THE VERTICAL STRUCTURE AND STABILITY OF ACCRETION DISKS SURROUNDING BLACK-HOLES AND NEUTRON-STARS

The structure and stability of the inner regions of accretion disks su rrounding neutron stars and black holes have been investigated. Within the framework of the alpha viscosity prescription for optically thick disks, we assume the viscous stress scales with gas pressure only, an d the alpha parameter, which is less than or equal to unity, is formul ated as alpha(0)(h/r)(n), where h is the local scale height and n and alpha(0) are constants. We neglect advective energy transport associat ed with radial motions and construct the vertical structure of the dis ks by assuming a Keplerian rotation law and local hydrostatic and ther mal equilibrium. The vertical structures have been calculated with and without convective energy transport, and it has been demonstrated tha t convection is important especially for mass accretion rates, M, grea ter than about 0.1 times the Eddington value, M(Edd). Although the eff iciency of convection is not high, convection significantly modifies t he vertical structure of the disk (as compared with a purely radiative model) and leads to lower temperatures at a given M. The results show that the disk can be locally unstable and that for n greater than or similar to 0.75, an S-shaped relation can exist between M and the colu mn density, Sigma, at a given radius. While the lower stable branch (d M/d Sigma > 0) and middle unstable branch (dM/d Sigma < 0) represent s tructures for which the gas and radiation pressure dominate respective ly, the stable upper branch (dM/d Sigma > 0) is a consequence of the s aturation of alpha. This saturation of alpha can occur for large alpha (0) and at M less than or similar to M(Edd). The instability is found to occur at higher mass accretion rates for neutron stars than for bla ck holes. In particular, the disk is locally unstable for M greater th an or similar to 0.5M(Edd) for neutron stars and for M greater than or similar to 0.1M(Edd) for black holes for a viscosity prescription cha racterized by n = 1 and alpha(0) = 10.