356 research outputs found
Testing common classical LTE and NLTE model atmosphere and line-formation codes for quantitative spectroscopy of early-type stars
It is generally accepted that the atmospheres of cool/lukewarm stars of
spectral types A and later are described well by LTE model atmospheres, while
the O-type stars require a detailed treatment of NLTE effects. Here model
atmosphere structures, spectral energy distributions and synthetic spectra
computed with ATLAS9/SYNTHE and TLUSTY/SYNSPEC, and results from a hybrid
method combining LTE atmospheres and NLTE line-formation with DETAIL/SURFACE
are compared. Their ability to reproduce observations for effective
temperatures between 15000 and 35000 K are verified. Strengths and weaknesses
of the different approaches are identified. Recommendations are made as to how
to improve the models in order to derive unbiased stellar parameters and
chemical abundances in future applications, with special emphasis on Gaia
science.Comment: 12 pages, 8 figures; accepted for publication in Journal of Physics:
Conference Series, GREAT-ESF Workshop: Stellar Atmospheres in the Gaia Er
HVS7: a chemically peculiar hyper-velocity star
Context: Hyper-velocity stars are suggested to originate from the dynamical
interaction of binary stars with the supermassive black hole in the Galactic
centre (GC), which accelerates one component of the binary to beyond the
Galactic escape velocity. Aims: The evolutionary status and GC origin of the
HVS SDSS J113312.12+010824.9 (HVS7) is constrained from a detailed study of its
stellar parameters and chemical composition. Methods: High-resolution spectra
of HVS7 obtained with UVES on the ESO VLT were analysed using state-of-the-art
NLTE/LTE modelling techniques that can account for a chemically-peculiar
composition via opacity sampling. Results: Instead of the expected slight
enrichments of alpha-elements and near-solar Fe, huge chemical peculiarities of
all elements are apparent. The He abundance is very low (<1/100 solar), C, N
and O are below the detection limit, i.e they are underabundant (<1/100, <1/3
and <1/10 solar). Heavier elements, however, are overabundant: the iron group
by a factor of ~10, P, Co and Cl by factors ~40, 80 and 440 and rare-earth
elements and Hg even by ~10000. An additional finding, relevant also for other
chemically peculiar stars are the large NLTE effects on abundances of TiII and
FeII (~0.6-0.7dex). The derived abundance pattern of HVS7 is characteristic for
the class of chemical peculiar magnetic B stars on the main sequence. The
chemical composition and high vsini=55+-2km/s render a low mass nature of HVS7
as a blue horizontal branch star unlikely. Conclusions: Such a surface
abundance pattern is caused by atomic diffusion in a possibly magnetically
stabilised, non-convective atmosphere. Hence all chemical information on the
star's place of birth and its evolution has been washed out. High precision
astrometry is the only means to validate a GC origin for HVS7.Comment: 9 pages, 3 figure
Quantitative spectroscopy of Deneb
Quantitative spectroscopy of luminous BA-type supergiants offers a high
potential for modern astrophysics. The degree to which we can rely on
quantitative studies of this class of stars as a whole depends on the quality
of the analyses for benchmark objects. We constrain the basic atmospheric
parameters and fundamental stellar parameters as well as chemical abundances of
the prototype A-type supergiant Deneb to unprecedented accuracy (Teff = 8525
+/- 75 K, log(g) = 1.10 +/- 0.05 dex, M_spec = 19 +/- 3 M_sun, L = 1.96 +/-
0.32 *10^5 L_sun, R = 203 +/- 17 R_sun, enrichment with CN-processed matter) by
applying a sophisticated hybrid NLTE spectrum synthesis technique which has
recently been developed and tested. The study is based on a high-resolution and
high-S/N spectrum obtained with the Echelle spectrograph FOCES on the Calar
Alto 2.2m telescope. Practically all inconsistencies reported in earlier
studies are resolved. Multiple metal ionization equilibria and numerous
hydrogen lines from the Balmer, Paschen, Brackett and Pfund series are brought
into match simultaneously for the stellar parameter determination. Stellar wind
properties are derived from H_alpha line-profile fitting using line-blanketed
hydrodynamic non-LTE models. A self-consistent view of Deneb is thus obtained,
allowing us to discuss its evolutionary state in detail by comparison with the
most recent generation of evolution models for massive stars. (abridged)Comment: 17 pages, 12 figures. Accepted for publication in A&
The chemical composition of the Orion star forming region: stars, gas and dust
We present a summary of main results from the studies performed in the series
of papers "The chemical composition of the Orion star forming region". We
reinvestigate the chemical composition of B-type stars in the Orion OB1
association by means of state-of-the-art stellar atmosphere codes, atomic
models and techniques, and compare the resulting abundances with those obtained
from the emission line spectra of the Orion nebula (M42), and recent
determinations of the Solar chemical composition.Comment: 5 pages, 4 figures, 2 tables. Poster contribution to the proceedings
of the LIAC2010 conference "The multi-wavelength view of hot, massive stars
Hydrogen and helium line formation in OB dwarfs and giants. A hybrid non-LTE approach
Aims: Hydrogen and helium line spectra are crucial diagnostic features for
the quantitative analysis of OB stars. We compute synthetic spectra based on a
hybrid non-LTE approach in order to test the ability of these models to
reproduce high-resolution and high-S/N spectra of dwarf and giant stars and
also to compare them with published grids of non-LTE (OSTAR2002) and LTE
(Padova) models. Methods: Our approach solves the restricted non-LTE problem
based on classical line-blanketed LTE model atmospheres. State-of-the-art model
atoms and line-broadening theories are employed to model the H and He I/II
spectra over the entire optical range and in the near-IR. Results: The
synthetic spectra match almost all measurable hydrogen and helium lines
observed in six test stars over a wide spectral range from the Balmer limit to
the NIR, except for only a few well-understood cases. Our approach reproduces
other published non-LTE calculations, however avoids inconsistencies with the
modelling of the He I singlets recently discussed in the literature. It
improves on the published LTE models in many aspects: non-LTE strengthening and
the use of improved line-broadening data result in overall significant
differences in the line profiles and equivalent widths of the Balmer and helium
lines. Where possible, systematic effects on the stellar parameter
determination are quantified, e.g. gravities derived from the Hgamma wings may
be overestimated by up to ~0.2 dex at our upper temperature boundary in LTE.
(abridged)Comment: 25 pages, 19 figures. Modified according to suggestions of the
referee. Accepted for publication in A&A. Several figures in low resolution.
A high-resolution pdf version of the preprint can be downloaded from
http://www.sternwarte.uni-erlangen.de/~ai97/preprints/HHe_nieva.pd
NLTE spectroscopic analysis of the He anomaly in subluminous B-type stars
Several B-type main-sequence stars show chemical peculiarities. A
particularly striking class are the He stars, which exhibit a remarkable
enrichment of He with respect to He. This isotopic anomaly has also
been found in blue horizontal branch (BHB) and subdwarf B (sdB) stars, which
are helium-core burning stars of the extreme horizontal branch. Using a hybrid
local/non-local thermodynamic equilibrium (LTE/NLTE) approach for B-type stars,
we analyzed high-quality spectra of two known He BHBs and nine known He
sdBs to determine their isotopic helium abundances and He/He abundance
ratios. We redetermined their atmospheric parameters and analyzed selected
neutral helium lines, including 4922 and 6678
, which are very sensitive to He/He. Most of the He
sdBs cluster in a narrow temperature strip between 26000 K and 30000 K and are
helium deficient in accordance with previous LTE analyses. BD+48 2721
is reclassified as a BHB star because of its low temperature
( 20700 K). Whereas He is almost absent
(He/He 0.25) in most of the known He stars, other sample stars
show abundance ratios up to He/He2.51. A search for He stars in
the ESO SPY survey led to the discovery of two new He sdB stars (HE
0929-0424 and HE 1047-0436). The observed helium line profiles of all BHBs and
of three sdBs are not matched by chemically homogeneous atmospheres, but hint
at vertical helium stratification. This phenomenon has been seen in other
peculiar B-type stars, but is found for the first time for sdBs. We estimate
helium to increase from the outer to the inner atmosphere by factors ranging
from 1.4 (SB 290) up to 8.0 (BD+48 2721).Comment: 19 pages, 79 figures submitted to Astronomy&Astrophysic
C II abundances in early-type stars: solution to a notorious non-LTE problem
We address a long-standing discrepancy between non-LTE analyses of the
prominent C II 4267 and 6578/82 A multiplets in early-type stars. A
comprehensive non-LTE model atom of C II is constructed based on critically
selected atomic data. This model atom is used for an abundance study of six
apparently slow-rotating main-sequence and giant early B-type stars.
High-resolution and high-S/N spectra allow us to derive highly consistent
abundances not only from the classical features but also from up to 18 further
C II lines in the visual - including two so far unreported emission features
equally well reproduced in non-LTE. These results require the stellar
atmospheric parameters to be determined with care. A homogeneous (slightly)
sub-solar present-day carbon abundance from young stars in the solar vicinity
(in associations and in the field) of log C/H +12= 8.29+/-0.03 is indicated.Comment: 8 pages, 5 figure
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