Time-dependent modelling of extended thin decretion disks of critically rotating stars

• Autoři: KURFÜRST Petr
• Rok publikování: 2013
• Druh: Konferenční abstrakty
• Popis: Gradually contracting massive stars may during their early evolution reach the phase of critical rotation when the further increase in rotational speed is no longer possible. The ejection of matter in the equatorial region can form the gaseous outflowing disk, which allows the star to remove the excess of its angular momentum. The outer part of the disk can extend up to very large distance from parent star, the size, structure and mass-loss rate as well as the time of the disk evolution depends on various physical parameters. We study the evolution of density, radial and azimuthal velocity and angular momentum loss rate of equatorial decretion disk even in very distant outer regions. We investigate how the physical characteristics of the disk depend on parameterized distribution of temperature and viscosity. We developed the numerical code for time-dependent hydrodynamical modelling based on explicit finite difference scheme on an Eulerian grid including viscosity and angular momentum. Advection steps are taken into account using van Leer's monotonic advection algorithm, in the source steps we apply the so-called ``operator-splitting'' method. According to parameterized disk temperature and viscosity distribution we calculate in various models the evolution of density, radial and azimuthal velocity and the angular momentum loss rate from the initial Keplerian state until the disk approaches the final stationary state. Consequently we obtain the corresponding locations of the disk outer radius and also the corresponding times of the disk evolution. From the models follows that for steeper outward decrease of temperature the sonic point radius and thus the angular momentum loss rate as well as the outer disk radius and the disk evolution time substantially increase. The similar dependence applies also in case of suggested outward decrease of viscosity. The model shows that rotational velocity of gaseous segments in the inner parts of disk remains nearly Keplerian, while in outer part this becomes angular-momentum conserving. In the outer disk radius region the rotational velocity drops to some very small, but persistently positive values. However, in case of the radial viscosity coefficient decrease we obtain more physically plausible solution.