430 research outputs found
NLTE wind models of hot subdwarf stars
We calculate NLTE models of stellar winds of hot compact stars (central stars
of planetary nebulae and subdwarf stars). The studied range of subdwarf
parameters is selected to cover a large part of these stars. The models predict
the wind hydrodynamical structure and provide mass-loss rates for different
abundances. Our models show that CNO elements are important drivers of subdwarf
winds, especially for low-luminosity stars. We study the effect of X-rays and
instabilities on these winds. Due to the line-driven wind instability, a
significant part of the wind could be very hot.Comment: 7 pages, to appear in Astrophysics and Space Science. The final
publication will be available at springerlink.com
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
Supernova explosions interacting with aspherical circumstellar material: implications for light curves, spectral line profiles, and polarization
Some supernova (SN) explosions show evidence for interaction with
pre-existing non-spherically symmetric circumstellar medium (CSM) in their
light curves, spectral line profiles, and polarization signatures. To better
understand the connection with binary stars and to aid in the interpretation of
observations, we perform two-dimensional axisymmetric hydrodynamic simulations
where an expanding spherical SN ejecta initialized with realistic density and
velocity profiles collide with various aspherical CSM distributions. We
consider CSM in the form of a circumstellar disk, colliding wind shells in
binary stars with different orientations and distances from the SN progenitor,
and bipolar lobes representing a scaled down version of the Homunculus nebula
of ~Car. We study how our simulations map onto observables, including
approximate light curves, indicative spectral line profiles at late times, and
estimates of polarization signature. We find that the SN--CSM collision layer
is composed of normal and oblique shocks, reflected waves, and other
hydrodynamical phenomena that lead to acceleration and shear instabilities. As
a result, the total shock heating power fluctuates in time, although the
emerging light curve might be smooth if the shock interaction region is deeply
embedded in the SN envelope. SNe with circumstellar disks or bipolar lobes
exhibit late-time spectral line profiles symmetric with respect to the rest
velocity and relatively high polarization. In contrast, SNe with colliding wind
shells naturally lead to line profiles with asymmetric and time-evolving blue
and red wings and low polarization. Given the high frequency of binaries among
massive stars, interaction of SN ejecta with a pre-existing colliding wind
shell must occur and the observed signatures could be used to characterize the
binary companion
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
First Stars. II. Evolution with mass loss
The first stars are assumed to be predominantly massive. Although, due to the
low initial abundances of heavy elements the line-driven stellar winds are
supposed to be inefficient in the first stars, these stars may loose a
significant amount of their initial mass by other mechanisms.
In this work, we study the evolution with a prescribed mass loss rate of very
massive, galactic and pregalactic, Population III stars, with initial
metallicities and , respectively, and initial masses
100, 120, 150, 200, and 250 during the hydrogen and helium burning
phases.
The evolution of these stars depends on their initial mass, metallicity and
the mass loss rate. Low metallicity stars are hotter, compact and luminous, and
they are shifted to the blue upper part in the Hertzprung-Russell diagram. With
mass loss these stars provide an efficient mixing of nucleosynthetic products,
and depending on the He-core mass their final fate could be either
pair-instability supernovae or energetic hypernovae. These stars contributed to
the reionization of the universe and its enrichment with heavy elements, which
influences the subsequent star formation properties.Comment: Accepted for publication in Astrophysics & Space Science. 15 pages,
18 figure
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
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