Time-dependent modeling of extended thin decretion disks of critically rotating stars
Kurfürst, P.; Feldmeier, A.; Krticka, J.
Abstract: Context. 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.
Aims: 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.
Methods: 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.
Results: 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.
Conclusions: 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.