THE EFFECTS OF FLOW-INHOMOGENEITIES ON MOLECULAR CLOUD FORMATION: LOCAL VERSUS GLOBAL COLLAPSE
THE EFFECTS OF FLOW-INHOMOGENEITIES ON MOLECULAR CLOUD FORMATION: LOCAL VERSUS GLOBAL COLLAPSE
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流动不均匀性对分子云形成的影响：局部崩溃与全局崩溃
DOI：
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发表时间：
2013
期刊：
影响因子：
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通讯作者：
F. Heitsch
中科院分区：
文献类型：
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作者：
J. Carroll;A. Frank;F. Heitsch
来自局部恒星形成区域的观测证据表明，恒星形成发生在分子云形成之后不久，甚至在分子云形成期间。分子云在大规模会聚流中形成的模型已经确定了驱动必要的快速分裂的物理机制。他们还指出，全球引力坍缩导致了分子云中的超音速湍流。以前的云形成模型主要关注湍流的产生、引力坍缩、磁场和反馈。在这里，我们将探讨气流中的结构对产生的云和随后的引力坍缩的影响。我们比较了两种极端情况，一种是两条光滑的溪流之间的碰撞，另一种是含有小团块的溪流。我们发现有结构的收敛流导致局部引力坍缩（“岩心形成”）的延迟。因此，云有更多的时间来积累质量，最终导致强烈的全球坍塌，从而导致高核形成率。均匀的汇聚流在早期就会在流体动力学上破碎，导致局部重力坍缩的快速发生和整体的低岩心形成速率。这也反映在岩心质量分布上：均匀初始条件比块状初始条件产生更多的低质量岩心。动能（Ek）和引力能（Eg）预算表明，只有在Ek > Eg的情况下才会阻止坍缩，这在大尺度的平滑流动中发生，而在小尺度的团块流动中发生。当Ek≈Eg时，我们观察到这些尺度上的引力坍缩。化学丰度变化的特征在气相和恒星群中演变不同。对于平滑的流动，形成的星云混合得很好，而它的恒星群保留了更多关于初始金属丰度的信息。对于团块流，气相混合得不太好，而恒星群已经失去了关于其起源的大部分信息。
Observational evidence from local star-forming regions mandates that star formation occurs shortly after, or even during, molecular cloud formation. Models of molecular cloud formation in large-scale converging flows have identified the physical mechanisms driving the necessary rapid fragmentation. They also point to global gravitational collapse driving supersonic turbulence in molecular clouds. Previous cloud formation models have focused on turbulence generation, gravitational collapse, magnetic fields, and feedback. Here, we explore the effect of structure in the flow on the resulting clouds and the ensuing gravitational collapse. We compare two extreme cases, one with a collision between two smooth streams, and one with streams containing small clumps. We find that structured converging flows lead to a delay of local gravitational collapse (“core formation”). Hence, the cloud has more time to accumulate mass, eventually leading to a strong global collapse, and thus to a high core formation rate. Uniform converging flows fragment hydrodynamically early on, leading to the rapid onset of local gravitational collapse and an overall low core formation rate. This is also mirrored in the core mass distribution: the uniform initial conditions lead to more low-mass cores than the clumpy initial conditions. Kinetic (Ek) and gravitational energy (Eg) budgets suggest that collapse is only prevented for Ek ≫ Eg, which occurs for large scales in the smooth flow, and for small scales for the clumpy flow. Whenever Ek ≈ Eg, we observe gravitational collapse on those scales. Signatures of chemical abundance variations evolve differently for the gas phase and for the stellar population. For smooth flows, the forming cloud is well mixed, while its stellar population retains more information about the initial metallicities. For clumpy flows, the gas phase is less well mixed, while the stellar population has lost most of the information about its origin.