热辐射吸积流的对流不稳定性
On the convective instability of hot radiative accretion flows
摘要：
有多少在外边界的气体最终会落到黑洞上，这是一个重要的问题。它决定了吸积流的观测形态，也与黑洞质量和自旋的演化有关。以往对热吸积流的二维流体动力学模拟发现，热吸积流是对流不稳定的，因为它的熵向内增加。结果，质量吸积率向内减小，即，只有一小部分的吸积气体可以落在黑洞上，而其余的则在对流漩涡中循环或在对流流出中丢失。在这些模拟中，辐射通常被忽略。然而，在许多情况下，辐射冷却是重要的。在发光热吸积流（LHAF）的制度，辐射冷却甚至比粘性耗散更强。在一维情况下，这意味着熵的向内增加将变得更慢，或者在LHAF的情况下熵甚至向内减小。因此，我们预计，对流不稳定性变得更弱或完全消失时，辐射冷却是重要的。为了检验这一预期的有效性，在本文中，我们进行了二维流体动力学模拟的热吸积流与强辐射冷却。我们发现，与可忽略辐射的情况相比，对流只是稍微变弱。即使LHAF仍然是强对流不稳定的，它的吸积率的径向轮廓相应地变化不大。我们发现原因是在二维情况下熵仍然向内增加。
英文摘要：
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.
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