Models of Vertically Stratified Two-Phase ISM Disks with MRI-Driven Turbulence
Models of Vertically Stratified Two-Phase ISM Disks with MRI-Driven Turbulence
复制标题
具有 MRI 驱动湍流的垂直分层两相 ISM 盘模型
DOI：
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发表时间：
2007
期刊：
影响因子：
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通讯作者：
E. Ostriker
中科院分区：
文献类型：
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作者：
R. Piontek;E. Ostriker
我们已经进行了随时间变化的星际介质（ISM）的数值模拟，其中占银河系的剪切和磁场，垂直重力，和原子气体的辐射冷却功能。这使我们能够研究多云，垂直分层磁盘的磁旋转不稳定性（MRI）。在以前的非分层模型中，我们发现，热不稳定性与MRI驱动的湍流和银河系剪切相互作用，产生一个网络的冷，密集，对流云嵌入在一个温暖的弥漫的环境介质。有显着的热不稳定的气体，但密度和温度分布保留了经典的两相ISM的双峰。当平均垂直磁场为0.26 μG时，不考虑气体总面密度和垂直重力水平，热磁压的中面比β = 0.3-0.6。我们分析了垂直分布的密度和各种压力项，并解决什么支持ISM垂直。所有的模式成为差分分层的温度，只有当冷的质量分数是小的湍流混合保持一个大的冷介质尺度高度。冷气体的湍流速度也随着冷质量分数的降低而增加，但在中平面附近通常较低（~1-3 km s-1）;在高平面处增加到>5 km s-1。温暖气体中的湍流振幅较高。所有型号的中心热压力相似，即使总重量在施加的垂直重力范围内变化7倍;在更高重力的型号中，增加的重量由增加的磁压力梯度支撑。近似垂直平衡适用于所有模型。最后，我们认为，在星系盘的外部，MRI可能能够防止自引力不稳定性的发展，从而抑制星星的形成，即使是冷气体的存在。
We have performed time-dependent numerical simulations of the interstellar medium (ISM) which account for galactic shear and magnetic fields, vertical gravity, and a radiative cooling function for atomic gas. This allows us to study the magnetorotational instability (MRI) in cloudy, vertically stratified disks. As in previous unstratified models, we find that thermal instability interacts with MRI-driven turbulence and galactic shear to produce a network of cold, dense, filamentary clouds embedded in a warm diffuse ambient medium. There is significant thermally unstable gas, but the density and temperature distributions retain the twin peaks of the classical two-phase ISM. Independent of the total gas surface density and vertical gravity levels adopted, the midplane ratios of thermal to magnetic pressure are β = 0.3-0.6, when the mean vertical magnetic field is 0.26 μG. We analyze the vertical distributions of density and various pressure terms and address what supports the ISM vertically. All models become differentially stratified by temperature; only when the cold mass fraction is small does turbulent mixing maintain a large cold-medium scale height. Turbulent velocities of the cold gas also increase as the cold mass fraction decreases, but are generally low (~1-3 km s-1) near the midplane; they increase to >5 km s-1 at high . Turbulent amplitudes are higher in the warm gas. The central thermal pressure is similar for all models even though the total weight varies by a factor of 7 for a range of imposed vertical gravity; in higher gravity models the increased weight is supported by increased magnetic pressure gradients. Approximate vertical equilibrium holds for all models. Finally, we argue that in the outer parts of galactic disks, MRI is likely able to prevent the development of self-gravitating instabilities and hence suppress star formation, even if cold gas is present.