GEOMETRICALLY THIN, HOT ACCRETION DISKS - TOPOLOGY OF THE THERMAL-EQUILIBRIUM CURVES

We present all the possible thermal equilibrium states of geometrically thin alpha-disks around stellar-mass black holes. We employ a (vertically) one-zone disk model and assume that a main energy source is viscous heating of protons and that cooling is due to bremsstrahlung and Compton scattering. Results can be summarized in the (SIGMA = surface density, M = accretion rate) plane. There exist various branches of the thermal equilibrium solution, depending on whether disks are effectively optically thick or thin, radiation pressure-dominated or gas pressure-dominated, composed of one-temperature plasmas or of two-temperature plasmas, and with high concentration of e+e- pairs or without pairs. The thermal equilibrium curves at high temperatures (T greater than or similar to 10(8) K) are substantially modified by the presence of e+e- pairs. The behavior of the pair-thermal equilibrium lines of the pair-dominated plasmas is well understood in the three-dimensional (SIGMA, M, z) space, where z is the ratio of positron density to proton density. We then examine the thermal stability of these branches. There are no thermally stable branches at high temperatures, T greater than or similar to 10(7) K, if the standard alpha-model is adopted and pair balance is assumed. A hard X-ray-emitting disk cannot therefore stay on any branches on time scales longer than the thermal time scales (of order 10 ms). It may be possible, however, that due to the limit cycle behavior, a disk stays around the high-temperature, pair-dominated solution in a quasi-steady manner.