107 research outputs found
Interpreting the Hydrogen IR Lines - Impact of Improved Electron Collision Data
We evaluate the effect of variations in the electron-impact excitation cross
sections on the non-LTE line formation for hydrogen in early-type stars. While
the Balmer lines are basically unaffected by the choice of atomic data, the
Brackett and Pfund series members allow us to discriminate between the
different models. Non-LTE calculations based on the widely-used approximations
of Mihalas, Heasley & Auer and of Johnson fail to simultaneously reproduce the
observed optical and IR spectra over the entire parameter range. Instead, we
recommend a reference model using data from ab-initio calculations up to
principal quantum number n<=7 for quantitative work. This model is of general
interest due to the ubiquity of the hydrogen spectrum.Comment: 6 pages, 6 figure
Present-day cosmic abundances. A comprehensive study of nearby early B-type stars and implications for stellar and Galactic evolution and interstellar dust models
Aims. A sample of early B-type stars in OB associations and the field within
the solar neighbourhood is studied comprehensively. Present-day abundances for
the astrophysically most interesting chemical elements are derived. Methods.
High-resolution and high-S/N spectra of early B-type stars are analysed in
NLTE. Atmospheric parameters are derived from the simultaneous establishment of
independent indicators, from multiple ionization equilibria and the hydrogen
Balmer lines. Results. Teff is constrained to 1-2% and logg to less than 15%
uncertainty. Absolute values for metal abundances are determined to better than
25% uncertainty. The synthetic spectra match the observations reliably over
almost the entire visual spectral range. Conclusions. A present-day cosmic
abundance standard is established. Our results i) resolve the discrepancy
between a chemical homogeneous local gas-phase ISM and a chemically
inhomogeneous young stellar component, ii) facilitate the amount of heavy
elements locked up in the interstellar dust to be constrained precisely:
carbonaceous dust is largely destroyed inside the Orion HII region, unlike the
silicates, and that graphite is only a minority species in interstellar dust -,
iii) show that the mixing of CNO-burning products in the course of massive star
evolution follows tightly the predicted nuclear path, iv) provide reliable
present-day reference points for anchoring Galactic chemical evolution models
to observation, and v) imply that the Sun has migrated outwards from the inner
Galactic disk over its lifetime from a birthplace at a distance around 5-6 kpc
from the Galactic Centre; a cancellation of the effects of Galactic chemical
evolution and abundance gradients leads to the similarity of solar and
present-day cosmic abundances in the solar neighbourhood, with a telltaling
signature of the Sun's origin left in the C/O ratio. (ABRIDGED)Comment: 20 pages, 11 figures, published in A&A. A complete version (57 pages,
17 figures) including online material can be downloaded from
http://www.sternwarte.uni-erlangen.de/~nieva/8158.pd
Quantitative Spectroscopy of Supergiants
Blue supergiants of spectral types B and A are the visually brightest stars
in spiral and irregular galaxies, with their most luminous members (at M_V=-10)
outshining entire dwarf galaxies. This characteristic allows us to use them as
probes to study the Local Universe in great detail. In principle, already the
existing large telescopes and instrumentation facilitate quantitative
spectroscopy of these objects as far as the Virgo and Fornax clusters of
galaxies. Beyond their challenging stellar atmospheres and opportunities for
testing sophisticated non-LTE physics they offer numerous applications to
modern astrophysics. Quantitative spectroscopy of supergiants can contribute to
improve our understanding of massive star evolution. Galactic abundance
gradients and abundance patterns, as can be obtained from studies of large
ensembles of supergiants in our own and other galaxies, will foster the
understanding of galactochemical evolution. Finally, they are promising
independent indicators for calibrating the extragalactic distance scale, by
application of the wind momentum-luminosity and the flux-weighted
gravity-luminosity relationships. In view of this large potential, the
objective of this thesis is to improve the status of quantitative spectroscopy
of BA-type supergiants and to provide first applications on a sample of
Galactic and extragalactic targets, within the Local Group and beyond.Comment: 2 pages. Dissertation Summary, accepted for publication in PASP.
Thesis may be retrieved from
http://edoc.ub.uni-muenchen.de/archive/00000082/01/Przybilla_Norbert.pd
A non-LTE spectral analysis of the 3He and 4He isotopes in the HgMn star kappa Cancri
We present a pilot study on non-local thermodynamic equilibrium (NLTE)
line-formation computations for the isotopes 3He and 4He in the
mercury-manganese star kappa Cancri. The impact of NLTE effects on the
determination of isotopic abundances and the vertical stratification of helium
in the atmosphere is investigated. Modern NLTE line-formation computations were
employed to analyse a high-resolution and high signal-to-noise ratio
ESO-VLT/UVES spectrum of kap Cnc. The atmospheric parameters were determined
from fitting the hydrogen Balmer lines and the spectral energy distribution.
Multiple HeI lines were investigated, including HeI 4921A and 6678A, which show
the widest isotopic splits. Half of the observed HeI lines in the spectrum of
kap Cnc show significant NLTE strengthening, the effects are strongest in the
red lines HeI 5875A and HeI 6678A. NLTE abundances from individual HeI lines
are up to a factor of about 3 lower than LTE values. Helium is found to be
stratified in the atmosphere of kap Cnc. While the LTE analysis indicates a
step-like profile of the helium abundance, a gradual decrease with height is
indicated by the NLTE analysis. A 3He/4He ratio of about 0.25-0.30 is found.
With the available data it cannot be decided whether the two isotopes follow
the same stratification profile, or not. This work implies that NLTE effects
may be ubiquitous in the atmospheres of HgMn stars and may have a significant
impact on abundance determinations and the interpretation of the vertical
abundance stratification of elements.Comment: 7 pages, 9 figure
Blue Supergiants as a Tool for Extragalactic Distances - Theoretical Concepts
Because of their enormous intrinsic brightness blue supergiants are ideal
stellar objects to be studied spectroscopically as individuals in galaxies far
beyond the Local Group. Quantitative spectroscopy by means of efficient
multi-object spectrographs attached to 8m-class telescopes and modern NLTE
model atmosphere techniques allow us to determine not only intrinsic stellar
parameters such as effective temperature, surface gravity, chemical composition
and absolute magnitude but also very accurately interstellar reddening and
extinction. This is a significant advantage compared to classical distance
indicators like Cepheids and RR Lyrae. We describe the spectroscopic
diagnostics of blue supergiants and introduce two concepts to determine
absolute magnitudes. The first one (Wind Momentum -- Luminosity Relationship)
uses the correlation between observed stellar wind momentum and luminosity,
whereas the second one (Flux-weighted Gravity -- Luminosity Relationship)
relies only on the determination of effective temperature and surface gravity
to yield an accurate estimate of absolute magnitude. We discuss the potential
of these two methods.Comment: 25 pages, 16 figures; Invited review at the International Workshop on
"Stellar Candles for the Extragalactic Distance Scale", held in Concepcion,
Chile, December 9-11, 2002, to be published in: "Stellar Candles", Lecture
Notes in Physics, Springer-Verla
Non-LTE line-formation for CNO
Accurate atomic data have become available in the recent past due to the
demands of astrophysics and fusion research. We report on the impact of such
data on non-LTE line-formation calculations for CNO in early-type stars.
Considerable improvement is achieved by the derivation of consistent results
from practically all available spectroscopic indicators, regardless of
ionization stage or spin system, and the uncertainties in the analyses are
drastically reduced. Moreover, systematic trends are revealed, e.g. an increase
of the NI abundances from previous studies of BA-type supergiants by a factor
of two is indicated. The present work promises stringent observational
constraints on chemical mixing in the course of massive star evolution. First
results on BA-type supergiants in the Galaxy and the Magellanic Clouds are
discussed.Comment: 6 pages, 2 figures; to be published in "CNO in the Universe", ASP
Conf. Series, eds. C. Charbonnel, D. Schaerer & G. Meyne
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