Two dimensional numerical simulations of Supercritical Accretion Flows revisited

Abstract

We study the dynamics of super-Eddington accretion flows by performing two-dimensional radiation-hydrodynamic simulations. Compared with previous works, in this paper we include the $T_{\theta\phi}$ component of the viscous stress and consider various values of the viscous parameter $\alpha$. We find that when $T_{\theta\phi}$ is included, the rotational speed of the high-latitude flow decreases, while the density increases and decreases at the high and low latitudes, respectively. We calculate the radial profiles of inflow and outflow rates. We find that the inflow rate decreases inward, following a power law form of $\dot{M}_{\rm in}\propto r^s$. The value of $s$ depends on the magnitude of $\alpha$ and is within the range of $\sim 0.4-1.0$. Correspondingly, the radial profile of density becomes flatter compared with the case of a constant $\dot{M}(r)$. We find that the density profile can be described by $\rho(r)\propto r^{-p}$, and the value of $p$ is almost same for a wide range of $\alpha$ ranging from $\alpha=0.1$ to $0.005$. The inward decrease of inflow accretion rate is very similar to hot accretion flows, which is attributed to the mass loss in outflows. To study the origin of outflow, we analyze the convective stability of slim disk. We find that depending on the value of $\alpha$, the flow is marginally stable (when $\alpha$ is small) or unstable (when $\alpha$ is large). This is different from the case of hydrodynamical hot accretion flow where radiation is dynamically unimportant and the flow is always convectively unstable. We speculate that the reason for the difference is because radiation can stabilize convection. The origin of outflow is thus likely because of the joint function of convection and radiation, but further investigation is required.