud that have been observed by HST and FUSE. The analysis,
which involves detailed NLTE atmospheric models recently constructed
by Lanz &amp; Hubeny, has the following goals: (1) calibration of
spectral properties in terms of fundamental parameters; (2) resolution
of the discrepancy between spectro- scopic masses and evolutionary
masses; and (3) evidence for mixing of nuclear-processed elements. We
derive a significantly lower temperature scale for O-type stars than
previously assumed; the lower temperatures help to resolve the “mass
discrepancy”. We describe the results of our analysis and discuss
their implications for stellar evolution at low metallicity. This work
was supported by NASA ADP, HST GO and FUSE grants.

---------------------------------------------------------
Title: Multidimensional Non-LTE Radiative Transfer. I. A Universal
Two-dimensional Short-Characteristics Scheme for Cartesian, Spherical,
and Cylindrical Coordinate Systems
Authors: van Noort, Michiel; Hubeny, Ivan; Lanz, Thierry
2002ApJ...568.1066V Altcode:
We have developed an efficient and robust two-dimensional non-LTE
radiation transfer solver appropriate for line transfer in the
equivalent two-level atom formalism. The numerical method applies
the accelerated lambda iteration technique together with the
short-characteristics scheme. The code presented in this paper
incorporates all three standard geometries (Cartesian, cylindrical,
and spherical) in a transparent way while allowing for arbitrary
(three-dimensional) velocity fields. The geometry-specific parts of
the radiative transfer solver are modularized so that the change of
geometry is accomplished by simply setting the appropriate switch. We
have also developed a parallel version of the code, in which we
use a parallelization in spatial subdomains, and showed that such a
scheme is sufficiently robust. We have performed a number of tests
of the performance of the solver in all three geometries. Finally,
we discuss the internal accuracy of the transfer solutions depending
on the number of spatial, angular, and frequency grid points.

---------------------------------------------------------
Title: The Origin of Hot Subluminous Horizontal-Branch Stars in ω
Centauri and NGC 2808
Authors: Sweigart, A. V.; Brown, T. M.; Lanz, T.; Landsman, W. B.;
Hubeny, I.
2002ASPC..265..261S Altcode: 2002ocuw.conf..261S; 2002astro.ph..3063S
Hot subluminous stars lying up to 0.7 mag below the extreme horizontal
branch (EHB) are found in the UV color-magnitude diagrams of omega Cen
and NGC 2808. Such stars are unexplained by canonical HB theory. In
order to explore the origin of these subluminous stars, we evolved a
set of low-mass stars from the main sequence through the helium-core
flash to the HB for a wide range in the mass loss along the red-giant
branch (RGB). Stars with the largest mass loss evolve off the RGB to
high effective temperatures before igniting helium in their cores. Our
results indicate that the subluminous EHB stars, as well as the gap
within the EHB of NGC 2808, can be explained if these stars undergo
a late helium-core flash on the white-dwarf cooling curve. Under
these conditions the flash convection will penetrate into the stellar
envelope, thereby mixing most, if not all, of the envelope hydrogen
into the hot helium- burning interior. This phenomenon is analogous
to the "born-again" scenario for producing hydrogen-deficient stars
during a very late helium-shell flash. "Flash mixing" greatly enhances
the envelope helium and carbon abundances and, as a result, leads to
an abrupt increase in the HB effective temperature. We argue that
the EHB gap in NGC 2808 is caused by this theoretically predicted
dichotomy in the HB morphology. Using new helium- and carbon-rich
stellar atmospheres, we show that the flash-mixed stars have the same
reduced UV flux as the subluminous EHB stars. Moreover, we demonstrate
that models without flash mixing lie, at most, ~0.1 mag below the EHB
and hence fail to explain the observations. Flash mixing may also
provide a new evolutionary channel for producing the high gravity,
He-rich sdO and sdB stars.

---------------------------------------------------------
Title: FUSE Observations of Key DBA and DAB White Dwarfs
Authors: Lanz, Thierry
2002fuse.prop.C026L Altcode:
We propose FUSE observations of three key white dwarfs having hybrid
helium and hydrogen spectra the DBA and DAB stars. These stars are
relatively rare, but may hold the key for understanding the chemical
evolution of white dwarf atmospheres along the cooling sequence, more
particularly the so-called DB gap. One possible explanation for these
white dwarfs with mixed atmospheres is convective dredge up, where
convection mixes the hydrogen and helium layers. Potentially other
elements such as carbon and nitrogen may also be dredged up. Strong C
and N lines are present in the FUSE band, and can serve as sensitive
measures of the presence of these elements. Alternatively, some DAB
stars have been found to be spectral composites consisting of DA and DB
double degenerate binary systems. The FUSE band is uniquely sensitive
to the spectral energy distribution of stars in the 20,000 to 30,000 K
range and will make it possible to distinguish two white dwarfs, even
if their effective temperatures are similar. Other possible models,
including interstellar accretion, stratified atmospheres and surface
inhomogeneities, can be tested with FUSE spectroscopy.

---------------------------------------------------------
Title: The D/H Abundance Ratio in Local Interstellar Gas
Authors: Sahu, M. S.; Landsman, W.; Bruhweiler, F. C.; Holberg, J.;
Hubeny, I.; Barstow, M.; Linsky, J.; Gull, T.; Lindler, D.; Lanz,
T.; Feggans, K.
2001AAS...199.1101S Altcode: 2001BAAS...33.1323S
Variations of the D/H ratio on scales of 100 pc are important for two
reasons: (1) they complicate the use of deuterium as a cosmological
probe, and (2) they imply non-uniform deuterium production/destruction
and an inefficient mixing of interstellar gas. The Local Interstellar
Medium (LISM) is an ideal location to test whether the D/H abundance
ratio varies or not, because the heating sources and radiation
fields are well-studied and we have a detailed knowledge of the
three-dimensional (3D) structure of the diffuse clouds in the LISM. This
detailed knowledge of the number, structure and velocities of the
absorbing clouds greatly helps in reducing errors in the derived
D/H ratios. We are working on a project to obtain high-precision
D/H abundance ratios in the interstellar gas within 100 pc. For this
purpose, we primarily use HST-STIS data towards nearby hot, white dwarfs
(WDs). We have also begun an HST archival research project to study
about 20 WD sightlines. In this poster, we present new STIS observations
for GD 153 and HZ 43 and report the results of our HST Archival project.

---------------------------------------------------------
Title: The Origin of Hot Subluminous Horizontal-Branch Stars in ω
Cen and NGC 2808
Authors: Sweigart, A. V.; Brown, T. M.; Moehler, S.; Lanz, T.;
Landsman, W. B.; Hubeny, I.; Dreizler, S.; Napiwotzki, R.
2001AAS...19913704S Altcode: 2001BAAS...33.1512S
Ultraviolet (UV) observations of the globular clusters ω Cen and NGC
2808 have revealed an unexpected population of hot subluminous stars
lying up to 0.7 mag below the extreme horizontal branch (EHB) in the
UV, which are not explained by canonical stellar models. In order to
explore the evolutionary status of these stars, we have evolved a set
of low-mass stars from the main sequence through the helium flash to
the horizontal branch (HB) for a wide range in the mass loss along the
red-giant branch (RGB). Stars with the largest mass loss evolve off
the RGB to high effective temperatures before igniting helium in their
cores. Our results indicate that the subluminous stars can be explained
if these stars undergo a late helium flash while descending the white
dwarf cooling curve. Under these conditions the convection zone produced
by the helium flash will penetrate into the stellar envelope, thereby
mixing the envelope hydrogen into the hot helium-burning interior, where
it is rapidly consumed. Such “flash-mixed” stars will have helium-
and carbon-rich envelopes and will lie at higher effective temperatures
than the hottest canonical (i.e., unmixed) EHB stars. Using new stellar
atmospheres, we show that these changes in the envelope abundances
will suppress the UV flux in the spectra of the flash-mixed stars by
the amount needed to explain the hot subluminous stars in ω Cen and
NGC 2808. To test this evolutionary scenario, we have obtained medium
resolution spectra of a sample of the hottest HB stars in ω Cen. We
find that these stars are indeed helium-rich compared to classical
EHB stars and also considerably hotter than the hottest EHB models
without flash mixing.

---------------------------------------------------------
Title: NLTE Models of Vertical structure of Accretion Disks around
Stellar Mass Black Holes
Authors: Hubeny, I.; Blaes, O.; Krolik, J. H.; Agol, E.; Lanz, T.
2001AAS...19915908H Altcode: 2002BAAS...34Q.569H
Recent upgrades of our computer program TLUSDISK are briefly
described. These include a self-consistent treatment of Compton
scattering, and the effects of X-ray continuum opacities of the most
important metal species (C, N, O, Ne, Mg, Si, S, Ar, Ca, Fe, Ni). In the
case the central degenerate object is a neutron star or a black hole,
we allow for a full general relativistic treatment. We show the effects
of Comptonization and metal opacities on the structure of disk under
various conditions. We also present a simple analytic prescription
for the vertical temperature structure of the disk in the presence
of Comptonization, and show under what conditions a hot outer layer
(a corona) is formed.

---------------------------------------------------------
Title: Solar UV Radiation and the Origin of Life on Earth
Authors: Heap, S. R.; Gaidos, E.; Hubeny, I.; Lanz, T. M.
2001AAS...199.0314H Altcode: 2001BAAS...33.1305H
We have embarked on a program aimed at understanding the atmosphere of
the early Earth, because of its importance as a greenhouse, radiation
shield, and energy source for life. Here, we give a progress report on
the first phase of this program: to establish the UV radiation from
the early Sun. We are presently obtaining ultraviolet spectra (STIS,
FUSE, EUVE) of carefully selected nearby, young solar-type stars,
which act as surrogates for the early Sun. We are currently making
detailed non-LTE analyses of the spectra and constructing models of
their photospheres + chromospheres. Once validated, these models will
allow us to extrapolate our theoretical spectra to unobserved spectral
regions, and to proceed to the next step: to develop photochemical
models of the pre-biotic and Archean atmosphere of the Earth.

---------------------------------------------------------
Title: NLTE Model Atmospheres of Supersoft X-Ray Sources
Authors: Lanz, T.; Hubeny, I.
2001AAS...199.1712L Altcode: 2001BAAS...33.1336L
Supersoft X-Ray Sources are most probably very hot white dwarfs
undergoing steady nuclear burning in their envelopes. They are high
luminosity (L &gt; 0.1 L Edd), soft (kT≈ 50 eV) low
mass binary with a white dwarf primary accreting at a sub-Eddington
rate. High-resolution spectra in the soft X-rays are now becoming
available with Chandra and XMM-Newton. Photospheric line spectroscopy
has the potential to provide accurate diagnostics of the white dwarf
properties. We have therefore embarked on a project aimed at providing
new NLTE model atmospheres of supersoft sources. We are building a
series of model atmospheres of hot white dwarfs (20&lt;= kT&lt;= 100
eV; 8&lt;= log g&lt;= 10) with various surface composition. The models
incorporate the most abundant species, H, He, C, N, O, Ne, Mg, Si, S,
and Fe, which are allowed to depart from Saha-Boltzmann statitics. Our
models include the influence of multi-level model atoms, metal line
blanketing, and Auger ionization on the atmospheric structure. A
preliminary comparison of models calculated assuming coherent (Thomson)
and non-coherent (Compton) scattering is presented. This work was
supported through a NASA ATP grant (NRA 00-01-ATP-153).

---------------------------------------------------------
Title: Flash Mixing on the White Dwarf Cooling Curve: Understanding
Hot Horizontal Branch Anomalies in NGC 2808
Authors: Brown, Thomas M.; Sweigart, Allen V.; Lanz, Thierry; Landsman,
Wayne B.; Hubeny, Ivan
2001ApJ...562..368B Altcode: 2001astro.ph..8040B
We present an ultraviolet color-magnitude diagram (CMD) spanning
the hot horizontal branch (HB), blue straggler, and white dwarf
populations of the globular cluster NGC 2808. These data were obtained
with the far-UV and near-UV cameras on the Space Telescope Imaging
Spectrograph (STIS). Although previous optical CMDs of NGC 2808 show
a high-temperature gap within the hot HB population, no such gap is
evident in our UV CMD. Instead, we find a population of hot subluminous
HB stars, an anomaly only previously reported for the globular cluster
ω Cen. Our theoretical modeling indicates that the location of these
subluminous stars in the UV CMD, as well as the high-temperature gap
along the HB in optical CMDs, can be explained if these stars underwent
a late helium-core flash while descending the white dwarf cooling
curve. We show that the convection zone produced by such a late helium
flash will penetrate into the hydrogen envelope, thereby mixing hydrogen
into the hot helium-burning interior, where it is rapidly consumed. This
phenomenon is analogous to the “born again” scenario for producing
hydrogen-deficient stars following a late helium-shell flash. The
flash mixing of the envelope greatly enhances the envelope helium and
carbon abundances, and leads, in turn, to a discontinuous increase in
the HB effective temperatures at the transition between canonical and
flash-mixed stars. We argue that the hot HB gap is associated with this
theoretically predicted dichotomy in the HB properties. Moreover, the
changes in the emergent spectral energy distribution caused by these
abundance changes are primarily responsible for explaining the hot
subluminous HB stars. Although further evidence is needed to confirm
that a late helium-core flash can account for the subluminous HB stars
and the hot HB gap, we demonstrate that an understanding of these stars
requires the use of appropriate theoretical models for their evolution,
atmospheres, and spectra. Based on observations with the NASA/ESA Hubble
Space Telescope obtained at the Space Telescope Science Institute,
which is operated by AURA, Inc., under NASA contract NAS5-26555.

---------------------------------------------------------
Title: Stellar Atmospheres
Authors: Lanz, T.
2001eaa..bookE1873L Altcode:
The atmosphere of a star is the boundary between the stellar interior
and the interstellar medium. It spans the layers from which photons can
escape to the surrounding space. Direct information about the physical
state of the stellar atmosphere can therefore be derived from the
emergent radiation. This is, however, not the case for deeper layers,
because photons that are emitted in these layers a...

---------------------------------------------------------
Title: A Comparative Study of the Atmospheric Composition of the DA
White Dwarfs Feige 24 and G191-B2B
Authors: Vennes, Stéphane; Lanz, Thierry
2001ApJ...553..399V Altcode:
We analyze Hubble Space Telescope Imaging Spectrograph spectra of
two white dwarfs, Feige 24 and G191-B2B, and we compare the measured
abundance patterns. Except for carbon, which appears overabundant in
G191-B2B by 0.3-0.5 dex relative to Feige 24, the average heavy-element
abundance in Feige 24 is 0.17 dex larger than in the cooler, hence
older, G191-B2B, indicating a slow but perceptible decline of
metallicity with time. We also found that the C IV λ1550 doublet
in G191-B2B is composed of a photospheric component and a second
component, either interstellar or circumstellar, separated by only
Δv=15 km s-1. We find a simple correspondence between the
measured element abundance, its solar abundance value, and its atomic
weight from which we deduce the likely presence of additional elements
such as neon and magnesium in the photospheres of Feige 24 and G191-B2B.

---------------------------------------------------------
Title: Understanding Horizontal Branch Anomalies in NGC 2808
Authors: Brown, T. M.; Sweigart, A. V.; Lanz, T.; Landsman, W. B.;
Hubeny, I.
2001AAS...198.4302B Altcode: 2001BAAS...33..844B
We present an ultraviolet color-magnitude diagram (CMD) spanning the hot
horizontal branch (HB), blue straggler, and white dwarf populations
of the globular cluster NGC 2808. These data, obtained with the
Space Telescope Imaging Spectrograph (STIS), demonstrate that NGC
2808 harbors a significant population of hot subluminous HB stars,
an anomaly only previously reported for the globular cluster Omega
Cen. Our theoretical modeling indicates that the location of these
subluminous stars in the CMD, as well as the high temperature gap along
the HB of NGC 2808, can be explained if these stars underwent a late
helium-core flash while descending the white dwarf cooling curve. We
show that the convective zone produced by such a late helium flash will
penetrate into the hydrogen envelope, thereby mixing hydrogen into
the hot helium-burning interior, where it is rapidly consumed. This
phenomenon is analogous to the "born again" scenario for producing
hydrogen-deficient stars following a late helium-shell flash. The flash
mixing of the envelope greatly enhances the envelope helium and carbon
abundances that, in turn, leads to a discontinuous increase in the HB
effective temperatures. We argue that the hot HB gap is associated with
this theoretically predicted dichotomy in the HB properties. Moreover,
the changes in the emergent spectral energy distribution caused by
these abundance changes are primarily responsible for explaining
the hot subluminous HB stars. Although further evidence is needed to
confirm that a late helium-core flash can account for the subluminous HB
stars and the hot HB gap, we demonstrate that an understanding of these
stars requires use of sophisticated models for the stellar evolution,
atmospheres, and synthetic spectra.

---------------------------------------------------------
Title: Determination of magnetic fields in stars based on polarized
hydrogen lines
Authors: Brillant, Stéphane; Stehlé, Chantal; Mathys, Gautier;
Lanz, Thierry
2001AIPC..559..205B Altcode:
The study of polarized light is a powerful tool for the determination
of magnetic fields in stellar objects. The most used methods are the
so-called “photographic” method [1] and Balmer line photopolarimetry
[2]. However, in various cases these two methods give different results
for the same star. To assess the validity of the interpretation of
photopolarimetric measurements of Balmer lines, we have developed a new
model of the formation of these lines in a magnetized plasma. This model
is based on the Simplified Unified Theory (SUT) for the computation of
the line shape [3-5]. We present this model, some numerical results
computed with it, and conclusions drawn from consideration of there
results [6]. We discuss the prospects for future developments in
continuation of this work. .

---------------------------------------------------------
Title: CMFGEN: A non-LTE Line-Blanketed Radiative Transfer Code for
Modeling Hot Stars with Stellar Winds
Authors: Hillier, D. John; Lanz, Thierry
2001ASPC..247..343H Altcode: 2001scpp.conf..343H
No abstract at ADS

---------------------------------------------------------
Title: NLTE Model Atmospheres for OB Stars
Authors: Lanz, Thierry; Hubeny, Ivan
2001ASPC..247..351L Altcode: 2001scpp.conf..351L
No abstract at ADS

---------------------------------------------------------
Title: Recent Developments for Balmer Line Photopolarimetry
Authors: Stehlé, C.; Mathys, G.; Brillant, S.; Lanz, T.
2001ASPC..248..357S Altcode: 2001mfah.conf..357S
No abstract at ADS

---------------------------------------------------------
Title: Simulation of Interfero-Polarimetric Observations for
Magnetic Stars
Authors: Rousselet-Perraut, K.; Stehlé, C.; Kilbinger, M.; Chesneau,
O.; Vakili, F.; Lanz, T.; Mathys, G.
2001ASPC..248..629R Altcode: 2001mfah.conf..629R
No abstract at ADS

---------------------------------------------------------
Title: Properties of the z = 2.73 galaxy, MS1512-cB58
Authors: Heap, Sara R.; Hubeny, Ivan; Lanz, Thierry M.
2001ApSSS.277..263H Altcode:
MS1512-cB58 is an z=2.73 galaxy whose apparent brightness is amplified
by30-50X due to gravitationally lensing. The restframe far-UV spectrum
of cB58 that was obtained by Keck-I/LRIS is `one of the best ultraviolet
spectra of a starburst galaxy obtained at any redshift' (Pettini et
al., 2000). We have analyzed and modeled the spectrum of this galaxy in
order to learn the properties of high-redshift galaxies. We find that
our model spectrum is a near match to the observed spectrum of cB58 if
the galaxy has a SMC-like metallicity, and has a Salpeter IMF extending
up to ~ 100 M. The spectrum of cB58 also shows many absorption lines
formed in a giant, expanding gas shell surrounding the star-forming
complex. We show preliminary findings of the properties of the giant
HII region based on our CLOUDSPEC model.

---------------------------------------------------------
Title: GALSPEC: A Tool for Computing the Spectra of Star-Forming
Galaxies of Low Metallicity
Authors: Heap, S. R.; Hubeny, I.; Lanz, T. M.; Lindler, D. J.
2000AAS...197.7813H Altcode: 2000BAAS...32R1531H
The far-ultraviolet spectral region is rich in diagnostics of
star formation in galaxies; it gives information not only about the
star-forming complex, but also about the dust and circumstellar gas. We
have therefore developed a tool, called GALSPEC, for computing far-UV
spectra of star-forming galaxies. The user controls the process through
a graphical user interface (GUI) by specifying the input parameters for
the stars, dust, and gas. GALSPEC computes the integrated spectrum of a
star-forming complex, given the rate of star formation, metallicity (0.2
Zsun or 0.5 Zsun), and IMF upper mass limit. To
compute the integrated spectrum, GALSPEC makes use of a library
of stellar spectra composed of observed spectra from HST and model
spectra calculated with the TLUSTY/SYNSPEC program (Hubeny &amp; Lanz
1995). GALSPEC then applies wavelength-dependent extinction according
to Calzetti's starburst extinction law. The mechanical luminosity from
massive stars and supernovae sweeps up interstellar material thereby
forming an expanding shell with a central cavity. GALSPEC computes the
absorption spectrum of the shell using the CLOUDSPEC program (Hubeny
2000), a combination of Ferland's CLOUDY program and SYNSPEC. This work
was supported by the STIS GTO program at Goddard and by STScI grants
(GO 7437, AR 7985).

---------------------------------------------------------
Title: Ultraviolet Spectroscopy of O and B Stars at 1/30 Solar
Metallicity: The Stars of the Local Group Galaxy Sextans A
Authors: Bruhweiler, F.; Miskey, C.; de Koter, A.; Smith Neubig, M.;
Walborn, N.; Lanz, T.
2000AAS...197.7805B Altcode: 2000BAAS...32.1530B
We have obtained slitless two-dimensional ultraviolet spectroscopy
using the Space Telescope Imaging Spectrograph (STIS) aboard the HST
of the brightest concentration of OB stars in the faint irregular
galaxy, Sextans A. The spectral image obtained with the G140L grating
samples a 25'x25" region spanning a wavelength range approximately
from 1170 to 1730 A. Sextans A lies at the edge of the Local Group at
a distance of 1.4 Mpc and has an extremely low metallicity, roughly
1/30 solar or about 1/3 that of the Small Magellanic Cloud. This
is within a factor of two of the most metal deficient galaxy known,
I Zw 18. The STIS dataset is unique in that it represents spectra of
individual O and B stars at the lowest metallicity ever obtained. The
earliest and one of the UV-brightest stars we classify as O6 V based
upon comparisons with UV spectra of SMC stars. This star exhibits a
definite N V 1240 P Cygni profile that appears only slightly weaker,
but with a lower terminal velocity than its SMC analogues. Both C IV
and Si IV are also very weak. Absorption from Fe V is also detectable
near 1430 and 1450 Angstroms. Other very UV-luminous stars are also
present. Several early B-supergiants are also detected as evidenced
by the pronounced Si IV 1400 A absorption. Further detailed modeling
of the UV spectra using sophisticated model atmosphere codes is planned.

---------------------------------------------------------
Title: Ultraviolet Spectra of Hot Horizontal Branch Stars in NGC 6752
Authors: Landsman, W. B.; Catelan, M.; Grundahl, F.; Lanz, T.;
Proffitt, C.; Sweigart, A. V.
2000AAS...197.4120L Altcode: 2000BAAS...32Q1464L
We have obtained STIS ultraviolet low-resolution spectra (1140 --
3100 Å) of eight hot horizontal branch (HB) stars with 10,500
&lt; Teff (K) &lt; 20,000 in the globular cluster NGC
6752 ([Fe/H = -1.6). The two coolest targets have temperatures just
coolward of Teff ~ 11,500 K, where a sudden jump in the iron
abundance has been observed in optical spectra of HB stars in NGC 6752
(Moehler et al. 2000, A&amp;A, 360, 120) presumably due to the onset
of radiative levitation. The spectra of the remaining six targets are
consistent with an iron abundance near solar. Because this dramatic
increase in the iron abundance appears to be characteristic of the
HB in all globular clusters (Grundahl et al. 1999, ApJ, 524, 242),
our data set can be used as a template for the ultraviolet spectral
synthesis of the HB of metal-poor populations. An unexpected result
is the apparent detection of the 1485 Å autoionizing feature of Si
II in four of the iron-rich stars.

---------------------------------------------------------
Title: A New Grid of NLTE Model Atmospheres of Hot Stars
Authors: Hubeny, I.; Lanz, T.
2000AAS...197.7812H Altcode: 2000BAAS...32Q1531H
We briefly describe improvements in our non-LTE model atmosphere code,
TLUSTY. The upgrades include opacity sampling, improved Fe collisional
strengths, resonance-averaged photoionization cross-sections, and a
number of numerical enhancements. Using TLUSTY, version 200, we have
calculated a grid of NLTE, fully-blanketed, model atmospheres covering
the range of O to early B-type stars ( 20000 &lt;= T eff
&lt;= 55000 K, 3.0 &lt;= log g &lt;= 4.75) with a solar or 1/5 solar
metallicity. The models incorporate over 70,000 NLTE atomic levels of
about 30 ions of H, He, C, N, O, Ne, Mg, Si, S, Fe and Ni, which are
grouped into about 700 superlevels. We compare our predicted EUV fluxes
to other existing grids of model atmospheres (Atlas9, CoStar models), as
well as to actual UV and optical observations of representative stars.

---------------------------------------------------------
Title: The D/H Abundance Ratio in Local Interstellar Gas
Authors: Sahu, M. S.; Landsman, W.; Bruhweiler, F. C.; Linsky, J.;
Hubeny, I.; Barstow, M.; Holberg, J.; Gull, T.; Bowers, C.; Lindler,
D.; Lanz, T.; Feggans, K.
2000AAS...197.0703S Altcode: 2000AAS...197..703S; 2000BAAS...32.1401S
Does the D/H ratio vary within ~ 100 pc of the Sun? If it does, what
is the range of these variations? Are the D/H variations correlated to
any physical property of the absorbing cloud? To answer these questions
we have begun a project to obtain high-precision D/H abundance ratios
in the Local Interstellar Medium within ~ 100 pc (Sahu et al. 1999,
ApJ 523, L159). For this purpose, we primarily use HST-Space Telescope
Imaging Spectrograph (STIS) data towards nearby hot, white dwarfs
(WDs). The advantages of using STIS for this study are: (1) The high
velocity resolution of STIS in the UV ( ~ 2 km s-1 in the
E140H and E230H modes) allows the velocity structure in the sightline
to be resolved and reliable component-to-component variations can be
studied (2) WDs provide a strong, smooth UV continuum against which
other diagnostic interstellar absorption lines can be measured. The
wider wavelength coverage ( ~ 200Å per setting) of STIS in the
high-resolution E140H mode allows important interstellar lines such as N
i, Si ii, O i and S ii to be measured in addition to D i and H i, at no
extra cost in HST time (3) The superior echelle scatter and background
corrections possible with the 2-dimensional STIS-MAMA detectors allow
more accurate D/H determinations from WD spectra. In this poster, we
present a status report of our project along with some recent results.

---------------------------------------------------------
Title: NGC 346-12, a Rapidly Rotating O9.5V Star in the SMC: Test
Case of Weak Winds
Authors: Lanz, T.; Bouret, J. -C.; Heap, S. R.; Hubeny, I.; Hillier,
D. J.; Lennon, D. J.; Smith, L. J.; Evans, C. J.; Owocki, S. P.
2000AAS...197.7811L Altcode: 2000BAAS...32R1531L
We have analyzed the UV and optical spectrum of the O9.5V Star, NGC
346-12, using sophisticated, NLTE line-blanketed model atmospheres
calculated with our code TLUSTY. The following stellar parameters
were derived: T eff=30,000 K, log g=3.5. An abundance
analysis yields a metallicity, [Fe/H]=-1.0. The N/C abundance ratio
is 25 times the solar ratio, indicating that material processed
through the CNO-cycle has been brought up to the surface. Assuming
a distance modulus, (m-M)=18.9, for the SMC, we have derived the
luminosity, radius, and mass of the star. We found, similarly to higher
luminosity galactic stars, a discrepancy between the mass derived
from the spectroscopic analysis and from stellar evolution theory. We
conclude that it is very likely that NGC 346-12 is a fast rotator,
whose evolution has been affected by rotation. Furthermore, the wind
of NGC 346-12 appears abnormal: while the C IV resonance lines do not
reveal any indication of a wind, a weak P-Cygni profile is observed
in N 5 1240. Various possible explanations for the low inferred ion
density in the stellar wind, including an enhanced degree of wind
ionization associated with ion frictional heating, or ion runaway
due to frictional decoupling from the hydrogen-helium bulk plasma,
are examined. This work was supported through a NASA/NRC RA award and
STScI grants (GO 7437, AR 7985).

---------------------------------------------------------
Title: NLTE Model Atmosphere Analysis of Main-Sequence O Stars in
NGC 346
Authors: Bouret, J. -C.; Heap, S. R.; Hubeny, I.; Lanz, T.; Hillier,
D. J.; Lennon, D. J.; Smith, L. J.; Evans, C. J.
2000AAS...197.7810B Altcode: 2000BAAS...32.1531B
To serve as spectral templates of young, low-metallicity populations,
high-resolution, high-quality, ultraviolet (STIS) and optical spectra
of several main-sequence O stars in NGC 346 have been obtained. Stellar
parameters and abundance of light elements are derived from the optical
spectrum using NLTE, line-blanketed, photospheric models calculated
with TLUSTY. In addition, the UV spectrum is analyzed with the NLTE,
line-blanketed wind model code, CMFGEN, to derive wind parameters
(mass-loss rate, clumping properties, terminal velocity). Iron
abundances, derived from fitting Fe 4 and Fe 5 lines with the two NLTE
codes, compare favorably. We achieve good matches of the observed
spectra, thus supporting our modeling work of spectra of starburst
galaxies at high redshift. We finally discuss the dependence of the
mass-loss rate in terms of metallicity and luminosity. This work
was supported through a NASA/NRC RA award and STScI grants (GO 7437,
AR 7985).

---------------------------------------------------------
Title: Hubble Space Telescope Imaging Spectrograph Observations of
the Hot White Dwarf in the Close Binary Feige 24
Authors: Vennes, Stéphane; Polomski, Elisha F.; Lanz, Thierry;
Thorstensen, John R.; Chayer, Pierre; Gull, Ted R.
2000ApJ...544..423V Altcode:
We obtained and analyzed two Hubble Space Telescope Imaging Spectrograph
spectra of the white dwarf in the DA plus dMe binary Feige 24. The
spectra, obtained at orbital quadratures, provide new estimates of the
white dwarf motion and gravitational redshift resulting in revised
white dwarf parameters. An analysis of interstellar absorption
lines reveals the presence of two clouds (+3.1 km s-1,
+17.6 km s-1) in the line of sight toward Feige 24;
one of these clouds (+17.6 km s-1) is identified with
the “local cloud.” A study of the Lyα H I and D I interstellar
medium lines shows that the deuterium-to-hydrogen abundance ratio
(D/H=1.3×10-5) is consistent with other measurements
supporting a relative constancy of this ratio throughout the local
interstellar medium. The total hydrogen column density measured with
Lyα (lognH=2.95×1018 cm-2) is
in agreement with EUV Lyman continuum flux measurements. Finally, we
present a complete abundance pattern for the white dwarf, demonstrating
the predominance of iron and nickel over lighter elements. Residual
ionization imbalance in the case of several elements, most notably
in the case of O IV/O V, which cannot be explained by temperature
or surface gravity variations, may indicate the presence of other
atmospheric constituents, inhomogeneous stratification of oxygen in
the photosphere, and/or remaining inaccuracies in the treatment of
model atoms. The abundance patterns in Feige 24 and in the hot DA
white dwarf G191-B2B are remarkably similar, indicating that the same
processes are operating equally in both stars.

---------------------------------------------------------
Title: Space Telescope Imaging Spectrograph Coronagraphic Observations
of β Pictoris
Authors: Heap, Sara R.; Lindler, Don J.; Lanz, Thierry M.; Cornett,
Robert H.; Hubeny, Ivan; Maran, S. P.; Woodgate, Bruce
2000ApJ...539..435H Altcode: 1999astro.ph.11363H
We present new coronagraphic images of β Pictoris obtained with the
Space Telescope Imaging Spectrograph (STIS) in 1997 September. The
high-resolution images (0.1") clearly detect the circumstellar disk as
close to the star as 0.75", corresponding to a projected radius of 15
AU. The images define the warp in the disk with greater precision and
at closer radii to β Pic than do previous observations. They show
that the warp can be modeled by the projection of two components:
the main disk and a fainter component, which is inclined to the main
component by 4°-5° and extends only as far as ~4" from the star. We
interpret the main component as arising primarily in the outer disk
and the tilted component as defining the inner region of the disk. The
observed properties of the warped inner disk are inconsistent with a
driving force from stellar radiation. However, warping induced by the
gravitational potential of one or more planets is consistent with the
data. Using models of planet-warped disks constructed by Larwood &amp;
Papaloizou, we derive possible masses of the perturbing object. Based
on observations with the NASA/ESA Hubble Space Telescope, obtained
at the Space Telescope Science Institute, which is operated by the
Association of Universities for Research in Astronomy, Inc., under
NASA contract NAS5-26555.

---------------------------------------------------------
Title: The physical foundations of stellar magnetic field diagnosis
from polarimetric observations of hydrogen lines
Authors: Mathys, G.; Stehlé, C.; Brillant, S.; Lanz, T.
2000A&A...358.1151M Altcode:
We have completed a new study of the formation of hydrogen lines
in dense magnetized plasmas. We have developed a global formalism
including the effects of the magnetic field and of the motional
electric field. The latter, resulting from the motion of the radiating
hydrogen atom in the magnetic field, modifies the structure of the
matrices appearing in the equation of transfer of the Stokes vector
and introduces a coupling between microscopic (“intrinsic”) and
macroscopic (Doppler) contributions to the line profiles. We have used
this new treatment of line formation to revisit the interpretation
of photopolarimetric (Stokes I and V) observations of Balmer lines in
terms of mean longitudinal magnetic field.

---------------------------------------------------------
Title: The D/H Abundance Ratio in Local Interstellar Gas
Authors: Sahu, M. S.; Landsman, W.; Bruhweiler, F. C.; Linsky, J. L.;
Hubeny, I.; Barstow, M.; Holberg, J.; Gull, T.; Bowers, C.; Lindler,
D.; Lanz, T.; Feggans, K.
2000AAS...196.2620S Altcode: 2000BAAS...32..713S
Does the D/H ratio vary within ~ 100 pc of the Sun? If it does, what
is the range of these variations? Are the D/H variations correlated to
any physical property of the absorbing cloud? To answer these questions
we have begun a project to obtain high-precision D/H abundance ratios
in the Local Interstellar Medium within ~ 100 pc (Sahu et al. 1999,
ApJ 523, L159). For this purpose, we primarily use HST-Space Telescope
Imaging Spectrograph (STIS) data towards nearby hot, white dwarfs
(WDs). The advantages of using STIS for this study are: (1) The high
velocity resolution of STIS in the UV ( ~ 2 km s-1 in the
E140H and E230H modes) allows the velocity structure in the sightline
to be resolved and reliable component-to-component variations can be
studied (2) WDs provide a strong, smooth UV continuum against which
other diagnostic interstellar absorption lines can be measured. The
wider wavelength coverage ( ~ 200 Angstroms per setting) of STIS in the
high-resolution E140H mode allows important interstellar lines such as N
i, Si ii, O i and S ii to be measured in addition to D i and H i, at no
extra cost in HST time (3) The superior echelle scatter and background
corrections possible with the 2-dimensional STIS-MAMA detectors allow
more accurate D/H determinations from WD spectra. In this poster, we
present a status report of our project along with some recent results.

---------------------------------------------------------
Title: Analysis of the Spectrum of the z=2.73 Galaxy, MS1512-cB58:
A Unified Model of the Stellar and Interstellar Contributions
Authors: Hubeny, I.; Heap, S. R.; Lanz, T. M.
2000AAS...196.2914H Altcode: 2000BAAS...32R.718H
We perform a detailed analysis of the restframe far-UV spectrum of
cB58 that was obtained by Pettini et al. (ApJ 528,96, 2000) using
the KECK-I/LRIS. In the previous paper (AAS 195, 9.19), we modeled
the spectrum of the galaxy and found that the observed spectrum is
well described by a starburst model having a SMC-like metallicity
and a constant star formation rate. Here, we extend the analysis to
model the total spectrum that includes stellar as well as interstellar
contributions. The latter is modeled using a newly developed program
CLOUDSPEC that combines the photoionization program CLOUDY (Ferland,
http://www.pa.uky.edu/ gary/cloudy) with our spectrum synthesis program
SYNSPEC (Hubeny &amp; Lanz, http://tlusty.gsfc.nasa.gov). With the
ionization structure of the interstellar medium given by CLOUDY,
CLOUDSPEC solves the radiative transfer along the line-of-sight to
the starburst and yields the profiles of the interstellar absorption
lines. We show that such a “unified” model of the stellar +
interstellar spectrum provides an excellent match to the observed
spectrum of cB58.

---------------------------------------------------------
Title: On the Age Estimation of LBDS 53W091
Authors: Yi, Sukyoung; Brown, Thomas M.; Heap, Sara; Hubeny, Ivan;
Landsman, Wayne; Lanz, Thierry; Sweigart, Allen
2000ApJ...533..670Y Altcode: 1999astro.ph.11067Y
The recent spectral analysis of LBDS 53W091 by Spinrad and his
collaborators has suggested that this red galaxy at z=1.552 is at
least 3.5 Gyr old. This imposes an important constraint on cosmology,
suggesting that this galaxy formed at z&gt;~6.5, assuming recent
estimates of cosmological parameters. While their analysis was heavily
focused on the use of some UV spectral breaks as age indicators,
we have performed χ2 tests to the continuum of this
galaxy using its UV spectrum and photometric data (R, J, H, and K:
2000-9000 Å in rest frame). We have used the updated Yi models
that are based on the Yale tracks. We find it extremely difficult to
reproduce such large age estimates, under the assumption of the most
probable input parameters. Using the same configuration as in Spinrad
et al. (conventional solar abundance models), our analysis suggests
an age of approximately 1.4-1.8 Gyr. We have improved our models over
conventional ones by taking into account convective core overshoot in
the stellar model calculations and realistic metallicity distributions
in the galaxy population synthesis. Overshoot affects the visible
continuum normalized to the UV and raises the photometry-based age
estimates by 25%. The use of metallicity mixtures affects the whole
spectrum and raises all continuum-based age estimates by up to a factor
of 2. If the mean metallicity of the stars in this galaxy is assumed
to be twice solar, the models including these two effects match the UV
spectrum and photometric data of LBDS 53W091 near the age of 1.5-2.0
Gyr. Our results cannot be easily reconciled with that of Spinrad et
al. The discrepancy between Spinrad et al.'s age estimate (based on
the Jimenez models) and ours originates from the large difference in
the model integrated spectrum: the Jimenez models are much bluer than
the Yi models and the Bruzual &amp; Charlot models. We propose to apply
some viable tests to them for verification and search for the origin
of the difference through a more thorough investigation. Considering
the significance of the age estimates of distant galaxies as probes
of cosmology, it would be an urgent task.

---------------------------------------------------------
Title: VizieR Online Data Catalog: Ages of main-sequence stars
(Lachaume+, 1999)
Authors: Lachaume, R.; Dominik, C.; Lanz, T.; Habing, H. J.
2000yCat..33480897L Altcode:
We have determined the age of a sample of nearby main-sequence stars
with spectral types B9-K9. We have derived the stellar ages from five
different age estimators: the location in the HR diagram compared
to theoretical isochrones, the rotational velocity, the strength of
chromospheric calcium emission lines, the stellar metallicity, and their
space velocity. New calibrations consistent with recent theoretical
isochrones are provided for the last four indicators. For hot stars,
isochrones are the best indicator, while stellar rotation is best
for cool stars. However, many stars require in fact a combination of
different methods to properly bracket their actual age. We also discuss
the uncertainties involved, in particular those in using isochrones,
and we find that these uncertainties are often underestimated in the
literature. (6 data files).

---------------------------------------------------------
Title: Model Atmospheres and Radiatively Driven Winds of Metal-Poor
Stars
Authors: Hubeny, Ivan; Lanz, Thierry; Heap, Sally
2000fist.conf..133H Altcode:
We present a grid of non-LTE metal-line-blanketed hydrostatic
model atmospheres for various effective temperatures, surface
gravities (luminosities), and metallicities. The metallicities range
from an essentially metal-free, primordial composition to a solar
composition. For each model, we calculate the exact radiation force and
compare to gravity acceleration. We thus find for each metallicity,
the maximum surface gravity as a function of effective temperature
required for the radiation force to exceed gravity and thus to initiate
a radiatively-driven wind.

---------------------------------------------------------
Title: Properties of the z=2.73 Galaxy, MS1512-cB58
Authors: Heap, S. R.; Bouret, J. -C.; Hubeny, I.; Lanz, T. M.
1999AAS...195.0919H Altcode: 1999BAAS...31.1387H
MS1512-cB58 is an z=2.73 galaxy whose apparent brightness is amplified
by 30X due to gravitationally lensing. The restframe far-UV spectrum of
cB58 that was obtained by Keck-I/LRIS is “one of the best ultraviolet
spectra of a starburst galaxy obtained at any redshift” (Pettini et
al. 1999, astro-ph/9908007). We have therefore analyzed and modelled the
spectrum of this galaxy in order to obtain information about the stellar
content of high-redshift galaxies. We find that our model spectrum is
a near match to the observed spectrum of cB58 if we assume a constant
rate of star formation, a SMC-like metallicity, and a Salpeter IMF
extending up to 100 Msun. In our paper, we will present
the details of the spectral synthesis (based on both observed and
model spectra of OB stars), and we will investigate the reliability
of current evolutionary tracks (based on single, non-rotating stars)
for interpreting the spectra of star-forming galaxies.

---------------------------------------------------------
Title: Age determinations of main-sequence stars: combining different
methods
Authors: Lachaume, R.; Dominik, C.; Lanz, T.; Habing, H. J.
1999A&A...348..897L Altcode:
We have determined the age of a sample of nearby main-sequence stars
with spectral types B9-K9. We have derived the stellar ages from five
different age estimators: the location in the HR diagram compared
to theoretical isochrones, the rotational velocity, the strength of
chromospheric calcium emission lines, the stellar metallicity, and their
space velocity. New calibrations consistent with recent theoretical
isochrones are provided for the last four indicators. For hot stars,
isochrones are the best indicator, while stellar rotation is best
for cool stars. However, many stars require in fact a combination of
different methods to properly bracket their actual age. We also discuss
the uncertainties involved, in particular those in using isochrones,
and we find that these uncertainties are often underestimated in
the literature.

---------------------------------------------------------
Title: UV Spectroscopic Dating of Stars and Galaxies
Authors: Heap, Sara; Brown, Thomas; Lanz, Thierry; Yi, Sukyoung
1999Ap&SS.265..531H Altcode: 1997astro.ph.11074H
An echelle spectrogram (R = 30,000) of the 2300-3100 A region in
the ultraviolet spectrum of the F8V star 9 Comae is presented. The
observation is used to calibrate features in the mid-ultraviolet spectra
of similar stars according to age and metal content. In particular, the
spectral break at 2640 A is interpreted using the spectral synthesis
code SYNSPEC. We use this feature to estimate the time since the
last major star formation episode in the z=1.55 early-type galaxy
LBDS 53W091, whose rest frame mid-ultraviolet spectrum, observed with
the Keck Telescope, is dominated by the flux from similar stars that
are at or near the main-sequence turnoff in that system (Spinrad et
al. 1997). Our result, 1 Gyr if the flux-dominating stellar population
has a metallicity twice solar, or 2 Gyr for a more plausible solar
metallicity, is significantly lower than the previous estimate and
thereby relaxes constraints on cosmological parameters that were
implied by the earlier work.

---------------------------------------------------------
Title: Hot stellar population synthesis from the UV spectrum: the
globular cluster M79 (NGC 1904)
Authors: Vink, Jorick S.; Heap, Sara R.; Sweigart, Allen V.; Lanz,
Thierry; Hubeny, Ivan
1999A&A...345..109V Altcode: 1999astro.ph..3258V
We have analyzed the far UV-spectrum of the globular cluster
M79. We show that the nearly Gaussian mass distribution of zero-age
horizontal-branch stars, as derived by Dixon et al. (1996), is able
to reproduce the far-UV Hopkins Utraviolet Telescope (HUT) spectrum,
if there is a luminous UV-bright star of about T_eff\ = 9,500 K
within the HUT entrance slit, or, more likely, if the horizontal
branch morphology becomes considerably redder in the core of M79, as
observed in some other centrally condensed globular clusters. Agreement
between the synthetic and observed far-UV spectra for M79 would also
be improved if the surface abundances of the heavy elements in the hot
horizontal-branch stars were enhanced by radiative diffusion. Contrary
to Dixon et al. (1996) we do not need extremely low gravities to
reproduce the width of Ly alpha .

---------------------------------------------------------
Title: Detailed non-LTE Analysis of the High-S/N STIS Spectrum of
the Hot Subdwarf, BD+28{(deg}4211)
Authors: Hubeny, I.; Lanz, T.; Haas, S.; Heap, S. R.; Lindler, D. J.;
Kaiser, M. E.
1999AAS...194.6702H Altcode: 1999BAAS...31..930H
During the course of commissioning the Space Telescope Imaging
Spectrograph (STIS), observations were obtained to demonstrate the
feasibility of achieving very high S/N spectra in the ultraviolet
with the MAMA detectors. The hot subdwarf, BD+28{(deg}4211) , was
selected as the test case. The two series of FP-SPLIT observations
(E140M and E230M) have been shown to provide spectra with S/N=370 per
resolution element in both the far-UV and near-UV. The observations
were reduced by an iterative procedure that uses models of all relevant
scatter sources including echelle scatter, MAMA detector halo and OTA
scatter. We have performed a detailed analysis of the spectrum, using
sophisticated non-LTE model atmospheres including several million
metal lines in non-LTE. First, we derive basic parameters of this
star (effective temperature, surface gravity, chemical composition)
and compare to earlier results. We present a detailed comparison of
predicted and observed spectra in order to provide a benchmark for
future detailed spectroscopic studies of hot compact objects. Finally,
we present a preliminary analysis of the effects of elemental abundance
stratification on the selected line profiles with the aim of studying
the detectability of non-homogeneous abundance patterns.

---------------------------------------------------------
Title: UV Spectroscopic Dating of Stars and Galaxies
Authors: Heap, S.; Brown, T.; Lanz, T.; Yi, S.
1999gecd.conf..531H Altcode:
We use STIS high-resolution spectra (R ~30,000) of well-studied F-type
stars to calibrate ultraviolet spectral features in terms of age and
metallicity. We apply our results to Keck spectra of "Extremely Red
Objects" such as the early-type galaxy LBDS 53W091 at a redshift of
z = 1.55.

---------------------------------------------------------
Title: Dating intermediate-age populations with main-sequence A and
F-type stars
Authors: Lanz, T.; Heap, S.; Brown, T. M.; Hubeny, I.; Yi, S.
1999ASPC..192..106L Altcode: 1999sdsg.conf..106L
No abstract at ADS

---------------------------------------------------------
Title: On the Age Estimation of High Redshift Galaxies
Authors: Yi, S.; Brown, T. M.; Heap, S.; Hubeny, I.; Landsman, W.;
Lanz, T.; Sweigart, A.
1999ASPC..192..126Y Altcode: 1999sdsg.conf..126Y
No abstract at ADS

---------------------------------------------------------
Title: On the Age Estimation of LBDS 53W091
Authors: Yi, S.; Brown, T.; Heap, S.; Hubeny, I.; Landsman, W.; Lanz,
T.; Sweigart, A.
1999ASPC..193..185Y Altcode: 1999hrug.conf..185Y
No abstract at ADS

---------------------------------------------------------
Title: Mid-UV Spectroscopic Dating of LBDS 53W091
Authors: Heap, S. R.; Lanz, T. M.; Brown, T.; Hubeny, I.
1999ASPC..193..167H Altcode: 1999hrug.conf..167H
No abstract at ADS

---------------------------------------------------------
Title: Non-LTE model atmospheres of hot DA white dwarfs
Authors: Hubeny, I.; Barstow, M. A.; Lanz, T.; Holberg, J. B.
1999ASPC..169..445H Altcode: 1999ewwd.conf..445H
No abstract at ADS

---------------------------------------------------------
Title: New insight in the diagnosis of stellar magnetic fields by
polarimetry of hydrogen lines
Authors: Brillant, S.; Lanz, T.; Mathys, G.; Stehl, S.
1999AAS...19312402B Altcode: 1999BAAS...31..668B
The study of hydrogen lines in polarized light is a powerful tool for
the detection and the study of stellar magnetic fields. In the case
of fast rotating stars it is even the only method (Landstreet, 1982,
ApJ, 258, 639). However, some discrepancies are observed between the
values of the field deduced from such observations and those obtained
from the observation of the polarization in metal lintes (Mathys,
1989, Fundam. Cosmic. Phys., 13, 143). In order to explain these
discrepancies, we used the formalism for the formation of hydrogen
lines in magnetized plasmas we had developed (Brillant et al., 1998,
A&amp;A, 339, 286), to compute synthetic line profiles in all the four
Stokes parameters for a typical magnetic Ap star model. We present
here the preliminary conclusions of the application of this method
for the diagnosis of stellar magnetic fields.

---------------------------------------------------------
Title: Age and Metallicity of F-type Stars
Authors: Lanz, T.; Heap, S. R.; Brown, T. M.; Hubeny, I.; Yi, S.
1998AAS...193.0302L Altcode: 1998BAAS...30.1246L
The rest-frame ultraviolet spectrum of intermediate-age stellar
populations (1 to few Gyr) is dominated by the UV flux of F-type
stars at the main sequence turn-off. We have obtained high-resolution
spectra of four well-studied F stars with STIS aboard HST to calibrate
ultraviolet spectral features in terms of age and metallicity. The UV
spectrum is mostly degenerate in terms of effective temperature and
metallicity, but we found that two spectral indices, [2850/2770] and
[2310/3040], allow a discrimination between these two parameters. The
first index measures mainly the strength of Mg I 2852, while the second
index is the mid-UV color. We find that the Mg I index is sensitive
to the effective temperature, but insensitive to metallicity; the
UV color is sensitive to both metallicity and T_eff. We discuss the
importance of departures from LTE and chromospheric activity on these
two spectral indices. We apply our results to estimate the time since
the last major star formation episode in the early-type galaxy LBDS
53W091 at redshift z=1.55.

---------------------------------------------------------
Title: Multidimensional, non-LTE Radiation Transport in Astrophysical
Media
Authors: Hubeny, I.; van Noort, M.; Lanz, T.
1998AAS...193.6918H Altcode: 1998BAAS...30.1352H
We present first results of our long-term program whose ultimate goal
is to develop multidimensional, self-consistent, non-LTE radiation
hydrodynamic and magnetohydrodynamic spectroscopic diagnostics of
astronomical objects. As the first step, we have developed an efficient
and robust 2-D and 3-D radiation transfer solver, based on the short
characteristics method. We assume the source function either to be
fully specified (i.e., the so-called formal solution of the transfer
equation), or to be given in terms of the equivalent-two-level
atom formalism. In the latter case, we have adopted the idea of
the Accelerated Lambda Iteration technique to treat iteratively the
non-LTE coupling of radiation and matter. We compare the efficiency and
numerical accuracy of several variants of the short-characteristics
scheme. Finally, we present some illustrative examples of radiative
transfer for 2-D and 3-D media with the specified temperature, density,
and velocity structure.

---------------------------------------------------------
Title: Fe II emission lines in the UV spectrum of Sirius-A and VEGA
Authors: van Noort, M.; Lanz, T.; Lamers, H. J. G. L. M.; Kurucz,
R. L.; Ferlet, R.; Hebrard, G.; Vidal-Madjar, A.
1998A&A...334..633V Altcode: 1998astro.ph..3127V
We present high-quality HST/GHRS spectra in the Hydrogen Lalpha
spectral region of Vega and Sirius-A. Thanks to the signal-to-noise
ratio achieved in these observations and to the similarity of the two
spectra, we found clear evidence of emission features in the low flux
region, lambda lambda 1190-1222 Angstroms. These emission lines can
be attributed unambiguously to Fe Ii and Cr Ii transitions. In this
spectral range, silicon lines are observed in absorption. We built
a series of non-LTE model atmospheres with different, prescribed
temperature stratification in the upper atmosphere and treating Fe
Ii with various degrees of sophistication in non-LTE. Emission lines
are produced by the combined effect of the Schuster mechanism and
radiative interlocking, and can be explained without the presence of
a chromosphere. Silicon absorption lines and the Lalpha profile set
constraints on the presence of a chromosphere, excluding a strong
temperature rise in layers deeper than tau_R ~ 10(-4) . Based on
observations with the NASA/ESA Hubble Space Telescope obtained at the
Space Telescope Science Institute, which is operated by AURA, Inc.,
under NASA contract NAS 5-26555.

---------------------------------------------------------
Title: Non-LTE Line-Blanketed Model Atmospheres of O Stars
Authors: Hubeny, I.; Heap, S. R.; Lanz, T.
1998ASPC..131..108H Altcode: 1998phls.conf..108H
No abstract at ADS

---------------------------------------------------------
Title: Ultraviolet Spectral Dating of Stars and Galaxies
Authors: Heap, S. R.; Brown, T. M.; Hubeny, I.; Landsman, W.; Yi,
S.; Fanelli, M.; Gardner, J. P.; Lanz, T.; Maran, S. P.; Sweigart,
A.; Kaiser, M. E.; Linsky, J.; Timothy, J. G.; Lindler, D.; Beck,
T.; Bohlin, R. C.; Clampin, M.; Grady, J.; Loiacono, J.; Krebs, C.
1998ApJ...492L.131H Altcode:
An echelle spectrogram (R = 30,000) of the 2300-3100 Å region in
the ultraviolet spectrum of the F8 V star 9 Comae is presented. The
observation is used to calibrate features in the mid-ultraviolet spectra
of similar stars according to age and metal content. In particular, the
spectral break at 2640 Å is interpreted using the spectral synthesis
code SYNSPEC. We use this feature to estimate the time since the last
major star formation episode in the early-type galaxy LBDS 53W091 at
redshift z=1.55, whose rest-frame mid-ultraviolet spectrum, observed
with the Keck Telescope, is dominated by the flux from similar stars
that are at or near the main-sequence turnoff in that system (Spinrad
et al.). Our result, 1 Gyr if the flux-dominating stellar population
has a metallicity twice solar, or 2 Gyr for a more plausible solar
metallicity, is significantly lower than the previous estimate and
thereby relaxes constraints on cosmological parameters that were implied
by the earlier work. Based on observations with the NASA/ESA Hubble
Space Telescope, obtained at the Space Telescope Science Institute,
which is operated by the Association of Universities for Research in
Astronomy, Inc., under NASA contract NAS5-2655.

---------------------------------------------------------
Title: Axisymmetric Model Atmospheres of B[e] Stars
Authors: Lanz, T.
1998ASSL..233..229L Altcode: 1998best.work..229L
No abstract at ADS

---------------------------------------------------------
Title: Non-LTE Line-blanketed Model Atmospheres of Hot Stars. III. Hot
Subdwarfs: The sdO Star BD +75°325
Authors: Lanz, Thierry; Hubeny, Ivan; Heap, Sara R.
1997ApJ...485..843L Altcode:
We have made a detailed comparison of results of spectroscopic analysis
using three differents types of model atmospheres: classical non-LTE
H-He models; approximate non-LTE line-blanketed models (with only
a subset of Fe and Ni lines: those originating from transitions
between levels with measured energies); and non-LTE fully blanketed
models. The three models were applied to the sdO star BD +75°325,
adopted as a test case. We demonstrate that the effects of
line blanketing are very important: the best fit of the observed H
and He lines is achieved for Teff = 58,000 K when using
H-He models, Teff = 55,000 K for approximate non-LTE
line-blanketed models, and Teff = 52,000 K for fully
blanketed non-LTE models. Using the high-resolution Goddard
High Resolution Spectrograph spectrum of BD +75°325 and our final
fully blanketed model, we have derived reliable abundances of He, C,
N, O, Si, Fe, and Ni. We find that BD +75°325 is an He-enriched star
(He/H = 1 by number), whose surface exhibits CNO-cycle products, i.e.,
N-rich (AN = 1.5 × 10-3 by number, or 4.2 ×
10-3 by mass fraction), and C and O deficient (by factor of
about 100 with respect to the solar value). We also find a significant
surface depletion of silicon and an enhancement of iron and nickel. We
argue that these anomalous abundances reflect some mixing with processed
material from the core, with subsequent modification at the surface by
diffusion processes. Finally, BD +75°325 possesses a weak wind. Using
a simplified description of the wind, we have derived a preliminary
value of the mass loss rate: Ṁ=1.5×10-11 M⊙
yr-1. Based on observations with the ESA/NASA Hubble
Space Telescope obtained at the Space Telescope Science Institute,
which is operated by AURA, Inc., under NASA contract NAS 5-26555.

---------------------------------------------------------
Title: Hubble Space Telescope Ultraviolet Spectroscopy of Two Hot
White Dwarfs
Authors: Holberg, J. B.; Barstow, M. A.; Lanz, T.; Hubeny, I.
1997ApJ...484..871H Altcode:
We have obtained Hubble Space Telescope high-dispersion spectroscopy of
two hot white dwarfs using the Goddard High-Resolution Spectrograph. The
stars were selected on the basis of EUV flux as examples of H-rich DA
stars containing high and low levels of short-wavelength opacity. The
high-opacity star, REJ 1614-085, possesses strong features due to C
IV, Si IV, and N V. Carbon and silicon in this star are underabundant
with respect to the predictions of radiative levitation; nitrogen,
however, is superabundant. This is in sharp contrast to the apparently
similar DA star GD 394, where silicon is in extreme overabundance. The
low-opacity star PG 1057+719, as expected, shows no clear evidence of
these or any other heavy ions. We find evidence of weak blueshifted
components in the REJ 1614-085 C IV and Si IV lines, which may
indicate the star is experiencing ongoing mass loss. Based
on observations with the NASA/ESA Hubble Space Telescope obtained
at the Space Telescope Science Institute, which is operated by the
Association of Universities for Research in Astronomy, Incorporated
under NASA contract NAS 5-26555. Observations reported here have
been obtained in part with the Multiple Mirror Telescope, a joint
facility of the University of Arizona and Smithsonian Institution.

---------------------------------------------------------
Title: The variations of the BP star HD 137509.
Authors: Mathys, G.; Lanz, T.
1997A&A...323..881M Altcode:
A refined value P=(4.4916+/-0.0002) d is derived for the rotation
period of the Bp star HD 137509, from the simultaneous consideration
of photometric data recorded in the Geneva system and of measurements
of the mean longitudinal magnetic field and of the crossover. The
variations of the magnetic field (one of the most intense fields
known in an Ap or Bp star) show the signature of a strong quadrupolar
component. This remarkable property is reflected in the brightness
and line equivalent width variations.

---------------------------------------------------------
Title: The mean magnetic field modulus of AP stars
Authors: Mathys, G.; Hubrig, S.; Landstreet, J. D.; Lanz, T.;
Manfroid, J.
1997A&AS..123..353M Altcode:
We present new measurements of the mean magnetic field modulus of
a sample of Ap stars with spectral lines resolved into magnetically
split components. We report the discovery of 16 new stars having this
property. This brings the total number of such stars known to 42. We
have performed more than 750 measurements of the mean field modulus
of 40 of these 42 stars, between May 1988 and August 1995. The best
of them have an estimated accuracy of 25 - 30 G. The availability
of such a large number of measurements allows us to discuss for the
first time the distribution of the field modulus intensities. A most
intriguing result is the apparent existence of a sharp cutoff at the
low end of this distribution, since no star with a field modulus
(averaged over the rotation period) smaller than 2.8 kG has been
found in this study. For more than one third of the studied stars,
enough field determinations well distributed throughout the stellar
rotation cycle have been achieved to allow us to characterize at least
to some extent the variations of the field modulus. These variations
are often significantly anharmonic, and it is not unusual for their
extrema not to coincide in phase with the extrema of the longitudinal
field (for the few stars for which enough data exist about the
latter). This, together with considerations on the distribution of
the relative amplitude of variation of the studied stars, supports
the recently emerging evidence for markedly non-dipolar geometry
and fine structure of the magnetic fields of most Ap stars. New or
improved determinations of the rotation periods of 9 Ap stars have
been achieved from the analysis of the variations of their mean
magnetic field modulus. Tentative values of the period have been
derived for 5 additional stars, and lower limits have been established
for 10 stars. The shortest definite rotation period of an Ap star
with magnetically resolved lines is 3.4 deg, while those stars that
rotate slowest appear to have periods in excess of 70 or 75 years. As
a result of this study, the number of known Ap stars with rotation
periods longer than 30 days is almost doubled. We briefly rediscuss
the slow-rotation tail of the period distribution of Ap stars. This
study also yielded the discovery of radial velocity variations in 8
stars. There seems to be a deficiency of binaries with short orbital
periods among Ap stars with magnetically resolved lines. Based on
observations collected at the European Southern Observatory (La Silla,
Chile; ESO programmes Nos. 43.7-004, 44.7-012, 49.7-030, 50.7-067,
51.7-041, 52.7-063, 53.7-028, 54.E-0416, and 55.E-0751), at the
Observatoire de Haute-Provence (Saint-Michel-l'Observatoire, France),
at Kitt Peak National Observatory, and at the Canada-France-Hawaii
Telescope. Tables 2, 3, and 4 are also available in electronic form
at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)
or via http://cdsweb.u-strasbg.fr/Abstract.html.

---------------------------------------------------------
Title: Interstellar and photospheric opacity from EUV spectroscopy
of DA white dwarfs
Authors: Barstow, M. A.; Dobbie, P. D.; Holberg, J. B.; Hubeny, I.;
Lanz, T.
1997MNRAS.286...58B Altcode:
We present a detailed analysis of the extreme-ultraviolet (EUV)
spectra of 13 hydrogen-rich DA white dwarfs, observed by the Extreme
Ultraviolet Explorer (EUVE) satellite, paying attention to the
possible sources of absorbing material along the lines of sight both
in the local interstellar medium (ISM) and in the photospheres of the
stars themselves. The range of interstellar column densities seen are
consistent with our previous understanding of the local distribution of
material. Absorption from interstellar Heii is found in the direction of
five stars, allowing us to measure directly the He ionization fraction
and estimate, indirectly, that of H. The weighted mean ionization
fractions along these lines of sight are 0.27+/-0.04 and 0.35+/-0.1
respectively. Where Heii is directly detected, the observed ionization
fractions are not correlated with direction or with the volume/column
density of material along the line of sight. Furthermore, the limits
on the amount of Heii established in all other directions completely
encompass the range of observed values. Indeed, all the data can be
consistent with more or less constant He and H ionization fractions
throughout the local ISM. It is clear that there is little photospheric
opacity, from either He or heavier elements, in the majority of the
stars we have studied. This poses further difficulties in explaining
the observed division of white dwarfs into H- and He-rich groups,
the temperature gap in the He-rich sequence and the detailed spectral
evolution of the H-rich DA white dwarfs as they cool. A striking
observational result is that our spectroscopic evidence indicates that
radiative levitation effects are only important at temperatures above
50000K, rather than the 40000K suggested by broad-band photometry. There
is clearly an urgent need for further theoretical work on the mechanisms
that determine the photospheric composition of white dwarf stars.

---------------------------------------------------------
Title: NLTE Line Blanketed Model Atmospheres of A-Type Stars
Authors: Hubeny, I.; Lanz, T.
1997IAUJD..16E.102H Altcode:
We present new non-LTE metal line-blanketed model atmospheres
of A-type stars. We show that thanks to the recent progress in
developing fast and robust numerical schemes, such as the hybrid
complete linearization/accelerated lambda iteration method, one may
now construct model atmospheres of unprecedented degree of complexity
and realism. Other basic ingredient of the recent progress are the
new atomic data calculations, like the Opacity and Iron Projects,
which provide a substantial quantity of high-accuracy data. In this
review we discuss the influence of the non-LTE approach, the influence
of metal line blanketing, and the impact of new atomic data on model
atmospheres and predicted spectra of A-type stars.

---------------------------------------------------------
Title: VizieR Online Data Catalog: Variations of HD 137509 (Mathys+
1997)
Authors: Mathys, G.; Lanz, T.
1997yCat..33230881M Altcode:
A refined value P=(4.4916+/-0.0002) d is derived for the rotation
period of the Bp star HD 137509, from the simultaneous consideration
of photometric data recorded in the Geneva system and of measurements
of the mean longitudinal magnetic field and of the crossover. The
photometric data are given in Table 1. For a description of the Geneva
photometric system, see e.g. &lt;GCPD/13&gt; (1 data file).

---------------------------------------------------------
Title: Heavy elements in white dwarf envelopes
Authors: Barstow, M. A.; Holberg, J. B.; Hubeny, I.; Lanz, T.
1997ASSL..214..237B Altcode: 1997whdw.work..237B
No abstract at ADS

---------------------------------------------------------
Title: VizieR Online Data Catalog: Mean magnetic field modulus of
Ap stars (Mathys, 1997)
Authors: Mathys, G.; Hubrig, S.; Landstreet, J. D.; Lanz, T.;
Manfroid, J.
1997yCat..41230353M Altcode:
The paper reports about a systematic study of the mean magnetic field
modulus of Ap stars with magnetically resolved split lines. Tables
2a and 2b present the main properties of all such stars presently
known. The results about their mean magnetic field modulus derived
in this study are summarized in Table 3, while the individual field
measurements are given in Table 4. (4 data files).

---------------------------------------------------------
Title: A Self-consistent Optical, Ultraviolet, and Extreme-Ultraviolet
Model for the Spectrum of the Hot White Dwarf G191-B2B
Authors: Lanz, T.; Barstow, M. A.; Hubeny, I.; Holberg, J. B.
1996ApJ...473.1089L Altcode:
The star G191-B2B is one of a number of hot DA white dwarfs whose
atmospheres have been found to contain significant quantities of
heavy elements, including C, N, 0, Si, Fe, and Ni. Several earlier
studies have measured their abundances using IUE echelle data in
conjunction with synthetic spectra derived from theoretical model
atmosphere calculations of varying degrees of sophistication. However,
predictions of the EUV spectrum based on these observations failed
completely to match either its shape or absolute flux level. We
present here the results of new non-LTE model calculations including
the effects of line blanketing from more than 9 million (mainly Fe and
Ni) transitions. For the first time, we are able to find an effective
temperature and composition that can consistently match the optical, FUV
and EUV data. However, to maintain this agreement below the He II λ228
Lyman limit, it is necessary to incorporate additional He II opacity
in the form of photo spheric, circumstellar or interstellar material.

---------------------------------------------------------
Title: Toward Resolving the “Mass Discrepancy” in O-Type Stars
Authors: Lanz, Thierry; de Koter, Alex; Hubeny, Ivan; Heap, Sara R.
1996ApJ...465..359L Altcode:
We show that metal line blanketing has an important effect on the
atmospheres of hot stars, and we argue that the omission of metal line
blanketing in previous non-LTE model atmospheres of 0-type stars has led
to underestimates of surface gravity and stellar mass. In addition to
wind emission, metal line blanketing contributes to the solution of the
long-standing discrepancy between spectroscopic and evolutionary masses
for 0-type stars. To support our argument, we calculated a series
of non-LTE stellar atmospheres for 0 and Of-type stars. We compared the
predicted profiles of hydrogen and helium lines produced by (1) a static
plane- parallel H-He model, (2) a metal line-blanketed static model,
and (3) an extended expanding model atmosphere. We find that simple
H-He models produce stronger lines than do our metal line-blanketed
models. Consequently, they lead to underestimated gravities. Wind
emission is more effective in filling in the wings of H or He lines in
the case of large mass-loss rates typical of extreme Of stars. These
findings apply equally to young, massive 0-type stars and to highly
evolved stars, i.e., central stars of planetary nebulae. The lower
gravities derived from H-He models yield underestimated masses for
young 0-type stars, and overestimated masses for highly evolved stars.

---------------------------------------------------------
Title: Photoionization resonances of SiII in stellar spectra.
Authors: Lanz, T.; Artru, M. -C.; Le Dourneuf, M.; Hubeny, I.
1996A&A...309..218L Altcode:
We have studied the influence of the Si^+^ photoionization on stellar
atmospheres. Recent calculations using the R-matrix code provide
detailed cross-sections with autoionization resonances for the first
50 energy levels of Si^+^. These photoionization cross-sections
are included in the calculation of the far ultraviolet synthetic
spectrum of A and late B-type stars. The opacity from the six lowest
excited states of Si^+^ provides observable features in the emergent
spectrum of a solar-composition A0 star. The photoionization from
higher states of Si^+^ should be included when modeling hotter or
silicon-rich stars. These new cross-sections increase dramatically
the opacity in the far UV spectrum of Ap Si stars and allow to
reproduce the most characteristic UV features of these stars, as
illustrated by a comparison with IUE data for the hot Ap Si star HD
34452. Broad features in its spectrum are definitely assigned to Si
ii autoionization resonances, and the flux deficiency below 130nm is
well matched with a predicted continuum edge near this wavelength. The
successful stellar spectrum synthesis brings a strong support to
the R-matrix photoionization calculations, which cannot otherwise be
compared to laboratory data.

---------------------------------------------------------
Title: Solving the mystery of the heavy-element opacity in the DA
white dwarf GD 394.
Authors: Barstow, M. A.; Holberg, J. B.; Hubeny, I.; Lanz, T.;
Bruhweiler, F. C.; Tweedy, R. W.
1996MNRAS.279.1120B Altcode:
The white dwarf GD 394 is one of a very small number of bright hot DAs
which have detectable quantities of highly ionized heavy elements in
their spectra. Whether this material resides in the photosphere of GD
394 or is in the form of a circumstellar shell has been the subject of
much debate, with the large discrepancy between the published radial
velocity of the star and the measured velocities of the Si III/IV
lines having apparently settled the argument in favour of the latter
idea. However, GD 394 is also the coolest DA white dwarf in the ROSAT
sky survey showing convincing evidence for the presence of absorbing
material which strongly suppresses the EUV continuum. This result has
reopened the argument about the nature of the silicon lines. From a
combined analysis of the IUE, HST and EUVE spectra, coupled with new
measurements of the photospheric radial velocity from Hα data, the
authors have established that the Si III and Si IV lines are indeed
photospheric. Furthermore, other heavy elements must be present to
account for the EUV opacity, but at levels below the thresholds for
detecting absorption lines in either far-UV or EUV bands.

---------------------------------------------------------
Title: Non-LTE spectroscopic analysis of the wind of the central
star of NGC 6543
Authors: de Koter, A.; Hubeny, I.; Heap, S. R.; Lanz, T.
1996ASPC...96..141D Altcode: 1996hds..conf..141D
No abstract at ADS

---------------------------------------------------------
Title: Spectroscopic Constraints on the Helium Abundance in Globular
Cluster Stars
Authors: Landsman, W. B.; Crotts, A. P. S.; Hubeny, I.; Lanz, T.;
O'Connell, R. W.; Whitney, J.; Stecher, T. P.
1996ASPC...99..199L Altcode: 1996coab.proc..199L
No abstract at ADS

---------------------------------------------------------
Title: EUV Radiation from Hot Star Photoepheres: Theory Versus
Observations
Authors: Hubeny, Ivan; Lanz, Thierry
1996aeu..conf..381H Altcode: 1996IAUCo.152..381H
No abstract at ADS

---------------------------------------------------------
Title: Fully line-blanketed NLTE model atmospheres for hot
hydrogen-deficient stars
Authors: Hubeny, I.; Lanz, T.
1996ASPC...96..249H Altcode: 1996hds..conf..249H
No abstract at ADS

---------------------------------------------------------
Title: The Composition and Structure of White Dwarf Atmospheres
Revealed by Extreme Ultraviolet Spectroscopy
Authors: Barstow, Martin A.; Hubeny, Ivan; Lanz, Thierry; Holberg,
Jay B.; Sion, Edward M.
1996aeu..conf..203B Altcode: 1996IAUCo.152..203B
No abstract at ADS

---------------------------------------------------------
Title: Atomic data for non-LTE model stellar atmospheres
Authors: Lanz, T.; Hubeny, I.; de Koter, A.
1996PhST...65..144L Altcode:
We briefly review the state-of-the-art in non-LTE modeling of hot
star atmospheres. During the last decade, the development of a
new class of radiation transfer methods, based on the Accelerated
Lambda Iteration method, have brought considerable progress in this
field. Severe limitations of the size of model atoms used in non-LTE
model atmospheres have been removed, so that realistic model atoms
can now be considered. In recent years, the problem of non-LTE line
blanketing, which aims to include the influence of thousands to millions
of lines on the atmospheric structure, has been attacked, and first
non-LTE line-blanketed models have been calculated. We illustrate
the importance of non-LTE line blanketing with the example of a hot
subdwarf, BD +75° 325. As a consequence of these developments,
there is a need for extensive radiative and collisional atomic databases
to build non-LTE models. Recent projects, like the Opacity Project,
provide a substantial quantity of the necessary data. We discuss the
current needs of atomic data for the purpose of building reliable
non-LTE line-blanketed model atmospheres. A critical evaluation of the
quality of such extensive sets of atomic data is essential. Finally,
we present a new, IDL-based, graphic tool intended to facilitate the
manipulation of large amounts of data needed to build the realistic
model atoms that we use.

---------------------------------------------------------
Title: Support for the Primordial Helium Abundance Derived from
Observation of Globular Cluster Stars
Authors: Stecher, T. P.; Landsman, W.; Crotts, A. P. S.; Whitney,
J.; O'Connell, R. W.; Lanz, T.; Hubeny, I.; Sweigart, A.
1995AAS...187.8606S Altcode: 1995BAAS...27.1414S
The UIT on Astro-1 found nearly 2000 hot stars in the globular cluster
Omega Centauri (Whitney et al. 1994, AJ, 108, 1350), of which 28 are at
least 1 mag brighter than the horizontal branch (HB). We have obtained
CTIO 4m and/or IUE low-dispersion spectra of eleven of the brightest
of these stars. All seven stars observed at CTIO are radial velocity
members of the clusters. Three of these stars have log Teff &gt; 60,000
K and show only Balmer lines and He II lines. These stars, which appear
to be in the post-asymptotic giant branch phase, are the hottest stars
ever found in a globular cluster. The other four stars have log Teff ~
20,000 K and show a B-type spectrum with numerous He I lines but none
of He II. We have estimated the helium abundance by comparison with
hydrogen-helium NLTE model stellar atmospheres. The He II lines in the
hottest three stars in our sample appear to be best fit by models with
a He/H ratio of about 0.1. This helium abundance should reflect the
primordial helium abundance of the gas from which Omega Cen formed
nearly 13 x 10(9) yr ago, except for the modest helium enrichment
caused by the first dredge-up during the red giant branch phase. By
analyzing these stars we have been able to derive a helium abundance
for a globular cluster which is independent of the well-known R-method
and which should provide an estimate of the helium abundance produced by
the Big Bang. The four cooler stars in our sample show a quite different
pattern of helium abundances. The three faintest of these stars have a
substantially lower helium abundance which we attribute to the effects
of diffusion during the preceding HB phase. These stars appear to
have recently evolved off of the blue end of the HB. In contrast,
the most luminous of these stars is very helium rich (He/H = 0.55),
indicating that some UV-bright stars undergo interior mixing during
their preceding AGB evolution.

---------------------------------------------------------
Title: VizieR Online Data Catalog: Photoelectric Photometry in
Johnson UBVRI System (Lanz 1986)
Authors: Lanz, T.
1995yCat.2116....0L Altcode:
The catalog is a compilation of photoelectric photometry in the Johnson
UBVRI system published prior to mid-1984. The catalog contains a total
of 6849 stars in the original Johnson system (Morel and Magnenat 1978),
467 stars in the system of Kunkel and Rydgren (1979), 1150 stars in
the system of Moffett and Barnes III (1979), and 252 stars in the
system of Neckel and Chini (1980). The catalog is in 14 files: The
first 12 files list observational data, mean values, and references
for each of the four standard systems. An additional two files contain
the numbering scheme of Mermilliod (1978) used to identify objects,
and introductory information. (9 data files).

---------------------------------------------------------
Title: HST/GHRS Observations of the beta Pictoris System: Basic
Parameters of the Age of the System
Authors: Lanz, Thierry; Heap, Sara R.; Hubeny, Ivan
1995ApJ...447L..41L Altcode:
We have reevaluated the basic parameters and age of the A5 IV--V star,
beta Pictoris, making use of new observations obtained by the Goddard
High-Resolution Spectrograph aboard the Hubble Space Telescope and
using modern methods of atmospheric modeling and spectral synthesis. We
derive stellar parameters appropriate to its spectral type (Teff =
8200 +/- 150 K, log g = 4.25 +/- 0.1) including a normal (solar)
metallicity. When dated by comparison with current evolutionary tracks,
we find that (1) the star is apparently "underluminous"---a situation we
attribute to (gray) extinction by the disk---and (2) the star is either
a pre--main-sequence (PMS) star nearing the zero-age main sequence
(ZAMS), or it is a main-sequence star older than 0.3 Gyr. Given the
apparent youth of the disk as inferred from its high density, we judge
the PMS interpretation as most likely.

---------------------------------------------------------
Title: The UV-Bright Stars of Omega Centauri
Authors: Landsman, W. B.; Crotts, A.; O'Connell, R. W.; Whitney,
J. H.; Lanz, T.; Stecher, T. P.
1995AAS...186.1203L Altcode: 1995BAAS...27..828L
Images of the globular cluster Omega Centauri obtained with the
Ultraviolet Imaging Telescope (UIT) in 1990 revealed numerous hot stars
more luminous than zero-age horizontal branch (Whitney et al. 1994, AJ,
108, 1350). We have obtained CTIO 4m and IUE low-dispersion spectra of
seven of the brightest stars in the Whitney et al. catalog. The target
stars include UIT-1 and UIT-2 in the core of Omega Cen (Landsman et
al. 1992 ApJL, 395, L21), as well as ROA 5342, Dk 3873, and Dk 3089 from
the catalog of Dickens (1988). All of the target stars are found to be
radial velocity members of the cluster. Three of the stars (ROA 5342,
UIT-151, Dk 3873) show strong He II lines in their spectra and are
probably very hot (&gt; 50,000 K) post-AGB stars. The remaining four
stars show strong He I lines, and UIT-1 also shows numerous nitrogen
lines. We present results of an atmospheric analysis to constrain the
reddening, effective temperatures, and helium abundances.

---------------------------------------------------------
Title: Eclipse observations of an accretion disc wind
Authors: Mason, K. O.; Drew, J. E.; Cordova, F. A.; Horne, Keith;
Hilditch, R.; Knigge, C.; Lanz, T.; Meylan, T.
1995MNRAS.274..271M Altcode:
The Goddard High Resolution Spectrometer on the Hubble Space Telescope
has been used to monitor, through eclipse, the C iv 1550-A and He ii
1640-A lines in the spectrum of the nova-like cataclysmic variable
UX UMa. Previous work has suggested that the Civ line arises in an
accretion disc wind. The HST data confirm this, in that the line shows
a broad, asymmetric emission profile. However, superimposed on the
emission are previously unsuspected narrow absorption components at
the rest wavelengths of the doublet. The absorption disappears during
the continuum eclipse, causing the net continuum-subtracted flux near
line centre to increase by about 20 per cent during this time. The
mid-eclipse line profile shows the classic asymmetric shape expected
for a bipolar wind viewed edge-on. The various wavelength segments
within the C iv line exhibit different eclipse light curves. The flux
at line centre declines gradually, beginning at least 0.05 in orbital
phase before the continuum eclipse, then rising sharply coincident with
continuum ingress due to loss of the absorption components. Flux is also
lost from the red wing of the C iv line beginning just before (Deltaphi
~ 0.02) the onset of continuum eclipse. The red wing flux declines more
slowly than the continuum itself, reaching a minimum after the time of
continuum mid-eclipse. This suggests that the emitting gas may have
a significant rotational component about the white dwarf. However,
the blue wing shows no comparable signature of rotation. These data
show that the shallowness of the line flux eclipse is primarily due to
occultation of regions of net absorption, with the implication that
the line-forming region can be smaller than the companion star. To
explain the appearance and eclipse evolution of the C iv line profile
solely in terms of outflow will require a highly collimated disc
wind. Alternatively the narrow, superposed absorption components may
originate elsewhere in the system. These data also provide the first
clear demonstration that He ii 1640 A is mainly a wind-formed feature.

---------------------------------------------------------
Title: Non-LTE Line-blanketed Model Atmospheres of Hot Stars. II. Hot,
Metal-rich White Dwarfs
Authors: Lanz, T.; Hubeny, I.
1995ApJ...439..905L Altcode:
We present several model atmospheres for a typical hot metal-rich DA
white dwarf, Teff = 60,000 K, log g = 7.5. We consider
pure hydrogen models, as well as models with various abundances
of two typical 'trace' elements-carbon and iron. We calculte a
number of Local Thermodynamic Equilibrium (LTE) and non-LTE models,
taking into account the effect of numerous lines of these elements
on the atmospheric structure. We demostrate that while the non-LTE
effects are not very significant for pure hydrogen models, except for
describing correctly the central emission in H-alpha they are essential
for predicting correctly the ionization balance of metals, such as
carbon and iron. Previously reported discrepancies in LTE abundances
determinations using C III and C IV lines are easily explained by
non-LTE effects. We show that if the iron abundance is larger than
10-5, the iron line opacity has to be considered not only for
the spectrum synthesis, but also in the model construction itself. For
such metal abundances, non-LTE metal line-blanketed models are needed
for detailed abundance studies of hot, metal-rich white dwarfs. We also
discuss the predicted Extreme Ultraviolet (EUV) spectrum and show that
it is very sensitive to metal abundances, as well as to non-LTE effects.

---------------------------------------------------------
Title: Non-LTE Line-blanketed Model Atmospheres of Hot
Stars. I. Hybrid Complete Linearization/Accelerated Lambda Iteration
Method
Authors: Hubeny, I.; Lanz, T.
1995ApJ...439..875H Altcode:
A new munerical method for computing non-Local Thermodynamic Equilibrium
(non-LTE) model stellar atmospheres is presented. The method, called the
hybird complete linearization/accelerated lambda iretation (CL/ALI)
method, combines advantages of both its constituents. Its rate of
convergence is virtually as high as for the standard CL method,
while the computer time per iteration is almost as low as for the
standard ALI method. The method is formulated as the standard complete
lineariation, the only difference being that the radiation intensity
at selected frequency points is not explicity linearized; instead,
it is treated by means of the ALI approach. The scheme offers a wide
spectrum of options, ranging from the full CL to the full ALI method. We
demonstrate that the method works optimally if the majority of frequency
points are treated in the ALI mode, while the radiation intensity at a
few (typically two to 30) frequency points is explicity linearized. We
show how this method can be applied to calculate metal line-blanketed
non-LTE model atmospheres, by using the idea of 'superlevels' and
'superlines' introduced originally by Anderson (1989). We calculate
several illustrative models taking into accont several tens of thosands
of lines of Fe III to Fe IV and show that the hybrid CL/ALI method
provides a robust method for calculating non-LTE line-blanketed model
atmospheres for a wide range of stellar parameters. The results for
individual stellar types will be presented in subsequent papers in
this series.

---------------------------------------------------------
Title: Non-LTE Line Blanketed Model Atmospheres of Hot, Metal-Rich
White Dwarfs
Authors: Hubeny, I.; Lanz, T.
1995LNP...443...98H Altcode: 1995whdw.conf...98H
No abstract at ADS

---------------------------------------------------------
Title: Interpreting the spectra of chemically peculiar stars
Authors: Lanz, T.
1995HiA....10..582L Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Atmospheres of Chemically Peculiar Stars: A Laboratory
for Atomic Physics
Authors: Lanz, T.
1995ASPC...78..423L Altcode: 1995aapn.conf..423L
No abstract at ADS

---------------------------------------------------------
Title: Magnetic Splitting and Identification of Spectral Lines in
AP Stars
Authors: Mathys, G.; Lanz, T.
1995ASPC...81..531M Altcode: 1995lahr.conf..531M
No abstract at ADS

---------------------------------------------------------
Title: The UV-bright stars of Omega Centauri.
Authors: Landsman, W. B.; Crotts, A.; O'Connell, R. W.; Whitney,
J. H.; Lanz, T.; Stecher, T. P.
1995BAAS...27S.828L Altcode:
No abstract at ADS

---------------------------------------------------------
Title: EUVE Spectroscopy of epsilon Canis Majoris (B2 II) from 70
to 730 Angstrom
Authors: Cassinelli, J. P.; Cohen, D. H.; Macfarlane, J. J.; Drew,
J. E.; Lynas-Gray, A. E.; Hoare, M. G.; Vallerga, J. V.; Welsh, B. Y.;
Vedder, P. W.; Hubeny, I.; Lanz, T.
1995ApJ...438..932C Altcode:
We present spectra of the brightest stellar source of extreme
ultraviolet (EUV) radiation longward of 400 A, the B2 II star, epsilon
CMa. These data were taken with the three spectrometers aboard the
NASA Extreme Ultraviolet Explorer satellite (EUVE) during the first
cycle of pointed observations. We report on our initial studies of
the continuum and line spectrum of the stellar photosphere in the
320 to 730 A region, and on the wind emission lines observed in the
170-375 A region. This is the first EUV spectrum of an early-type
star, and thus makes epsilon CMa the most comprehensively observed B
star from the X-ray to infrared regimes. The radiation in both the H
Lyman continuum and He I continuum (shortward of 504 A) are found to
be significantly greater than predicted by both Local Thermodynamic
Equilibrium (LTE) and non-LTE model atmospheres. Since epsilon CMa
also exhibits a mid-infrared excess, this points to the outer layers
being warmer than the models indicate. The anomalously large Lyman
continuum flux, combined with the very low column density measured in
the direction toward this star implies that it is the dominant source
of hydrogen ionization of the local interstellar medium in the immediate
vicinity of the sun. All of the lines predicted to be strong from model
atmospheres are present and several wind absorption features are also
identified. We have detected emission lines from highly ionized iron
that are consistent with the ROSAT Position Sensitive Proportional
Counter (PSPC) observations if a multi-temperature emission model is
used, and the assumption is made that there is significant absorption
beyond that of the neutral phase of the ISM. The spectrum shows strong
O III 374 A line emission produced by the Bowen flourescence mechanism,
which has not previously been observed in the spectra of hot stars.

---------------------------------------------------------
Title: HST/GHRS Observations of beta Pictoris
Authors: Heap, S. R.; Lanz, T.; Hubeny, I.; Lindler, D.
1994AAS...185.4812H Altcode: 1994BAAS...26.1389H
In September 1994, we obtained HST/GHRS spectra and maps of beta
Pic (HD 39060) and its surrounding disk. In this paper, we report
our preliminary findings concerning the star and circumstellar gas
disk. Both Stromgren photometry and Geneva photometry (as listed
by SIMBAD) yield the following parameters for the star: T_eff=8200
+/- 50 K log g = 4.25 +/- 0.05 [Fe/H] = -0.1 +/- 0.2 To refine the
estimated metallicity, we have compared the GHRS ultraviolet spectra
with model spectra. Our model of beta Pic incorporates both a LTE
stellar atmosphere plus a circumstellar gas disk, treated in a first
approximation as a homogeneous, absorbing slab at T=5,000 K with
a column density, N_H=2x10(19) cm(-2) . We confirm that the star
has a near-solar metallicity. We have compared the derived stellar
parameters with evolutionary models for Z=0.017. For reference, a ZAMS
star having a T_eff=8,200 K has a mass, M ~1.72 M_sun, and a gravity,
log g= 4.37. The somewhat low surface gravity of beta Pic suggests that
the star has evolved slightly off the ZAMS or is still in the pre-main
sequence phase. In either case, it should be somewhat more luminous
than a 1.72M_sun ZAMS star. However, its absolute magnitude, M_V= 2.70
(for a distance of 17 pc) is 0.21 mag fainter than a M=1.72 M_sun star
on the ZAMS. We therefore conclude that (1) the dust disk must diminish
light from the star, and (2) it is entirely possible that beta Pic is
a very young star. Both conclusions raise problems for interpreting
observations of the dust disk (c.f. Paresce, 1991, A&amp;A, 247, L25).

---------------------------------------------------------
Title: HST/FOS Spectroscopy of the Central Star of NGC 7027
Authors: Hubeny, I.; Heap, S. R.; Lanz, T.
1994AAS...185.4710H Altcode: 1994BAAS...26.1385H
With its rich UV-optical-IR spectrum, NGC 7027 has been the target
of choice for study of important physical processes including
charge-exchange, dust-quenching of nebular lines, PDR's, extended-red
emission, etc. Surprisingly little, though, is known about the central
star, because the star is embedded in such a bright nebula. With
the success of the HST servicing mission, it is now possible to
carry out direct studies of the central star. Here, we report on
HST/FOS spectroscopy of the central star. In August 1994, we obtained
FOS/G400H, G570H spectra of NGC 7027 with the central star centered
in the 1farcs 0 aperture. The spectra cover the wavelength range,
3250-6750 Angstroms. The spectra are of excellent quality. For example,
at lambda 4800 Angstroms, the S/N=76 per resolution element, and the
star contributes 94% of the observed continuum flux. Other very hot
stars, i.e. PG 1159 stars, are known to show high-ionization lines,
such as O VI lambda lambda 3811,3834, C IV lambda 4441, or C V lambda
4945, etc. To our surprise, these lines are absent in the spectrum
of the central star. In fact, the stellar spectrum is apparently
featureless. We have therefore used the absence of spectral features
to put limits on effective temperature and chemical composition of the
central star. We are constructing a small grid of line-blanketed NLTE
model atmospheres for the effective temperatures range 100,000 K &lt;
T_eff &lt; 250,000 K, 5.8 &lt; log g &lt; 7.5, and for two compositions:
(i) a H-rich (solar) composition, and (ii) a C-O enriched atmosphere,
typical of PG 1159 stars. We will report on the results of this
analysis.

---------------------------------------------------------
Title: NLTE Line-blanketed Model Atmosphere Analysis of the hot
Subdwarf BD +75° 325
Authors: Lanz, T.; Hubeny, I.; Heap, S. R.
1994AAS...185.8005L Altcode: 1994BAAS...26.1447L
With the Goddard High Resolution Spectrograph aboard HST, we have
recorded a high-resolution, high signal-to-noise ratio, far ultraviolet
spectra of the hot subdwarf, BD +75° 325. Because of the low apparent
rotational velocity (V sin i &lt; 20 km/s) of the star, the high
quality of the spectra and our newly developed NLTE line-blanketed
model atmospheres, we have an unprecedented opportunity to derive
accurate stellar parameters. With our hybrid CL/ALI method, we have
constructed a small grid of NLTE line-blanketed model atmospheres for
hot subdwarfs. These model atmospheres include consistently about
200 NLTE levels (or superlevels) of H, He, C, N, O, Si, Fe and Ni,
and all transitions between them. Individual atomic levels of iron and
nickel are merged into a relatively small number of superlevels. The
cross-sections of the transitions between superlevels are represented by
NLTE Opacity Distribution Functions. This grid of NLTE model atmospheres
is used for a thorough analysis of BD +75° 325, which then provides
in addition a “real” check of our methodology. Our results show that
BD +75° 325 is a He enriched star, whose surface exhibit CNO-cycle
products (N-rich and C-deficient). The very rich Fe v and Fe vi line
spectra can only be reproduced by a significant overabundance of iron
(about 5 to 10 times solar). With our metal line blanketed NLTE model
atmospheres, we are now able to reproduce the deep line cores in the
ultraviolet spectrum assuming a microturbulence of 5-10 km/s. Moreover,
the line blanketed models are necessary to match the H i/He ii line
cores in the visual spectrum (models including only H and He produce
too shallow lines). Finally, we have found a weak asymmetry of the
N v resonance line profiles, which can be attributed to a weak wind
with M = 9+/- 2times 10(-11) M_sun /yr.

---------------------------------------------------------
Title: (Erratum) The nature of the F STR λ4077
stars. III. Spectroscopy of the barium dwarfs and other CP stars.
Authors: North, P.; Berthet, S.; Lanz, T.
1994A&A...292..350N Altcode:
Erratum to Astron. Astrophys. 281, 775-796 (1994)

---------------------------------------------------------
Title: Theoretical Modeling of GHRS Observations of the Of/WN-Type
Star R136a5
Authors: de Koter, Alex; Hubeny, Ivan; Heap, Sara R.; Lanz, Thierry
1994ApJ...435L..71D Altcode:
We present a theoretical analysis of the Hubble Space Telescope (HST)
Goddard High Resolution Spectrograph (GHRS) spectrum of R136a5,
an O3fWN star in the R136a cluster in 30 Doradus. Using non-LTE
extended and expanding model atmospheres, we find a surprisingly high
mass-loss rate dot-M = 1.8 +/- 0.5 x 10-5 solar mass/yr and
an effective temperature Teff = 42.5 +/- 2.5 kK. With the
observed visual magnitude, this implies a radius R* = 17 +/-
2 solar radius and a solar luminosity L = 8.5 +/- 1 x 105
solar luminosity. We discuss possible sources of the uncertainties
in the derived stellar parameters and conclude that the effective
temperature may be underestimated; however, if so, the value of the
mass-loss rate would not be affected.

---------------------------------------------------------
Title: Dynamic Processes in Be Star Atmospheres. II. He i 2P--nD
Line Formation in lambda Eridani (Outburst)
Authors: Smith, Myron A.; Hubeny, Ivan; Lanz, Thierry; Meylan, Thomas
1994ApJ...432..392S Altcode:
The He I lambda 6678 line of early Be stars generally shows violet (V)
and red (R) emission whenever hydrogen alpha emission is present, but
its use as a diagnostic has been handicapped by a poor understanding of
the processes that drive it into emission. In an attempt to address this
problem we obtained three series of eschelle spectra of the first two
members of the singlet and triplet 2P-nD series of lambda Eri (B2e)
during 1992 November 3-5 at Kitt Peak. During these observations
lambda 6678 showed substantial emission variability in both the
wings and central profile, providing an opportunity to compare its
behavior with that of the lambda 4922, lambda 5876, and lambda 4471
lines. We found that the responses of the lines were different in
several respects. Whereas the emissions in the V wings of all four
lines scaled together, the R wing of the lambda 4922 line invariably
responded with increased absorption whenever the R wing of lambda 6678
line showed increased emission. These same trends occurred within
the central photospheric profiles. The R-wing behavior shows that
much, but not all of the emission in lambda 6678 is caused by matter
projected against the stellar disk. The excitation temperatures of
the neighboring 21 P transitions, lambda 6678 and lambda
4922 must be greater than and less than the photospheric continuum
temperature, respectively. We have investigated departures from local
thermodynamic equilibrium (LTE) for the He I spectrum in a variety of ad
hoc, perturbed model atmospheres. We have found only one way to cause
the source function of lambda 6678 to increase so strongly, namely,
by increasing the atmospheric temperature in the line formation region
to 30,000 - 40,000 K. This effect was discovered by Auer and Mihalas
for O3-O4 atmospheric models, but it has not been applied to active
B stars. Our models suggest that lambda 6678 emission in Be stars can
be used as a sensitive monitor of localized hot spots on these stars'
surfaces. The energies involved in heating the active portions of the
atmosphere are too high to be produced by gravitational infall. This
leaves magnetically induced flares among the few known processes on
the surfaces of stars capable of sustaining this energy level.

---------------------------------------------------------
Title: NLTE Model Stellar Atmospheres: The Hybrid CL/ALI Method
Authors: Lanz, T.; Hubeny, I.
1994AAS...184.4011L Altcode: 1994BAAS...26..924L
A new numerical method for computing sophisticated NLTE model
stellar atmospheres is presented. The method, called the hybrid
Complete Linearization/Accelerated Lambda Iteration (CL/ALI) method,
combines advantages of both its constituents. Its rate of convergence
is virtually as high as for the standard CL method, while the computer
time per iteration is almost as low as for the standard ALI method. The
method is formulated as the standard complete linearization; the only
difference being that the radiation intensity at selected frequency
points is not explicitly linearized; instead, it is treated by means
of the ALI approach. The scheme offers a wide spectrum of options,
ranging from the full CL to the full ALI method. We demonstrate
that the method works optimally if the majority of frequency points
are treated in the ALI mode, while the radiation intensity at a few
(typically 2 - 30) frequency points is explicitly linearized. We show
how this method can be applied to calculate metal line blanketed NLTE
model atmospheres of hot stars. We use the idea of “superlevels” and
“superlines” introduced originally by Anderson (1989). We calculate
several illustrative models taking into account several tens of thousand
of lines of Fe iii to Fe vi, and show that the hybrid CL/ALI method
provides a robust method for calculating NLTE line blanketed model
atmospheres for a wide range of stellar parameters. This work was
supported in part by NASA grants NAGW-3025 and NAGW-3834.

---------------------------------------------------------
Title: GHRS observations and theoretical modeling of early type
stars in R136a
Authors: de Koter, A.; Heap, S.; Hubeny, I.; Lanz, T.; Hutchings,
J.; Lamers, H. J. G. L. M.; Maran, S.; Schmutz, W.
1994AAS...184.3106D Altcode: 1994BAAS...26..909D
We present the first spectroscopic observations of individual stars in
R136a, the most dense part of the starburst cluster 30 Doradus in the
LMC. Spectra of two stars are scheduled to be obtained with the GHRS
on board the HST: R136a5, the brightest of the complex and R136a2,
a Wolf-Rayet star of type WN. The 30 Doradus cluster is the only
starburst region in which individual stars can be studied. Therefore,
quantitative knowledge of the basic stellar parameters will yield
valuable insight into the formation of massive stars in starbursts and
into their subsequent evolution. Detailed modeling of the structure
of the atmosphere and wind of these stars will also lead to a better
understanding of the mechanism(s) that govern their dynamics. We
present the first results of our detailed quantitative spectral
analysis using state-of-the-art non-LTE model atmospheres for stars
with extended and expanding atmospheres. The models are computed using
the Improved-Sobolev Approximation wind code (ISA-WIND) of de Koter,
Schmutz &amp; Lamers (1993, A&amp;A 277, 561), which has been extended
to include C, N and Si. Our model computations are not based on the
core-halo approximation, but use a unified treatment of the photosphere
and wind. This approach is essential for Wolf-Rayet stars. Our synthetic
spectra, dominated by the P Cygni profiles of the UV resonance lines,
also account for the numerous weak metal lines of photospheric origin.

---------------------------------------------------------
Title: The Discovery of NI V in the Photospheres of the Hot DA White
Dwarfs RE 2214-492 and G191-B2B
Authors: Holberg, J. B.; Hubeny, I.; Barstow, M. A.; Lanz, T.; Sion,
E. M.; Tweedy, R. W.
1994ApJ...425L.105H Altcode:
We have co-added six recently obtained International Ultraviolet
Explorer (IUE) echelle spectra of the hot DA white dwarf RE 2214-492
and 10 existing archive spectra of the well-known hot DA, G191-B2B. We
find that both stars contain numerous weak features due to Ni V. Nickel
is thus the second iron-group element to be found in the spectra of the
very hottest DA white dwarfs. In addition to Ni V, we also observe Al
III in both stars and present evidence for the possible presence of
Ni IV and Fe IV in RE 2214-492. The presence of Ni and Al, together
with previously reported elements, will contribute significantly to
both the EUV opacity and to the apparent complexity of the UV spectra
of these stars. Using Non-Local Thermodynamic Equilibrium (NLTE) model
atmospheres we estimate the Ni abundances in RE 2214-492 the G191-B2B
to be log(Ni/H) = -5.5 +/- 0.3 and -6.0 +/- 0.3, respectively.

---------------------------------------------------------
Title: The nature of the F STR λ4077 stars. V. Spectroscopic data
Authors: North, P.; Berthet, S.; Lanz, T.
1994A&AS..103..321N Altcode:
We report first high-resolution spectroscopic observations of 17 stars
classified F str λ4077 by Bidelman. They are compared to different
types of stars, which have been observed at the same time, including 3
CH subgiants, 3 barium stars, 5 λ Bootis stars, Ap/Am stars, and normal
main-sequence stars. The detailed equivalent widths and abundances are
given for all measured spectral lines, as well as the mean abundance per
species for each object. These data and their significance are described
in detail and discussed in an article published in the Main Journal.

---------------------------------------------------------
Title: NLTE model stellar atmospheres with line blanketing near the
series limits.
Authors: Hubeny, I.; Hummer, D. G.; Lanz, T.
1994A&A...282..151H Altcode:
In this paper we study the influence of line-merging regions at the
intermediate long-wavelength side of a continuum threshold on the
computed model atmosphere structure and predicted spectrum. In order
to model these regions sufficiently accurately, we have developed two
concepts. First, we have extended the occupation probability formalism
of Hummer and Mihalas to non-local thermodynamic equilibrium (non-LTE)
plasmas. Second, in order to treat the very complicated opacity in
the line merging region, we have generalized the concept of opacity
distribution functions to treat non-LTE situations. All Rydberg states
are consistently included within this framework, so that no arbitrary
cutoff of high (LTE) levels is made. We have calculated several
pure hydrogen models atmospheres for two effective temperatures,
Teff = 20000 and 35000 K, and discussed the differences
between models calculated with various treatments of the line
merging. In particular, we have shown that the error in the predicted
profiles of Balmer lines resulting from the neglect of line merging
is typically of the order of 3-4%, while the errors in the far-UV
portion of the Balmer continuum reaches 15-35%. The errors generally
decrease with increasing effective temperature. At the same time,
the internal accuracy of the models is shown to be about or below 0.5%
for all predicted spectral features. We conclude that for interpreting
current high-accuracy spectrophotometric observations models including
the line merging are necessary, and that the formalism developed in
this paper is capable of providing a sufficiently accurate and robust
modeling technique.

---------------------------------------------------------
Title: Puzzling problems of He I line formation in early B stars
Authors: Smith, M. A.; Hubeny, I.; Lanz, T.
1994IAUS..162..273S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: HST Eclipse Mapping of the Dwarf Nova OY Carinae in
Quiescence: an "Fe II Curtain" with Mach approximately equals 6
Velocity Dispersion Veils the White Dwarf
Authors: Horne, K.; Cheng, F. H.; Marsh, T.; Hubeny, I.; Lanz, T.
1994ASPC...56..283H Altcode: 1994ibs..conf..283H
No abstract at ADS

---------------------------------------------------------
Title: Iron-Peak Elements in Hot Subdwarfs: CYCLE4 Medium
Authors: Lanz, Thierry
1994hst..prop.5305L Altcode: 1994hst..prop.1753L
Hot evolved subdwarfs are the immediate progenitors of white dwarfs,
and are therefore of great importance for our understanding of the late
stages of stellar evolution. Recently, significant abundances of heavy
metals have been found in some hot DA white dwarfs, formely thought
of having hydrogen-pure atmospheres. Iron and nickel now appear to be
the major opacity sources in the FUV and EUV spectrum of these white
dwarfs as well as in the hot evolved sdO subdwarfs. We will observe one
sdO stars to redetermine most accurately its stellar parameters. We
will analyze the spectrum using our new most sophisticated NLTE
line-blanketed model atmospheres, including the blanketing of tens of
thousands of Fe IV, Fe V, Fe VI, Ni IV and Ni V lines. We will derive
consistently the abundances of the iron-peak elements. Contrary to
the abundances of light elements which are primarily signatures of
nucleosynthetic processes, the iron-peak elements may shed light
on the relative importance of the individual mixing processes in
the photospheres of sdO stars, and give us a better insight on the
connection between these subdwarfs and the white dwarfs.

---------------------------------------------------------
Title: HST/GHRS Observations of β Pictoris
Authors: Heap, S.; Lanz, T.; Hubeny, Y.
1994cddp.conf..367H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The nature of the F STR lambda 4077 stars. III. Spectroscopy
of the barium dwarfs and other CP stars.
Authors: North, P.; Berthet, S.; Lanz, T.
1994A&A...281..775N Altcode:
The abundances of C, O, Al, Ca, iron-peak and s-process elements
have been derived from high-resolution spectra for a sample of
stars classified as F str lambda 4077 by Bidelman. Among the 20
stars mentioned by Bidelman, we have discovered 8 barium dwarfs
(or CH subgiants, according to Bond's terminology), while a 9th
star, HD 182274, was already known as a CH subgiant. In addition,
we have analyzed three barium stars taken from the list of Lu et
al. (1983) which are probably dwarfs rather than giants, and three CH
subgiants. The other 11 F str lambda 4077 stars resemble either the
delta Delphini stars, since their iron abundance is enhanced while Ca is
normal, or are probably spectrum composites. A few Am, Ap, lambda Bootis
and normal stars have been analyzed for comparison. In particular,
we have included three lambda Boo candidates, selected from their
photometric properties, and their iron deficiency is confirmed. The
spectroscopic, photometric and statistical evidences concerning the
Ba dwarfs, support the idea that these stars may be the main sequence
counterparts, and possibly the progenitors of the Ba giants. The C/O
ratio varies in these stars from normal values to a maximum of 1.5,
but mostly within 0.6 and 1.2. Some of these objects may therefore
be considered, in this sense, as carbon stars. On the other hand,
the abundances of carbon and s-process elements relative to iron are
inversely correlated with metallicity, and may even exceed significantly
those of typical, solar-metallicity carbon stars. Metal-deficient C
stars must therefore have (C/Fe) greater than or approximately equal
to 1 and (s/Fe) greater than or approximately equal to 1.5 as soon as
(Fe/H) less than or approximately equal to -1. The neutron exposure is
shown to increase when the metallicity decreases, which is compatible
with the C-13 (alpha, n) O-16 neutron source, but not with the Ne-22
(alpha, n) Mg-25 one. The evolutionary state (within the main sequence)
of the Ba dwarfs, is rediscussed in relation with their photometric
and spectroscopic surface gravity, but it remains unclear.

---------------------------------------------------------
Title: A search for magnetic fields in AM stars.
Authors: Lanz, T.; Mathys, G.
1993A&A...280..486L Altcode:
Although Am stars have been assumed to be nonmagnetic stars, we
found recently that the hot Am stars omicron Peg has a magnetic
field of the order of 2 kG, with a complex structure. Therefore,
we reconsidered the question of magnetism among upper main sequence
chemically peculiar stars. We report here new observations of a pair
of magnetically sensitive Fe II lines in the red spectrum of Am stars
and of HgMn stars. Two other hot Am stars, HD 29173 and HD 195479A, are
likely to possess also a magnetic field of about the same strength. It
is striking that 3 out of 4 hot Am stars observed up to now could be
magnetic. On the other hand, this fast method is limited by blends and
cannot provide a satisfactory diagnosis for classical Am stars and
for HgMn stars. A multi-line approach must be the next step towards
our understanding of magnetism among these chemically peculiar stars.

---------------------------------------------------------
Title: Erratum: The nature of the F STR A4077 stars. IV. Search for
white dwarfs around barium dwarfs
Authors: North, P.; Lanz, T.
1993A&A...273..720N Altcode:
No abstract at ADS

---------------------------------------------------------
Title: NLTE Line Blanketed Model Atmospheres for Hot, Metal-rich
White Dwarfs
Authors: Hubeny, I.; Lanz, T.
1993AAS...182.4701H Altcode: 1993BAAS...25..878H
Recent observations of some hot DA white dwarfs (Feige 24, G191 B2B -
Sion et al. 1992, Ap.J. 391, L29; Vennes et al. 1992, Ap.J. 392, L27)
and subsequent analyses have demonstrated that their atmospheres are
contaminated with heavy metal species with appreciable abundances. So
far, modeling was mostly limited to calculating synthetic spectra
(usually in LTE), based on previously calculated model atmospheres that
were constructed assuming a simplified chemical composition. However,
in reality the metal lines may also significantly influence the
temperature structure, and consequently the ionization balance of some
important species. Since the effective temperature is rather high
(55000 to 60000 K), the NLTE effects may be important despite the
high gravity. Therefore, in order to settle the question on metal
abundances of hot DA white dwarfs, we need to calculate NLTE fully
line blanketed model atmospheres. Using our previously developed
hybrid complete linearization/accelerated lambda iteration method,
we have calculated a set of NLTE models including H, He, C, N, O, and
Fe, with some 13000 lines of Fe IV, Fe V and Fe VI taken into account
explicitly in model construction. The NLTE departure coefficients
for all levels of Fe IV, Fe V, and Fe VI are then employed in the
spectrum synthesis program SYNSPEC. In the first part of this study,
we present a theoretical comparison between models computed with adding
more and more opacity sources, and show how the temperature structure
and synthetic spectra develop. We also discuss possible errors arising
from using an inconsistent spectrum synthesis (the metals considered
only in the spectrum synthesis, not in the model construction). In the
second part, we compare the computed UV spectra with with available
observations of Feige 24 and G191 B2B, and deduce limits for the iron
abundance of these stars.

---------------------------------------------------------
Title: The GA II lines in the red spectrum of AP stars.
Authors: Lanz, T.; Artru, M. -C.; Didelon, P.; Mathys, G.
1993A&A...272..465L Altcode:
We report first observations of the Ga II λ633.4 line in the
red spectrum of Ap stars. This line is present in the spectrum of
16 out of 45 Ap stars of our sample, translating to large gallium
overabundances. While most HgMn stars are gallium-rich, we have observed
strong gallium lines in several magnetic stars. We have derived LTE
abundances from this line after reviewing all needed atomic data and
stellar parameters. In particular, a new calibration of the Geneva
photometry for the effective temperature of the magnetic Ap stars is
presented. LTE abundances arc compared to previous analyses of the
ultraviolet resonance lines, and they arc about 1 dex larger. This
large discrepancy might possibly be explained by departures from LTE or
by the stratification of gallium predicted by the radiative diffusion
theory. We present the variations of the Ga II line strength during
the rotation period of three magnetic Ap stars; from the variations
of silicon and of the longitudinal component of the magnetic field
it appears that gallium accumulates around magnetic poles. Moreover,
we suggest that secular changes in the gallium surface abundance on
time-scales of about 1-2 108 yr might explain why some Ap
HgMn stars arc not gallium-rich.

---------------------------------------------------------
Title: Observations of Magnetically Split Lines in Ap-Stars
Authors: Mathys, G.; Landstreet, J. D.; Lanz, T.
1993ASPC...44..300M Altcode: 1993IAUCo.138..300M; 1993pvnp.conf..300M
No abstract at ADS

---------------------------------------------------------
Title: Similarities and Differences Between Magnetic and Non-Magnetic
Cp-Stars
Authors: Lanz, T.
1993ASPC...44...60L Altcode: 1993IAUCo.138...60L; 1993pvnp.conf...60L
No abstract at ADS

---------------------------------------------------------
Title: Modelling A-Type Atmospheres - NLTE Models
Authors: Hubeny, I.; Lanz, T.
1993ASPC...44...98H Altcode: 1993IAUCo.138...98H; 1993pvnp.conf...98H
No abstract at ADS

---------------------------------------------------------
Title: Spectral Diagnoses of Chromospheres and Winds in A-Type Stars
Authors: Lanz, T.; Hubeny, I.
1993ASPC...44..517L Altcode: 1993IAUCo.138..517L; 1993pvnp.conf..517L
No abstract at ADS

---------------------------------------------------------
Title: Line Blanketed Model Atmospheres for Metal-rich White Dwarfs
Authors: Lanz, T.; Hubeny, I.; Cheng, F. -H.; Horne, K.
1992AAS...181.5004L Altcode: 1992BAAS...24.1203L
In many cases of a close binary system composed of a white dwarf, a
companion main-sequence or giant star, and an accretion disk around the
white dwarf, the UV spectrum of the system contains contributions from
both the white dwarf and the disk. In order to derive information about
the system from its UV spectrum, the white dwarf atmosphere has to be
accurately modeled. The chemical composition of the white dwarf is very
likely to be quite unusual, however, because it may have nearly solar
composition due to accretion. Due to its high surface gravity, the white
dwarf's lines are generally very broad, and thus metal line blanketing
may play a very important role. We have therefore begun a project of
calculating a grid of non-LTE, fully line-blanketed model atmospheres
for white dwarfs, for a large range of metal abundances, from zero (pure
DA) to solar composition. The method is described in a companion paper
(Hubeny and Lanz) at this meeting. We consider a representative model
atmosphere with T_eff=15000 K and log g = 8. In this preliminary study,
only hydrogen and iron (Fe II and Fe III) opacity was considered. The
models take into account 45815 Fe II and 23059 Fe III lines (all lines
between the levels with measured energies). As expected, the NLTE
effects are rather small for this temperature, but the effect of metal
line blanketing is very important. In particular, we demonstrate how
the metal line blanketing influences the UV spectrum and the hydrogen
Lyman and Balmer lines profiles. The work on the project was supported
in part by the NASA grant No.65 under program NRA 91-OSSA-12.

---------------------------------------------------------
Title: Non-LTE Line Blanketed Model Atmospheres: Application to
A-stars
Authors: Hubeny, I.; Lanz, T.
1992AAS...181.1911H Altcode: 1992BAAS...24.1152H
We present a new method for calculating realistic line blanketed model
atmospheres without the assumption of local thermodynamic equilibrium
(LTE). The method is based on complete linearization, with the
following two important modifications with respect to the standard
variant. i) Instead of dealing separately with individual energy
levels of a complicated metal species (e.g. Fe II), several levels
with the same parity and close enough energy are grouped together to
form a “superlevel". Transitions between superlevels, the so-called
“superlines", are treated by resampling the detailed absorption
cross-section and forming a partial “NLTE opacity distribution
function", which is representable by a relatively small number of
frequency points (typically 15 - 30 points per superline). ii) The
radiative rates (together with the heating/cooling rates, and the
contributions to the rates in other important transitions - e.g. the
hydrogen and carbon continua) are not fully linearized. However,
in contrast to the earlier approaches, they are not held fixed. The
only fixed quantity here is the approximate lambda operator. The new
method is therefore a hybrid combining the complete linearization
and the class of modern methods called accelerated lambda iteration
(ALI). We have contructed several non-LTE model atmospheres for T_eff
= 10000 K, log g = 4, with H, C I, C II, Mg II, and Fe II treated in
NLTE. A variable number of Fe II lines, up to 45815 (i.e. all lines
originating between the levels with measured energies), have been
included, to form 221 superlines. The most interesting preliminary
result is that the first 8 Fe II superlevels (about 13000 lines)
produce most of the total blanketing effect. The work on the project
was supported by the NASA grant No.65 under program NRA 91-OSSA-12.

---------------------------------------------------------
Title: Non-LTE Analysis of the Hot Subdwarf: BD 75 325
Authors: Heap, S. R.; Hubeny, I.; Lanz, T.; Altner, B.
1992AAS...181.5003H Altcode: 1992BAAS...24.1203H
No abstract at ADS

---------------------------------------------------------
Title: Stratification of chemical elements in the atmosphere of the
AP star 53 Camelopardalis.
Authors: Babel, J.; Lanz, T.
1992A&A...263..232B Altcode:
A detailed abundance analysis of Cr, Fe, Mn, Ca, and Ti in the Ap star
53 Cam from IUE high-resolution spectra has been performed. It is found
that UV lines give significantly lower abundance than visible lines
for Cr and Fe. Theoretical spectra of Cr II line calculated with the
stratified chromium abundance distributions predicted a priori by the
diffusion-mass loss model match well both visible and UV lines. The
spectral shape of the Ca II K line and its variation are well accounted
for by the diffusion model with mass loss. A spectrum synthesis with a
stratified abundance well reproduces the observations. These findings
provide convincing evidence for stratification in the photosphere of
53 Cam.

---------------------------------------------------------
Title: Accelerated complete-linearization method for calculating
NLTE model stellar atmospheres
Authors: Hubeny, I.; Lanz, T.
1992A&A...262..501H Altcode:
Two approaches to accelerating the method of complete linearization
for calculating NLTE model stellar atmospheres are suggested. The
first one, the so-called Kantorovich variant of the Newton-Raphson
method, consists of keeping the Jacobi matrix of the system fixed,
which allows us to calculate the costly matrix inversions only a few
times and then keep them fixed during the subsequent computations. The
second method is an application of the Ng acceleration. Both methods
are extremely easy to implement with any model atmosphere code based
on complete linearization. It is demonstrated that both methods, and
especially their combination, yield a rapidly and globally convergent
algorithm, which takes 2 to 5 times less computer time, depending
on the model at hand and the required accuracy, than the ordinary
complete linearization. Generally, the time gain is more significant
for more complicated models. The methods were tested for a broad range
of atmospheric parameters, and in all cases they exhibited similar
behavior. Ng acceleration applied on the Kantorovich variant thus offers
a significant improvement of the standard complete-linearization method,
and may now be used for calculating relatively involved NLTE model
stellar atmospheres.

---------------------------------------------------------
Title: Mass loss in main-sequence A-type stars?
Authors: Lanz, T.; Catala, C.
1992A&A...257..663L Altcode:
We present new observations of the H-alpha line profile for five
main-sequence A-type stars, where very high SNRs were achieved. A
search for weak asymmetries was carried out to detect a stellar wind,
but the many telluric absorption lines in this range prevent us from
taking the full benefit of the high SNRs. This situation was improved
by modeling the telluric absorptions to remove them from the observed
spectra, but the H-alpha profiles were nevertheless found to be quite
symmetric. We calculated the profile of the H-alpha line for a grid of
model atmospheres of an A-type star including a weak wind, in order to
assess an upper limit on the mass loss rate. The asymmetry is found
to be sensitive to the velocity law and to the turbulent velocity of
the wind, but its first moment depends much less on them. The upper
limits deduced on the mass loss rate are between 1 and 2 x 10 exp -10
solar mass/yr. An improvement up to a factor 10 could be anticipated
if new observations could be secured from a much drier location,
achieving similar SNRs.

---------------------------------------------------------
Title: Ap stars with resolved magnetically split lines.
Authors: Mathys, G.; Lanz, T.
1992A&A...256..169M Altcode:
New high-resolution spectroscopic observations of Ap stars with resolved
magnetically split spectral lines are presented. Six new such stars
have been discovered: HD 2453, HD 9996, HD 18078, HD 50169, HD 137949,
and HD 192678. This raises the total number of known Ap stars with
resolved magnetically split lines to 21. The spectral line profiles
observed in two more stars, HD 111133 and HD 147010, are strongly
affected by the magnetic field, but they are not fully resolved, due
to non-negligible rotational Doppler effect. Observations of fairly
sharp-lined Ap stars whose lines are unresolved are also presented. The
relative difference between the equivalent widths of the Fe II lines
λ6147.7 and λ6149.2 is shown to increase with the mean magnetic field
modulus. The existence of a simple, unambiguous correlation between
the mean magnetic field modulus and the index Z of Geneva photometry,
which had previously been advocated, is not confirmed. At most, stars
with large negative values of Z may be more prone to have strong
magnetic fields, but the evidence for this is not statistically strong.

---------------------------------------------------------
Title: Compilation of atomic oscillator strengths for carbon, nitrogen
and oxygen ions. II - Addendum for the beryllium isoelectronic
sequence
Authors: Allard, N.; Le Dourneuf, M.; Artru, M. -C.; Lanz, T.
1991A&AS...91..399A Altcode:
This addendum complements the recent compilation of oscillator
strengths for C III, N IV and O V by Allard et al. (1990). It concerns
high-excitation multiplets of C III and N W Adopted g f-values are
proposed, following the same scheme of averaging. Additional references
are given to recent theoretical results. In particular we present a
comparative analysis of the data of Nussbaumer &amp; Storey (1984)
which were not previously considered.

---------------------------------------------------------
Title: The Rotation Period of the A0p Star HD 133652
Authors: Lanz, T.; Bohlender, D. A.; Landstreet, J. D.
1991IBVS.3678....1L Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The nature of the F STR lambda 4077 stars. IV. Search for
white dwarfs around barium dwarfs.
Authors: North, P.; Lanz, T.
1991A&A...251..489N Altcode:
The energy distribution of three stars classified as F str
4077-A by Bidelman has been observed in the ultraviolet with the
IUE satellite. Although the stars examined have the same kind
of abundance anomalies as the Ba giants and Bond's CH subgiants
(i.e. an overabundance of s-process elements), no UV excess related
with the presence of the expected white dwarf companion was found. The
implications for the mass-transfer scenario are discussed.

---------------------------------------------------------
Title: Reduced upper limits on the equivalent width of interstellar
Li I 670.8 towards SN 1987A.
Authors: Baade, D.; Cristiani, S.; Lanz, T.; Malaney, R. A.; Sahu,
K. S.; Vladilo, G.
1991A&A...251..253B Altcode:
A new search for interstellar Li I 670.8 along the line of sight to SN
1987A has been carried out using all 34 known (and previously published)
high-resolution Reticon spectra obtained at the European Southern
Observatory. In spite of the large number and high quality of the
observations, the line remains undetected even in the carefully weighted
and filtered mean spectrum. The most probable 3-σ detection limit in
equivalent width is 7.3 10^-6^ nm. For the cloud at radial velocity
+286 km s^-1^, standard relations then imply a maximum abundance,
Li/H, of 1.7 10^-11^ in the gaseous interstellar medium of the LMC. If
depletion to dust grains is the same for both lithium and potassium,
this figure has to be increased by 1.1 dex. Accordingly, conventional
present-day reasoning suggests that the elemental abundance of lithium
does not exceed 2.7 10^-10^. However, the cumulative uncertainties are
such that a primordial abundance which is an order of magnitude higher
is not firmly ruled out. Constraints on primordial nucleosynthesis
models are, therefore, only weak.

---------------------------------------------------------
Title: Photometric variations of some magnetic AP stars.
Authors: Lanz, T.; Mathys, G.
1991A&AS...90..365L Altcode:
New photometric observations of five magnetic Ap stars in the Geneva
system are reported. New or improved parameters of the light curves
are presented. Magnetic data confirm the adopted rotation periods and
make it possible in some cases to remove ambiguities.

---------------------------------------------------------
Title: A New Look at the Standard Teff = 10000K, log g=4, Non-LTE
Model Atmosphere
Authors: Lanz, T.; Hubeny, I.; Altner, B.
1991BAAS...23R1381L Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Photometric Variability of the B9p Star HD 137509
Authors: Lanz, T.; Mathys, G.
1991IBVS.3655....1L Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Compilation of atomic oscillator strengths for carbon,
nitrogen and oxygen ions - The beryllium isoelectronic sequence
(C III, N IV, and O V)
Authors: Allard, N.; Artru, M. -C.; Lanz, T.; Le Dourneuf, M.
1990A&AS...84..563A Altcode:
Oscillator strengths for the observed lines of C III, N IV, and O V are
compiled. All the data published between 1966 and 1989 are reported
and critically analyzed. Recent exhaustive results of the OPACITY
project are included. The level lifetimes measured for these ions
are also compiled and compared to the theoretical lifetimes. Adopted
gf-values are proposed from a weighted mean of the best theoretical
results. These data concern 204, 190, and 140 multiplets of C III,
N IV, and O V respectively, with an accuracy in the 10 percent range
in most cases.

---------------------------------------------------------
Title: Nitrogen abundances in chemically peculiar stars
Authors: Lanz, T.; Artru, M. -C.
1990ESASP.310..275L Altcode: 1990eaia.conf..275L
No abstract at ADS

---------------------------------------------------------
Title: The magnetic field of the AM star omicron Pegasi.
Authors: Mathys, G.; Lanz, T.
1990A&A...230L..21M Altcode:
Evidence is presented that the hot Am star Omicron Peg (= HD 214994)
has a magnetic field of the order of 2 kG, with a complex structure. Two
independent approaches support this conclusion. The first one is an
analysis by the Stenflo-Lindegren (1977) technique of the widths of
a statistical sample of lines. The other relies on a line-intensity
anomaly that is found in magnetic stars. The implications of this
probable detection of a field in a member of a class of stars that
were not previously known as magnetic is discussed.

---------------------------------------------------------
Title: A detailed study of a 6° diameter field in Scutum
Authors: Reichen, M.; Lanz, T.; Golay, M.; Huguenin, D.
1990Ap&SS.163..275R Altcode:
A 6-deg diameter field centered on the star ɛ Scutum has been selected
from the SCAP-2000 balloon-borne UV survey for its high apparent stellar
density contrast and relatively good transparency on a small projected
portion of the galactic plane. In view of a detailed study of the
absorption and interstellar matter distribution in this field, a large
collection of data has been made and is presented here. Ultraviolet
magnitudes at 2000 Å have been obtained from the reduction of three
SCAP frames. In addition 322 stars among the 487 stars detected on
the UV frames have been mesured in the Geneva photometric system. For
a small number of photometrically distant objects, medium resolution
spectra have been obtained in order to determine spectral types and
confirm the distances determination. An overall description of the
distribution of interstellar material in the ultraviolet is made using
star counting methods, and is confronted to similar studies in the
visible range. A discussion of the variations of the extinction law in
different parts of the field is made. A new distance determination of
the Sct OB2 asociation is proposed, as well as a list of additional
probable members of the association. A certain number of distant hot
stars seem to indicate the presence of an unknown OB association at the
edge of the Scutum spiral, arm at a distance of over 3 kpc from the Sun.

---------------------------------------------------------
Title: The IUE spectral atlas of two normal B stars : pi Ceti and
nu Capricorni (125-198 nm).
Authors: Artu, M. -C.; Borsenberger, J.; Lanz, T.
1989A&AS...80...17A Altcode:
The ultraviolet spectra of two main-sequence B stars, Pi Cet (HD 17081)
and Nu Capricorni (HD 193432) recorded by the IUE satellite have been
used to elaborate a reference atlas, with line identification, in the
low wavelength range 125-198 nm. To obtain realistic identifications
and to select the dominant contributors in blends, a synthetic
calculation of most of the detected lines has been performed in the LTE
approximation. About 80 percent of the 1460 measured stellar lines have
a definite identification, 57 percent of them corresponding to a single
assignment. The presence of 20 different elements (33 atoms or ions)
is firmly established and the elimination of spurious identifications
is justified. The observed spectra are displayed between 125 and 198
nm, with comparison to the calculated ones.

---------------------------------------------------------
Title: Modelling of Magnetic Field Geometries and Chemical Abundance
Distributions of Ap Si Stars
Authors: Landstreet, J. D.; Lanz, T.
1989BAAS...21.1199L Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The chemically peculiar magnetic stars.
Authors: Mégessier, C.; Lanz, T.; Artru, M. C.
1989musi.work...61M Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Les étoiles magnetiques chimiquement particulières.
Authors: Megessier, C.; Artru, M. -C.; Lanz, T.
1989JAF....34R...7M Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The IUE spectral atlas of two normal B stars: πCet and νCap
Authors: Artru, M. -C.; Borsenberger, J.; Lanz, T.
1988ESASP.281a.389A Altcode: 1988IUE88...1..389A; 1988uvai....1..389A
An atlas for B stars pi Cet (HD17081, B7V) and nu Cap (HD192432,
B9.5V) was prepared in the range 125 to 200 nm. By means of an
improved software to process the IUE images and by the coaddition of
10 high-resolution images, the best possible resolution and signal to
noise ratio are obtained. The identification of the lines is based on
updated laboratory lists with a selection of the dominant contributors
for each absorption line. About 80 percent of the measured lines have
a definite identification.

---------------------------------------------------------
Title: Stark broadening of visible SI II lines in stellar atmospheres
Authors: Lanz, T.; Dimitrijevic, M. S.; Artru, M. -C.
1988A&A...192..249L Altcode:
The Stark broadening of visible Si II lines is studied and its
consequence on the intensity of the stellar absorption is analyzed. A
complete set of atomic data concerning the Stark widths, is elaborated
for 19 multiplets of Si II of astrophysical importance. Previous
available determinations are reviewed and new Stark widths are
calculated by means of the semi-classical impact theory. The accuracy
of different theoretical and empirical methods are discussed. The new
adopted Stark widths are applied, in several examples, to the analysis
of stellar absorption lines. These widths are generally smaller than
the previous theoretical estimates for the low-excitation transitions
of Si II.

---------------------------------------------------------
Title: The Period of Two Bp Si Mg Stars: HD 60431 and CoD -51 3378
Authors: North, P.; Babel, J.; Lanz, T.
1988IBVS.3155....1N Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Magnetic Field and Silicon Diffusion in Bp-Si Stars
Authors: Megessier, C.; Lanz, T.; Landstreet, J. D.
1988IAUS..132..329M Altcode:
Si II lines of magnetic Bp-Si stars in open clusters have been observed
with the CAT (ESO) in order to get a mapping of the silicon abundance
distribution over the stellar surface, in the frame of the oblique
rotator model. The authors point out the influence of the Zeeman
splitting and of the abundance inhomogeneities on the line profiles.

---------------------------------------------------------
Title: Critical Overview of Atomic Data for Stellar Abundance Analyses
Authors: Lanz, T.; Artru, M. -C.
1988eaa..conf..156L Altcode:
The atomic data which enter into LTE abundance analyses are
reviewed. Improvements permitted by the current progress in
physics are examined, with emphasis on oscillator strengths and
line widths. Different compilation efforts are reviewed. The set of
available oscillator strengths is shown to be satisfactory for light
elements, but some examples of the many problems which remain for the
iron sequence ions are detailed. The need for theoretical or empirical
evaluations of many Stark widths is pointed out.

---------------------------------------------------------
Title: Radiation in moving gaseous media
Authors: Chmielewski, Y.; Lanz, T.
1988rmgm.conf.....C Altcode: 1988SAAS...18.....C
No abstract at ADS

---------------------------------------------------------
Title: Abundance Analysis of Phi Herculis
Authors: Lanz, T.
1988eaa..conf...82L Altcode:
Contents: 1. Computer programs. 2. Measurements of
spectrograms. 3. Line identifications. 4. Atomic data. 5. Photospheric
parameters. 6. Microturbulent velocity. 7. Metal abundances.

---------------------------------------------------------
Title: The Measurement of Equivalent Widths: A Comparison for 100
Selected Lines
Authors: Lanz, T.
1988eaa..conf..120L Altcode:
Contents: 1. Introduction. 2. The selected lines. 3. The methods. 4. The
comparison. 5. Conclusion.

---------------------------------------------------------
Title: Elemental abundance analyses
Authors: Adelman, Saul J.; Lanz, T.
1988eaa..conf.....A Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Silicon absorption in UV spectra of AP SI stars.
Authors: Artru, M. -C.; Lanz, T.
1987A&A...182..273A Altcode:
The authors investigate the role of silicon UV absorption occurring
in the atmosphere of early-type stars. A detailed analysis of the
λλ1250 - 1850 Å range is presented. High-resolution spectra are
obtained from the IUE database for typical silicon stars and for a few
normal main-sequence stars. The strongest absorption in these spectra
are the resonance lines of Si II, and three large features (λ1400,
λ1560 and λ1770 Å) which are strongly enhanced in the spectra of
ApSi stars. For the hottest silicon stars, a discontinuity at 1310
Å is observed. The atomic data are carefully reviewed for Si II to
perform spectrum synthesis calculations showing out more precisely
the silicon absorption.

---------------------------------------------------------
Title: The interstellar spectrum toward SN 1987A.
Authors: Vidal-Madjar, A.; Andreani, P.; Cristiani, S.; Ferlet, R.;
Lanz, T.; Vladilo, G.
1987A&A...177L..17V Altcode:
The supernova 1987A offers a unique opportunity to investigate the
visible interstellar absorption spectrum toward the Large Magellanic
Cloud at high resolution and high signal to noise ratio. The authors
report in this letter the first results of these observations. A Ca II
"forest" is detected through all the velocity range between the Galaxy
and the LMC and, in particular, a component is seen at a velocity of
≡215 km s-1 which may correspond to cooler gas in the
halo. According to the Na I/Ca II ratio, the galactic (and LMC) gas is
clearly different from the "halo" one at intermediate velocities. For
the first time, neutral lithium, calcium and potassium are detected in
an extragalactic interstellar medium. Six diffuse interstellar bands
are also seen in both the Galaxy and the LMC.