Collision between dissimilar clouds: stability of the bow-shock, and the formation of pre-stellar cores
Collision between dissimilar clouds: stability of the bow-shock, and the formation of pre-stellar cores
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DOI：
10.1111/j.1365-2966.2010.16541.x
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发表时间：
2010-07
影响因子：
4.8
通讯作者：
S. Anathpindika
中科院分区：
文献类型：
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作者：
S. Anathpindika
我们用光滑粒子流体力学(SPH)模拟了两个物理上不同的云之间的迎头碰撞，并研究了碰撞后弓激波的动力学稳定性。激波阵面似乎容易受到许多流体动力学不稳定性的影响，如开尔文-亥姆霍兹不稳定性、瑞利-泰勒不稳定性和薄壳不稳定性。我们最初通过逐渐增加SPH粒子的数量从而增加分辨率来执行模拟的三种实现。观察到，缺乏足够的分辨率往往会抑制激波锋面上扰动的增长，进而抑制碎裂。因此，较差的分辨率有利于连续结构的形成，而瑞利-泰勒不稳定性的特征指状结构几乎看不到。然而，大体物理特征似乎与分辨率无关，三种情况下的弓激波对TSI都变得不稳定，一旦触发，TSI迅速增长，最终成为非线性。尽管如此，TSI的增长速度似乎比分析预测的要慢得多。TSI有助于弯曲激波阵面内气体动能的耗散，通过内部激波，最终弓激波坍塌形成沿碰撞轴定向的暗条，在该暗条中形成了前恒星核。弓激波的机翼对TSI也是不稳定的，因此看起来像是丝状的，有几个团块，这些团块在几倍于105年的时间内迅速破坏。这种过渡性团块的形成可以解释为什么通常在丝状区域发现的无星核心的出现。从完全稳定的初始条件开始，所报道的弓激波中的不稳定性纯粹是由数值噪声引起的。然而，激波阵面内的湍流似乎在局部抑制了引力不稳定性。我们证明，全球引力收缩可以产生湍流，湍流大到足以容纳前恒星核心。最后，我们还讨论了碰撞前云之间的初始密度对比度进一步增加一个数量级的情况，同时它们以低得多的速度碰撞。
We use smoothed particle hydrodynamics (SPH) to simulate a head-on collision between two physically dissimilar clouds, and investigate the dynamical stability of the post-collision bow-shock. The shock-front appears susceptible to a number of hydrodynamical instabilities such as the Kelvin―Helmholtz instability, the Rayleigh-Taylor instability, and the thin shell instability (TSI). We initially perform three realizations of the simulation by progressively increasing the number of SPH particles, and therefore the resolution. It is observed that lack of sufficient resolution tends to damp the growth of perturbations on the shock-front which in turn suppresses fragmentation. Thus poorer resolution favours formation of contiguous structure while the fingers, characteristic of the Rayleigh-Taylor instability, hardly appear. However, the gross physical features seem independent of resolution, the bow-shock in each of the three cases becomes unstable to the TSI which once triggered, grows rapidly and eventually becomes non-linear. Albeit, the TSI appears to grow at a rate much slower than that predicted analytically. The TSI contributes to dissipation of gas kinetic energy within the curved shock-front via internal shocking, eventually the bow-shock collapses to form a filament oriented along the collision axis; pre-stellar cores form in this filament. The wings of the bow-shock are also unstable to the TSI and consequently appear filamentary with a few clumps, which rapidly disrupt over a period of a few times 10 5 yr. Formation of such transitional clumps may explain the occurrence of starless cores often found in filamentary regions. Having commenced with perfectly stable initial conditions, the reported instabilities in the bow-shock originate purely through numerical noise. Turbulence within the shock-front, however, appears to locally suppress the gravitational instability. We demonstrate that a global gravitational contraction can produce turbulence, large enough to contain pre-stellar cores. Finally, we also discuss a case in which the initial density contrast between the pre-collision clouds was further increased by an order of magnitude, while colliding them at a much lower velocity.