A POLYTROPIC MODEL OF AN ACCRETION DISK, A BOUNDARY-LAYER, AND A STAR

A model of a thin polytropic disk accreting onto a polytropic star through a polytropic boundary layer is presented. No a priori distinction is made in the manner various parts of the system are treated. A one-parameter series of models is calculated, with the stellar angular velocity varying from small to somewhat supercritical values along the series. All subcritical models accrete mass as well as angular momentum. For the most supercritical models accretion of mass is accompanied by the loss of angular momentum from the star to the disk, mediated by the viscous stresses. No termination of the series is noticed near the critical rotation rate. If anything, the supercritical models are simpler and easier to calculate than the models of disk accretion onto a slowly rotating star. The "no torque boundary condition" arises naturally in the accretion onto a star with a subcritical rotation. There is no such condition for a star with a supercritical rotation, as angular velocity decreases monotonically with radius throughout the whole disk-star system.