Saturated-State Turbulence and Structure from Thermal and Magnetorotational Instability in the ISM: Three-dimensional Numerical Simulations
Saturated-State Turbulence and Structure from Thermal and Magnetorotational Instability in the ISM: Three-dimensional Numerical Simulations
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ISM 中热不稳定和磁旋转不稳定造成的饱和态湍流和结构：三维数值模拟
DOI：
10.1086/431549
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发表时间：
2005
期刊：
影响因子：
--
通讯作者：
E. Ostriker
中科院分区：
文献类型：
--
作者：
R. Piontek;E. Ostriker
本文报道了星际扩散介质(ISM)中动力学和热力学的三维数值模拟。我们的模型是本地的，考虑了剪切的银河系旋转、磁场和真实的冷却，并分辨率≈1-200PC。这种结合使得在代表冷/暖原子ISM的云介质中研究准稳态湍流成为可能。湍流是由磁旋转不稳定性(MRI)驱动的；我们的模型首次研究了云/云间密度和温度对比度为~100的强不均匀条件下MRI的饱和状态。当体积平均密度为0.2 5~4 cm-3时，平均饱和态速度弥散的范围为8~1 km S-1，标度δv∝为0.77。因此，核磁共振很可能在驱动ISM低密度区域的湍流方面非常重要，无论是远离内银河系中面(如在高纬度观察到的那样)，还是整个遥远的外部星系(在那里平均密度下降和盘状耀斑)。在螺旋星系中，核磁共振甚至可能是抑制大半径恒星形成的关键，那里的气压可能足够高，如果没有核磁共振驱动的湍流，就会形成一个重力不稳定的冷层。不出所料，我们发现湍流影响了ISM的热结构。在我们的所有模拟中，热不稳定气体的比例随着磁共振成像的发展而增加，并且在高δv模型中处于饱和状态的比例最大。热稳定气体和不稳定气体的质量分数通常是可比的，这与观测结果一致。虽然与目前的模型相比，加入磁场的阻性耗散可以增加热不稳定气体的数量，但我们目前的结果表明，即使是高水平的湍流也不能消除热不稳定的特征，而且转变到“相连续”描述可能是不必要的。相反，我们发现温度和密度PDF被加宽(并包括对平衡的极端偏离)，但保留了经典两相描述的双峰特征。我们的介绍还包括关于团块质量分布的结果(~100M☉处的质谱峰)，饱和状态磁共振定标与单相模拟结果的比较(我们发现⟨B2⟩是独立的)，以及合成HI线剖面图的例子(表明物理团块在速度分量中不易区分，反之亦然)。
This paper reports on three-dimensional numerical simulations of dynamics and thermodynamics in the diffuse interstellar medium (ISM). Our models are local, account for sheared galactic rotation, magnetic fields, and realistic cooling, and resolve scales ≈1-200 pc. This combination permits the study of quasi-steady state turbulence in a cloudy medium representing the warm/cold atomic ISM. Turbulence is driven by the magnetorotational instability (MRI); our models are the first to study the saturated state of MRI under strongly inhomogeneous conditions, with cloud/intercloud density and temperature contrasts of ~100. For volume-averaged densities = 0.25-4 cm-3, the mean saturated-state velocity dispersion ranges from 8 to 1 km s-1, with a scaling δv ∝ -0.77. The MRI is therefore likely quite important in driving turbulence in low-density regions of the ISM, both away from the midplane in the inner Galaxy (as observed at high latitudes) and throughout the far outer Galaxy (where the mean density drops and the disk flares). The MRI may even be key to suppressing star formation at large radii in spiral galaxies, where the pressure can be high enough that without MRI-driven turbulence, a gravitationally unstable cold layer would form. As expected, we find that turbulence affects the thermal structure of the ISM. In all our simulations, the fraction of thermally unstable gas increases as the MRI develops and in the saturated state is largest in high-δv models. The mass fractions of warm stable and unstable gas are typically comparable, in agreement with observations. While inclusion of resistive dissipation of magnetic fields could enhance the amount of thermally unstable gas compared to current models, our present results indicate that even high levels of turbulence cannot wipe out the signature of thermal instability and that a shift to a "phase continuum" description is probably unwarranted. Instead, we find that temperature and density PDFs are broadened (and include extreme departures from equilibrium), but retain the bimodal character of the classical two-phase description. Our presentation also includes results on the distribution of clump masses (the mass spectrum peaks at ~100 M☉), comparisons of saturated-state MRI scalings with single-phase simulation results (we find that ⟨B2⟩ is independent of ), and examples of synthetic H I line profile maps (showing that physical clumps are not easily distinguished in velocity components and vice versa).