MRI turbulence and thermal instability in accretion disks
MRI turbulence and thermal instability in accretion disks
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DOI：
10.1093/mnras/stx564
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发表时间：
2017-03
期刊：
影响因子：
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通讯作者：
J. Ross;H. Latter;M. Tehranchi
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文献类型：
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作者：
J. Ross;H. Latter;M. Tehranchi
黑洞吸积研究中长期存在的一个难题是热不稳定性是否存在。经典理论预测环绕吸积盘是不稳定的，一些自洽的 MHD 流动模拟也是如此。然而，对强烈吸积源的观测通常无法在预期的时间尺度上表现出循环或不稳定的动态。本文检查了湍流波动是否会阻碍热不稳定性。它还询问在湍流背景下进行线性稳定性分析是否有意义。通过在热不稳定的局部盒子中进行一组 MRI 模拟并结合近似磁盘能量学的随机方程来探索这些问题。这些模型表明，磁盘的热行为明显偏离层流理论，尽管最终确实发生了热失控。我们发现磁盘温度以有偏随机游走的方式演变，而不是呈指数增长，从而产生广泛的结果，不稳定通常会延迟几个热时间。我们构建了一个统计理论来描述其中一些行为，强调“逃逸时间”及其相关概率分布的重要性。总之，湍流波动本身无法稳定圆盘，但它们可以减弱和延迟热不稳定性。
A long-standing puzzle in the study of black-hole accretion concerns the presence or not of thermal instability. Classical theory predicts the encircling accretion disk is unstable, as do some self-consistent MHD simulations of the flow. Yet observations of strongly accreting sources generally fail to exhibit cyclic or unstable dynamics on the expected timescales. This paper checks whether turbulent fluctuations impede thermal instability. It also asks if it makes sense to conduct linear stability analyses on a turbulent background. These issues are explored with a set of MRI simulations in thermally unstable local boxes in combination with stochastic equations that approximate the disk energetics. These models show that the disk's thermal behaviour deviates significantly from laminar theory, though ultimately a thermal runaway does occur. We find that the disk temperature evolves as a biased random walk, rather than increasing exponentially, and thus generates a broad spread of outcomes, with instability often delayed for several thermal times. We construct a statistical theory that describes some of this behaviour, emphasising the importance of the `escape time' and its associated probability distribution. In conclusion, turbulent fluctuations on there own cannot stabilise a disk, but they can weaken and delay thermal instability.