Collapse of Rotating Magnetized Molecular Cloud Cores and Mass Outflows
Collapse of Rotating Magnetized Molecular Cloud Cores and Mass Outflows
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DOI：
10.1086/341133
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发表时间：
2001-05
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影响因子：
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通讯作者：
K. Tomisaka
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文献类型：
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作者：
K. Tomisaka
通过轴对称磁流体动力学 (MHD) 模拟研究了旋转磁化分子云核的塌缩。由于星际气体状态方程的变化，分子云核心在塌陷过程中经历了几个阶段。在最早的等温失控塌陷（n≲1010H2cm-3）中，形成了一个假盘，并且它继续收缩，直到在中心形成一个不透明的核心。在这个圆盘中，出现了许多MHD快慢激波对，它们的波前与圆盘平行。我们假设星际气体遵循多方状态方程，其指数 Γ > 1 高于临界密度，在临界密度下，核心在来自尘埃 ncr ~ 1010 cm-3 的热辐射下变得光学厚。状态方程变硬后，在中心（第一核）形成绝热准静态核心，该核心通过径向向外的吸积激波锋面与等温收缩赝盘分开。通过磁张力的作用，角动量从磁盘中面转移到表面。表面附近角动量过剩的气体最终被喷射出来，这解释了分子双极流出。发现了两种类型的流出。当极向磁场较强时（其能量与热磁场相当），形成U形流出，其中气体主要通过距旋转轴有限距离的形状像大写字母U的区域流出。气体通过离心力和环形部件的磁压力梯度而加速。另一种是湍流流出，其中磁场线和速度场似乎是随机定向的。在这种情况下，气体总体上几乎垂直地从圆盘中移出，流出看起来像一个大写字母I。在这种情况下，虽然气体是通过离心力发射的，但沿极向磁力线作用的磁力在扩大流出方面起着重要作用。连续的质量吸积导致第一核心的准静态收缩。由于 H2 解离而发生第二次塌陷。最后，另一个质量较小的准静态核心由原子氢形成（第二个核心）。同时，人们发现第二个原子核周围喷射出另一种流出物，这似乎与光学射流或快速中性风相对应。
The collapse of rotating magnetized molecular cloud cores is studied with axisymmetric magnetohydrodynamic (MHD) simulations. Because of the change of the equation of state of the interstellar gas, molecular cloud cores experience several phases during the collapse. In the earliest isothermal runaway collapse (n ≲ 1010 H2 cm-3), a pseudodisk is formed, and it continues to contract until an opaque core is formed at the center. In this disk, a number of MHD fast and slow shock pairs appear whose wave fronts are parallel to the disk. We assume that the interstellar gas obeys a polytropic equation of state with the exponent of Γ > 1 above the critical density at which the core becomes optically thick against the thermal radiation from dusts ncr ~ 1010 cm-3. After the equation of state becomes hard, an adiabatic quasi-static core forms at the center (the first core), which is separated from the isothermal contracting pseudodisk by the accretion shock front facing radially outward. By the effect of the magnetic tension, the angular momentum is transferred from the disk midplane to the surface. The gas with an excess angular momentum near the surface is finally ejected, which explains the molecular bipolar outflow. Two types of outflows are found. When the poloidal magnetic field is strong (its energy is comparable to the thermal one), a U-shaped outflow is formed, in which gas is mainly outflowing through a region whose shape looks like a capital letter U at a finite distance from the rotation axis. The gas is accelerated by the centrifugal force and the magnetic pressure gradient of the toroidal component. The other is a turbulent outflow in which magnetic field lines and velocity fields seem to be randomly oriented. In this case, globally the gas moves out almost perpendicularly from the disk, and the outflow looks like a capital letter I. In this case, although the gas is launched by the centrifugal force, the magnetic force working along the poloidal field lines plays an important role in expanding the outflow. The continuous mass accretion leads to a quasi-static contraction of the first core. A second collapse due to the dissociation of H2 occurs in it. Finally, another less massive quasi-static core is formed by atomic hydrogen (the second core). At the same time, it is found that another outflow is ejected around the second atomic core, which seems to correspond to the optical jets or the fast neutral winds.