48 research outputs found
Time-dependent modeling of extended thin decretion disks of critically rotating stars
During their evolution massive stars can reach the phase of critical rotation
when a further increase in rotational speed is no longer possible. Direct
centrifugal ejection from a critically or near-critically rotating surface
forms a gaseous equatorial decretion disk. Anomalous viscosity provides the
efficient mechanism for transporting the angular momentum outwards. The outer
part of the disk can extend up to a very large distance from the parent star.
We study the evolution of density, radial and azimuthal velocity, and angular
momentum loss rate of equatorial decretion disks out to very distant regions.
We investigate how the physical characteristics of the disk depend on the
distribution of temperature and viscosity. We calculated stationary models
using the Newton-Raphson method. For time-dependent hydrodynamic modeling we
developed the numerical code based on an explicit finite difference scheme on
an Eulerian grid including full Navier-Stokes shear viscosity. The sonic point
distance and the maximum angular momentum loss rate strongly depend on the
temperature profile and are almost independent of viscosity. The rotational
velocity at large radii rapidly drops accordingly to temperature and viscosity
distribution. The total amount of disk mass and the disk angular momentum
increase with decreasing temperature and viscosity. The time-dependent
one-dimensional models basically confirm the results obtained in the stationary
models as well as the assumptions of the analytical approximations. Including
full Navier-Stokes viscosity we systematically avoid the rotational velocity
sign change at large radii. The unphysical drop of the rotational velocity and
angular momentum loss at large radii (present in some models) can be avoided in
the models with decreasing temperature and viscosity
Two-dimensional modeling of density and thermal structure of dense circumstellar outflowing disks
Context. Evolution of massive stars is affected by a significant loss of mass
either via (nearly) spherically symmetric stellar winds or by aspherical
mass-loss mechanisms, namely the outflowing equatorial disks. However, the
scenario that leads to the formation of a disk or rings of gas and dust around
massive stars is still under debate. Aims. We study the hydrodynamic and
thermal structure of optically thick, dense parts of outflowing circumstellar
disks that may be formed around various types of critically rotating massive
stars, for example, Be stars, B[e] supergiant (sgB[e]) stars or Pop III stars.
Methods. We specify the optical depth of the disk along the line-of-sight from
stellar poles. Within the optically thick dense region we calculate the
vertical disk thermal structure using the diffusion approximation while for the
optically thin outer layers we assume a local thermodynamic equilibrium with
the impinging stellar irradiation. We use two of our own types of hydrodynamic
codes: two-dimensional operator-split numerical code and unsplit code based on
the Roe's method. Results. Our models show the geometric distribution and
contribution of viscous heating that begins to dominate in the central part of
the disk. In the models of dense viscous disks the viscosity increases the
central temperature up to several tens of thousands of Kelvins. The high
mass-loss rates and high viscosity lead to instabilities with significant waves
or bumps in density and temperature in the very inner disk region. Conclusions.
The two-dimensional radial-vertical models of dense outflowing disks including
the full Navier-Stokes viscosity terms show very high temperatures that are
however limited to only the central disk cores inside the optically thick area,
while near the edge of the optically thick region the temperature may be low
enough for the existence of neutral hydrogen.Comment: 24 pages, 21 figure
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
A binary nature of the marginal CP star Sigma Sculptoris
The A2 V star σ Scl was suspected of being a low-amplitude rotating variable of the Ap-type star by several authors. Aiming to decide whether the star is a variable chemically peculiar (CP) star, we searched for the photometric and spectroscopic variability, and determined chemical abundances of σ Scl. The possible variability was tested using several types of periodograms applied to the photometry from Long-Term Photometry of Variables project (LTPV) and Hipparcos. Sixty spectrograms of high signal-to-noise (S/N) were obtained and used for chemical analysis of the stellar atmosphere and for looking for spectral variability that is symptomatic for the CP stars. We did not find any signs of the light variability or prominent chemical peculiarity, that is specific for the CP stars. The only exception is the abundance of scandium, which is significantly lower than the solar one and yttrium and barium, which are strongly overabundant. As a by-product of the analysis, and with the addition of 29 further spectra, we found that σ Scl is a single-lined spectroscopic binary with orbital period of 46.877(8) d. We argue that σ Scl is not an Ap star, but rather a marginal Am star in SB1 system. The spectral energy distribution of the binary reveals infrared excess due to circumstellar material.Fil: Janík, Jan. Masaryk University; República ChecaFil: Krtička, Jiří. Masaryk University; República ChecaFil: Mikulášek, Zdeněk. Masaryk University; República ChecaFil: Zverko, Juraj. Masaryk University; República ChecaFil: Pintado, Olga Ines. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán. Instituto Superior de Correlación Geológica. Universidad Nacional de Tucumán. Facultad de Ciencias Naturales e Instituto Miguel Lillo. Departamento de Geología. Cátedra Geología Estructural. Instituto Superior de Correlación Geológica; Argentin
Mass and angular momentum loss of first stars via decretion disks
Although the first stars were likely very hot and luminous, their low or zero metallicity implies that any mass loss through winds driven by line-scattering of radiation in metal ions was likely small or non-existent. Here we examine the potential role of another possible mechanism for mass loss in these first stars, namely via decretion disks associated with near-critical rotation induced from evolution of the stellar interior. In this case the mass loss is set by the angular momentum needed to keep the stellar rotation at or below the critical rate. In present evolutionary models, that mass loss is estimated by assuming effective release from a spherical shell at the surface. Here we examine the potentially important role of viscous coupling of the decretion disk in outward angular momentum transport, emphasizing that the specific angular momentum at the outer edge of the disk can be much larger than at the stellar surface. The net result is that, for a given stellar interior angular momentum excess, the mass loss required from a decretion disk can be significantly less than invoked in previous models assuming a direct, near-surface releas
Mass and angular momentum loss of fast rotating stars via decretion disks
The spinup of massive stars induced by evolution of the stellar interior can bring the star to near-critical rotation. In critically rotating stars the decrease of the stellar moment of inertia must be balanced by a net loss of angular momentum through an equatorial decretion disk. We examine the nature and role of mass loss via such disks. In contrast to the usual stellar wind mass loss set by exterior driving from the stellar luminosity, such decretion-disk mass loss stems from the angular momentum loss needed to keep the star near and below critical rotation, given the interior evolution and decline in the star's moment of inertia. Because the specific angular momentum in a Keplerian disk increases with the square root of the radius, the decretion mass loss associated with a required level of angular momentum loss critically depends on the outer radius for viscous coupling of the disk, and can be significantly less than the spherical, wind-like mass loss commonly assumed in evolutionary calculation
X-ray photoionized bubble in the wind of Vela X-1 pulsar supergiant companion
Vela X-1 is the archetype of high-mass X-ray binaries, composed of a neutron
star and a massive B supergiant. The supergiant is a source of a strong
radiatively-driven stellar wind. The neutron star sweeps up this wind, and
creates a huge amount of X-rays as a result of energy release during the
process of wind accretion. Here we provide detailed NLTE models of the Vela X-1
envelope. We study how the X-rays photoionize the wind and destroy the ions
responsible for the wind acceleration. The resulting decrease of the radiative
force explains the observed reduction of the wind terminal velocity in a
direction to the neutron star. The X-rays create a distinct photoionized region
around the neutron star filled with a stagnating flow. The existence of such
photoionized bubbles is a general property of high-mass X-ray binaries. We
unveiled a new principle governing these complex objects, according to which
there is an upper limit to the X-ray luminosity the compact star can have
without suspending the wind due to inefficient line drivingComment: accepted for publication in ApJ, 6 pages, 4 figure
Influence of XUV radiation on Pv ionization fraction in hot star winds
Different diagnostics of hot star wind mass-loss rates provide results that
are difficult to reconcile with each other. The widely accepted presence of
clumping in hot star winds implies a significant reduction of observational
mass-loss rate estimates from diagnostics that depend on the square of the
density. Moreover, the ultraviolet Pv resonance lines indicate a possible need
for even stronger reduction of hot star mass-loss rates, provided that Pv is a
dominant ionization stage of phosphorus at least in some hot stars. The latter
assumption is challenged by a possible presence of the XUV radiation.
Here we study the influence of the XUV radiation on the Pv ionization
fraction in the hot star winds. By a detailed solution of the hydrodynamical,
radiative transfer, and statistical equilibrium equations we confirm that
sufficiently strong XUV radiation source may decrease the Pv ionization
fraction, possibly depreciating the Pv lines as a reliable mass-loss rate
indicator. On the other hand, the XUV radiation influences also the ionization
fraction of heavier ions that drive the wind, leading to a decrease of the wind
terminal velocity. Consequently, we conclude that the XUV radiation alone can
not bring theory and observations in accord.
We fit our predicted wind mass-loss rates by a suitable formula and compare
the results with the observational mass-loss rate diagnostics. We show that for
supergiants and giants the theoretical predictions do not contradict the
mass-loss rate estimates based on X-ray line profiles or density squared
diagnostics. On the other hand, for main-sequence stars the predicted mass-loss
rates are still significantly higher than that inferred from Pv or X-ray lines.
This indicates that the "weak wind problem" recently detected in low-luminosity
main-sequence stars may occur to some extent also for the stars with higher
luminosity.Comment: Accepted for publication in MNRAS. The definitive version is
available at www.blackwell-synergy.com. 8 pages, 5 figure