On the convective instability of hot radiative accretion flows

TL;DR

This study investigates how radiative cooling influences convective instability in hot accretion flows onto black holes, revealing that convection remains significant even with strong radiative cooling, contrary to prior expectations.

Contribution

The paper provides the first two-dimensional hydrodynamical simulations showing that radiative cooling only slightly weakens convection in hot accretion flows, challenging previous assumptions.

Findings

Convection remains strong in hot accretion flows despite radiative cooling.

Radiative cooling only slightly reduces convective instability.

The inward increase of entropy persists even with strong radiative cooling.

Abstract

How many fraction of gas available at the outer boundary can finally fall onto the black hole is an important question. It determines the observational appearance of accretion flows, and is also related with the evolution of black hole mass and spin. Previous two-dimensional hydrodynamical simulations of hot accretion flows find that the flow is convectively unstable because of its inward increase of entropy. As a result, the mass accretion rate decreases inward, i.e., only a small fraction of accretion gas can fall onto the black hole, while the rest circulates in the convective eddies or lost in convective outflows. Radiation is usually neglected in these simulations. In many cases, however, radiative cooling is important. In the regime of the luminous hot accretion flow (LHAF), radiative cooling is even stronger than the viscous dissipation. In the one dimensional case, this implies that the inward increase of entropy will become slower or the entropy even decreases inward in the case of an LHAF. We therefore expect that convective instability becomes weaker or completely disappears when radiative cooling is important. To examine the validity of this expectation, in this paper we perform two-dimensional hydrodynamical simulations of hot accretion flows with strong radiative cooling. We find that compared to the case of negligible radiation, convection only becomes slightly weaker. Even an LHAF is still strongly convectively unstable, its radial profile of accretion rate correspondingly changes little. We find the reason is that the entropy still increases inward in the two-dimensional case.