Magnetohydrodynamics of Cloud Collisions in a Multiphase Interstellar Medium
Magnetohydrodynamics of Cloud Collisions in a Multiphase Interstellar Medium
复制标题
多相星际介质中云碰撞的磁流体动力学
DOI：
10.1086/306599
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发表时间：
1998
期刊：
影响因子：
--
通讯作者：
T. Jones
中科院分区：
文献类型：
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作者：
F. Miniati;D. Ryu;A. Ferrara;T. Jones
我们通过二维磁流体动力学 (MHD) 数值模拟开始研究磁场的作用，从而扩展了先前对星际云碰撞物理学的研究。特别是，我们研究了与我们之前的研究类似的等质量、轻度超音速、扩散云之间的正面碰撞。这里，我们包括一个中等磁场，对应于 β=pg/pb=4，以及两个极限场几何形状，其中场线平行（对齐）和垂直（横向）于碰撞云运动。我们探索绝热和辐射 (η=τrad/τcoll≃0.38) 情况，并模拟通过云间介质中的先前运动演化的云之间的碰撞。除了进化相同云的碰撞（对称情况）之外，我们还研究最初相同但具有不同进化年龄的云的碰撞（非对称情况）。与流体动力学 (HD) 情况相比，磁场可以显着改变碰撞结果，具体取决于它们的几何形状。 (1) 在对齐的情况下，绝热碰撞与 HD 对应物一样，无论对称性如何，都具有很大的破坏性。然而，与 HD 计算不同，当考虑辐射过程时，即使在不对称情况下也会发生部分聚结。 (2) 在横向情况下，磁场的影响更加显着，未演化云和演化云之间存在显着差异。 （最初相邻的）未演化云之间的碰撞几乎不受磁场的影响。然而，在碰撞前演化过程中与磁化云间气体的相互作用在云前产生了一个具有非常高磁能的区域。在具有横向场几何形状的演化云之间的碰撞中，该区域就像保险杠一样，防止云之间的直接接触并最终反转它们的运动。弹性，定义为每个云的最终动能与初始动能之比，在我们考虑的情况下约为 0.5-0.6。这种行为在绝热和辐射情况下都存在。
We extend previous studies of the physics of interstellar cloud collisions by beginning an investigation of the role of magnetic fields through two-dimensional magnetohydrodynamical (MHD) numerical simulations. In particular, we study head-on collisions between equal mass, mildly supersonic, diffuse clouds similar to those in our previous study. Here we include a moderate magnetic field, corresponding to β=pg/pb=4, and two limiting field geometries, with the field lines parallel (aligned) and perpendicular (transverse) to the colliding cloud motion. We explore both adiabatic and radiative (η=τrad/τcoll≃0.38) cases, and we simulate collisions between clouds evolved through prior motion in the intercloud medium. In addition to the collision of evolved identical clouds (symmetric cases), we also study collisions of clouds that are initially identical but have different evolutionary ages (asymmetric cases). Depending on their geometry, magnetic fields can significantly alter the outcome of the collisions compared to the hydrodynamic (HD) case. (1) In the aligned case, adiabatic collisions, like their HD counterparts, are very disruptive independently of the symmetry. However, when radiative processes are taken into account, partial coalescence takes place even in the asymmetric case, unlike the HD calculations. (2) In the transverse case, the effects of the magnetic field are even more dramatic, with remarkable differences between unevolved and evolved clouds. Collisions between (initially adjacent) unevolved clouds are almost unaffected by magnetic fields. However, the interaction with the magnetized intercloud gas during precollision evolution produces a region of very high magnetic energy in front of the cloud. In collisions between evolved clouds with transverse field geometry, this region acts like a bumper, preventing direct contact between the clouds and eventually reversing their motion. The elasticity, defined as the ratio of the final to the initial kinetic energy of each cloud, is about 0.5-0.6 in the cases we considered. This behavior is found in both adiabatic and radiative cases.