220 research outputs found
Statistical equilibrium equations for trace elements in stellar atmospheres
The conditions of thermodynamic equilibrium, local thermodynamic equilibrium,
and statistical equilibrium are discussed in detail. The equations of
statistical equilibrium and the supplementary equations are shown together with
the expressions for radiative and collisional rates with the emphasize on the
solution for trace elements.Comment: presented at the workshop held in Nice, France, 30.7.-4.8.2007, to
appear in Non-LTE Line Formation for Trace Elements in Stellar Atmospheres,
R. Monier et al. eds., EAS Publ.Se
Radiative transfer in moving media II. Solution of the radiative transfer equation in axial symmetry
A new method for the formal solution of the 2D radiative transfer equation in
axial symmetry in the presence of arbitrary velocity fields is presented. The
combination of long and short characteristics methods is used to solve the
radiative transfer equation. We include the velocity field in detail using the
Local Lorentz Transformation. This allows us to obtain a significantly better
description of the photospheric region, where the gradient of the global
velocity is too small for the Sobolev approximation to be valid. Sample test
calculations for the case of a stellar wind and a rotating atmosphere are
presented.Comment: 11 pages, 19 figures. accepted by Astronomy and Astrophysic
On multicomponent effects in stellar winds of stars at extremely low metallicity
We calculate multicomponent line-driven wind models of stars at extremely low metallicity suitable for massive first generation stars. For most of the models we find that the multicomponent wind nature is not important for either wind dynamics or for wind temperature stratification. However, for stars with the lowest metallicities we find that multicomponent effects influence the wind structure. These effects range from pure heating to possible fallback of the nonabsorbing wind component. We present a simple formula for the calculation of metallicity for which the multicomponent effects become important. We show that the importance of the multicomponent nature of winds of low metallicity stars is characterised not only by the low density of driving ions, but also by lower mass-loss rate
Current status of NLTE analysis of stellar atmospheres
Various available codes for NLTE modeling and analysis of hot star spectra
are reviewed. Generalizations of standard equations of kinetic equilibrium and
their consequences are discussed.Comment: in Determination of Atmospheric Parameters of B-, A-, F- and G-Type
Stars, E. Niemczura et al. eds., Springer, in pres
Macroclumping as solution of the discrepancy between H{\alpha} and P v mass loss diagnostics for O-type stars
Recent studies of O-type stars demonstrated that discrepant mass-loss rates
are obtained when different diagnostic methods are employed - fitting the
unsaturated UV resonance lines (e.g. P v) gives drastically lower values than
obtained from the H{\alpha} emission. Wind clumping may be the main cause for
this discrepancy. In a previous paper, we have presented 3-D Monte-Carlo
calculations for the formation of scattering lines in a clumped stellar wind.
In the present paper we select five O-type supergiants (from O4 to O7) and test
whether the reported discrepancies can be resolved this way. In the first step,
the analyses start with simulating the observed spectra with Potsdam Wolf-Rayet
(PoWR) non-LTE model atmospheres. The mass-loss rates are adjusted to fit best
to the observed H{\alpha} emission lines. For the unsaturated UV resonance
lines (i.e. P v) we then apply our 3-D Monte-Carlo code, which can account for
wind clumps of any optical depths, a non-void inter-clump medium, and a
velocity dispersion inside the clumps. The ionization stratifications and
underlying photospheric spectra are adopted from the PoWR models. From fitting
the observed resonance line profiles, the properties of the wind clumps are
constrained. Our results show that with the mass-loss rates that fit H{\alpha}
(and other Balmer and He II lines), the UV resonance lines (especially the
unsaturated doublet of P v) can also be reproduced without problem when
macroclumping is taken into account. There is no need to artificially reduce
the mass-loss rates, nor to assume a sub-solar phosphorus abundance or an
extremely high clumping factor, contrary to what was claimed by other authors.
These consistent mass-loss rates are lower by a factor of 1.3 to 2.6, compared
to the mass-loss rate recipe from Vink et al. Macroclumping resolves the
previously reported discrepancy between H{\alpha} and P v mass-loss
diagnostics.Comment: 18 pages, 14 figures, 5 tables, accepted for publication in
Astrononomy & Astrophysic
Low-metallicity massive single stars with rotation. II. Predicting spectra and spectral classes of chemically-homogeneously evolving stars
Context. Metal-poor massive stars are supposed to be progenitors of certain
supernovae, gamma-ray bursts and compact object mergers, potentially
contributing to the early epochs of the Universe with their strong ionizing
radiation. However, they remain mainly theoretical as individual spectroscopic
observations of such objects have rarely been carried out below the metallicity
of the SMC.
Aims. This work aims at exploring what our state-of-the-art theories of
stellar evolution combined with those of stellar atmospheres predict about a
certain type of metal-poor (0.02 Z) hot massive stars, the chemically
homogeneously evolving ones, called TWUIN stars.
Methods. Synthetic spectra corresponding to a broad range in masses (20-130
M) and covering several evolutionary phases from the zero-age
main-sequence up to the core helium-burning stage were computed.
Results. We find that TWUIN stars show almost no emission lines during most
of their {core hydrogen-burning} lifetimes. Most metal lines are completely
absent, including nitrogen. During their core helium-burning stage, lines
switch to emission and even some metal lines (oxygen and carbon, but still
almost no nitrogen) show up. Mass loss and clumping play a significant role in
line-formation in later evolutionary phases, particularly during core
helium-burning. Most of our spectra are classified as an early O type giant or
supergiant, and we find Wolf-Rayet stars of type WO in the core helium-burning
phase.
Conclusions. An extremely hot, early O type star observed in a
low-metallicity galaxy could be the outcome of chemically homogeneous evolution
and therefore the progenitor of a long-duration gamma-ray burst or a type
Ic supernova. TWUIN stars may play an important role in reionizing the Universe
due to their being hot without showing prominent emission lines during the
majority of their lifetimes.Comment: Accepted by Astronomy and Astrophysics. In Pres
NLTE analysis of spectra: OBA stars
Methods of calculation of NLTE model atmosphere are discussed. The NLTE trace
element procedure is compared with the full NLTE model atmosphere calculation.
Differences between LTE and NLTE atmosphere modeling are evaluated. The ways of
model atom construction are discussed. Finally, modelling of expanding
atmospheres of hot stars with winds is briefly reviewed.Comment: in Determination of Atmospheric Parameters of B-, A-, F- and G-Type
Stars, E. Niemczura et al. eds., Springer, in pres
Improved velocity law parameterization for hot star winds
The velocity law of hot star winds is usually parameterized via the so-called
beta velocity law. Although this parameterization stems from theoretical
considerations, it is not the most accurate description of the wind velocity
law that follows from hydrodynamical calculations. We show that the velocity
profile of our hydrodynamical wind models is described much better by
polynomial approximation. This approximation provides a better fit than the
beta velocity law already for the same number of free parameters.Comment: 3 pages, 2 figures, accepted for publication in Astronomy &
Astrophysic
- …