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 α-parameter of viscosity is smaller than a critical value, αcr. In this paper we show that the maximum accretion rate is reduced by pair production and that the value of αcr(∼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 α < αcr 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 is about 1 when α = 1, where n+ and np are the number densities of positrons and protons, respectively. When α > αcr, 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.