Hypercritical advection-dominated accretion flow

In this paper we study the accretion disk that arises in hypercritical accretion 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 carried out by Chevalier, who had earlier suggested that the neutron star would 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 neutrino emission, Chevalier found a small value of the alpha-parameter, where 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 then predominate. With these larger alpha-values, the temperatures of the accreting material are much lower, greater than or similar to 0.35 MeV. The result is that neutrino cooling during the flow is negligible, satisfying very 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 neutron star, at a radius less than or similar to 10(8) cm, much as in the earlier 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 pressure high enough so that neutrinos can carry off the energy.