Hypercritical advection-dominated accretion flow

In this paper we study the accretion disk that arises in hypercritical accr etion of (M)over dot similar to 10(8) M-Edd onto a neutron star while it is in common envelope evolution with a massive companion. Such a study was ca rried out by Chevalier, who had earlier suggested that the neutron star wou ld go into a black hole in common envelope evolution. In his later study ha found that the accretion could possibly be held up by angular momentum. In order to raise the temperature high enough that the disk might cool by n eutrino emission, Chevalier found a small value of the alpha-parameter, whe re the kinematic coefficient of sheer viscosity is v = alpha c(s)H, with c( s) the velocity of sound and H the disk height; namely, alpha similar to 10 (-6) was necessary for gas pressure to dominate. He also considered results with higher values of alpha, pointing out that radiation pressure would th en predominate. With these larger alpha-values, the temperatures of the acc reting material are much lower, greater than or similar to 0.35 MeV. The re sult is that neutrino cooling during the flow is negligible, satisfying ver y well the advection-dominating conditions. The low temperature of the accreting material means that it cannot get rid of its energy rapidly by neutrino emission, so it piles up, pushing its way through the accretion disk. An accretion shock is formed, far beyond the n eutron star, at a radius less than or similar to 10(8) cm, much as in the e arlier spherically symmetric calculation, but in rotation. Two-dimensional numerical simulation shows that an accretion disk is reformed inside of the accretion shock, allowing matter to accrete onto the neutron star with pre ssure high enough so that neutrinos can carry off the energy.