Effects of electron-positron pairs in advection-dominated disks

Disk structure with advective cooling and pair production is studied, using a recently obtained local solution by Abramowicz and coworkers and a self-similar solution by Narayan and Yi. They found that there is a maximum accretion rate above which no steady state solution is allowed, when the value of the alpha-parameter of viscosity is smaller than a critical value, alpha(cr). In this paper we show that the maximum accretion rate is reduced by pair production and that the value of alpha(cr) (similar to 1) increases slightly because of radiative cooling by pairs. These effects are, however, only important when the accretion rate is near the Eddington rate; otherwise, the disk structure with alpha < alpha(cr) is not significantly changed by pair production. This is because the surface density is kept low as a result of advective cooling, and photon-photon collisions do not contribute to pair production significantly. The maximum value of n(+)/n(p) is about 1 when alpha = 1, where n(+) and n(p) are the number densities of positrons and protons, respectively. When alpha > alpha(cr), on the other hand, disk structure is strongly changed by pairs; there are multiple solutions for certain values of accretion rate in the (surface density, accretion rate)-plane. The additional solutions exist because of a high pair density.