The surprisingly small impact of magnetic fields on the inner accretion flow of Sagittarius A* fueled by stellar winds
The surprisingly small impact of magnetic fields on the inner accretion flow of Sagittarius A* fueled by stellar winds
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DOI：
10.1093/mnras/stz3605
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发表时间：
2020-01
影响因子：
4.8
通讯作者：
S. Ressler;E. Quataert;J. Stone
中科院分区：
文献类型：
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作者：
S. Ressler;E. Quataert;J. Stone
我们研究了三维磁流体动力学（MHD）模拟的吸积到人马座A* 通过轨道沃尔夫-拉叶星的磁化风的流动结构。这些模拟覆盖了半径超过3个数量级，达到$\约$300的引力半径，只有一个约束较差的参数（恒星风中的磁场）。即使对于具有相对弱的磁场的风（例如，等离子体$\beta$ $\sim $$10^6 $），流入气体中的通量冻结/压缩在其到达事件视界之前将场放大到$\beta$ $\sim$ few。总的来说，动力学、吸积速率和球形平均流动剖面（例如，密度，速度）在我们的MHD模拟是非常相似的类似流体动力学模拟。我们把这归因于恒星风提供的角动量的广泛分布，即使没有太多的角动量传输，也会产生吸积。我们发现，磁旋转不稳定性并不重要，因为i）强磁场被通量冻结/压缩放大，ii）气体的快速流入/流出时间和低效的辐射冷却排除了循环。磁场的主要作用是驱动流体动力学中不存在的极地外流。在我们的模拟中发现的吸积流的动力学状态是不同的旋转支持环面作为初始条件的地平线尺度模拟，这可能会影响模型被用来解释事件地平线望远镜和重力观测的Sgr A*。
We study the flow structure in 3D magnetohydrodynamic (MHD) simulations of accretion onto Sagittarius A* via the magnetized winds of the orbiting Wolf-Rayet stars. These simulations cover over 3 orders of magnitude in radius to reach $\approx$ 300 gravitational radii, with only one poorly constrained parameter (the magnetic field in the stellar winds). Even for winds with relatively weak magnetic fields (e.g., plasma $\beta$ $\sim$ $10^6$), flux freezing/compression in the inflowing gas amplifies the field to $\beta$ $\sim$ few well before it reaches the event horizon. Overall, the dynamics, accretion rate, and spherically averaged flow profiles (e.g., density, velocity) in our MHD simulations are remarkably similar to analogous hydrodynamic simulations. We attribute this to the broad distribution of angular momentum provided by the stellar winds, which sources accretion even absent much angular momentum transport. We find that the magneto-rotational instability is not important because of i) strong magnetic fields that are amplified by flux freezing/compression, and ii) the rapid inflow/outflow times of the gas and inefficient radiative cooling preclude circularization. The primary effect of magnetic fields is that they drive a polar outflow that is absent in hydrodynamics. The dynamical state of the accretion flow found in our simulations is unlike the rotationally supported tori used as initial conditions in horizon scale simulations, which could have implications for models being used to interpret Event Horizon Telescope and GRAVITY observations of Sgr A*.