Models of extended decretion disks of critically rotating stars

Massive stars may during their 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 forms 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 large distance from the parent star. We study the evolution of density, radial and azimuthal velocity and angular momentum loss rate of equatorial decretion disk up to quite distant outer regions. We calculate the evolution of density, radial and azimuthal velocity from the initial Keplerian state until the disk approaches the final stationary state. 1 Viscous decretion disks of critically rotating stars The basic scenario follows the model of the viscous decretion disk proposed by Lee et al. (1991), which naturally leads to the formation of Keplerian disk, assuming the outward transport of matter from star’s near-equatorial surface occurs through the gradual drifting and feeding of the disk due to the viscous torque. The disk mass loss rate of critically rotat-ing star is determined by the angular momentum loss rate needed to keep the star at critical rotation (Krtička et al., 2011). The main uncertainties are the viscous coupling and the radial temperature distribution. The modelling of the viscosity is typically constrained to regions close to the star (Penna et al., 2013), consequently we assume a power low decrease of the viscous coupling. In this model we maintain the temperature distribution as a free parame-ter. Following the stationary models (Kurfürst & Krtička, 2012) we introduce the results of calculations of the time-evolving converging solutions for governing physical quantities in the disk up to quite distant supersonic regions from the parent star. a