26 research outputs found
Runaway of Line-Driven Winds Towards Critical and Overloaded solutions
Line-driven winds from hot stars and accretion disks are thought to adopt a
unique, critical solution which corresponds to maximum mass loss rate and a
particular velocity law. We show that in the presence of negative velocity
gradients, radiative-acoustic (Abbott) waves can drive shallow wind solutions
towards larger velocities and mass loss rates. Perturbations introduced
downstream from the wind critical point lead to convergence towards the
critical solution. By contrast, low-lying perturbations cause evolution towards
a mass-overloaded solution, developing a broad deceleration region in the wind.
Such a wind differs fundamentally from the critical solution. For sufficiently
deep-seated perturbations, overloaded solutions become time-dependent and
develop shocks and shells.Comment: Latex, 2 postscript figures Astrophysical Journal Letters, in pres
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
Dynamics of Line-Driven Winds from Disks in Cataclysmic Variables. II. Mass Loss Rates and Velocity Laws
We analyze the dynamics of 2D stationary line-driven winds from accretion
disks in cataclysmic variables (CVs), by generalizing the Castor, Abbott and
Klein theory. In paper 1, we have solved the wind Euler equation, derived its
two eigenvalues, and addressed the solution topology and wind geometry. Here,
we focus on mass loss and velocity laws. We find that disk winds, even in
luminous novalike variables, have low optical depth, even in the strongest
driving lines. This suggests that thick-to-thin transitions in these lines
occur. For disks with a realistic radial temperature, the mass loss is
dominated by gas emanating from the inner decade in r. The total mass loss rate
associated with a luminosity 10 Lsun is 10^{-12} Msun/yr, or 10^{-4} of the
mass accretion rate. This is one order of magnitude below the lower limit
obtained from P Cygni lines, when the ionizing flux shortwards of the Lyman
edge is supressed. The difficulties with such small mass loss rates in CVs are
principal, and confirm our previous work. We conjecture that this issue may be
resolved by detailed nonLTE calculations of the line force within the context
of CV disk winds, and/or better accounting for the disk energy distribution and
wind ionization structure. We find that the wind velocity profile is well
approximated by the empirical law used in kinematical modeling. The
acceleration length scale is given by the footpoint radius of the wind
streamline in the disk. This suggests an upper limit of 10 Rwd to the
acceleration scale, which is smaller by factors of a few as compared to values
derived from line fitting.Comment: 14 pages, 3 Postscript figures, also from
http://www.pa.uky.edu/~shlosman/publ.html. Astrophysical Journal, submitte