[论文解读] The early stages of molecular cloud evolution in the magnetised interstellar medium: clump morphology and evolution

Robi Banerjee, Enrique Vázquez-Semadeni+2 more|arXiv (Cornell University)|Aug 7, 2008

Astrophysics and Star Formation Studies被引用 1

一句话总结

本研究利用三维磁流体动力学模拟，研究在汇聚流作用下弱磁化、热双稳态的暖中性介质（WNM）中团块的形成过程。结果表明，团块通过热不稳定性压缩形成，主要通过湍流相变界面的吸积增长，且与周围环境保持压力平衡，导致其内部马赫数与流入流马赫数一致；磁场与速度方向对齐并增强团块的稳定性。

ABSTRACT

Abridged: We study the properties of clumps formed in three-dimensional weakly magnetized magneto-hydrodynamic simulations of converging flows in the thermally bistable, warm neutral medium (WNM). We find that: (1) Similarly to the situation in the classical two-phase medium, cold, dense clumps form through dynamically-triggered thermal instability in the compressed layer between the convergent flows, and are often characterised by a sharp density jump at their boundaries though not always. (2) However, the clumps are bounded by phase-transition fronts rather than by contact discontinuities, and thus they grow in size and mass mainly by accretion of WNM material through their boundaries. (3) The clump boundaries generally consist of thin layers of thermally unstable gas, but these layers are often widened by the turbulence, and penetrate deep into the clumps. (4) The clumps are approximately in both ram and thermal pressure balance with their surroundings, a condition which causes their internal Mach numbers to be comparable to the bulk Mach number of the colliding WNM flows. (5) The clumps typically have mean temperatures 20 < T < 50 K, corresponding to the wide range of densities they contain (20 < n < 5000 pcc) under a nearly-isothermal equation of state. (6) The turbulent ram pressure fluctuations of the WNM induce density fluctuations that then serve as seeds for local gravitational collapse within the clumps. (7) The velocity and magnetic fields tend to be aligned with each other within the clumps, although both are significantly fluctuating, suggesting that the velocity tends to stretch and align the magnetic field with it. (8) The typical mean field strength in the clumps is a few times larger than that in the WNM. (9) The magnetic field strength has a mean value of B ~ 6 mu G ...

研究动机与目标

• 理解磁化、热双稳态星际介质中分子云团块的早期演化过程。
• 研究磁场与湍流如何影响汇聚流中团块的形态与生长机制。
• 确定热不稳定性与冲压压力涨落对团块内引力坍缩的触发作用。
• 考察团块内磁场相对于周围暖中性介质的取向与强度。

提出的方法

• 在热双稳态暖中性介质（WNM）中开展三维、弱磁化、理想磁流体动力学（MHD）模拟，研究汇聚流过程。
• 采用近等温状态方程模拟星际介质，以捕捉热气体与冷气体之间的相变。
• 通过压缩界面层的动力触发热不稳定性，追踪致密团块的形成与演化。
• 分析团块边界结构，以区分接触间断面与相变界面。
• 测量内部马赫数、压力平衡状态及磁场相对于速度场的对齐情况。
• 量化湍流冲压压力涨落及其在团块内局部引力坍缩中的作用。

实验结果

研究问题

• RQ1 在热双稳态WNM中，弱磁场存在下，汇聚流如何影响团块的形成与演化？
• RQ2 团块的主要生长机制是通过相变界面的吸积，还是其他过程？
• RQ3 团块在多大程度上与周围环境保持压力平衡，这种平衡如何影响其内部动力学？
• RQ4 团块内磁场与速度场如何对齐，其对应的磁场强度如何？
• RQ5 湍流冲压压力涨落如何在团块内触发引力坍缩？

主要发现

• 团块通过汇聚流之间压缩层的热触发不稳定性形成，其边界常表现出明显的密度跃迁。
• 团块主要通过湍流相变界面从暖中性介质（WNM）吸积物质而增长，而非通过接触间断面。
• 团块边界由薄而热不稳定的层构成，常被湍流拓宽并穿透。
• 团块与周围环境在冲压和热压力上均近似保持平衡，导致其内部马赫数与流入WNM的总体马赫数相当。
• 团块的平均温度在20 K至50 K之间，密度范围为20至5000 pcc，符合近等温状态方程。
• WNM中的湍流冲压压力涨落引发密度涨落，这些涨落成为团块内局部引力坍缩的种子。
• 团块内速度场与磁场显著对齐，速度场使磁场发生拉伸与对齐。
• 团块的平均磁场强度为几倍于周围WNM，典型值约为B ~ 6 μG。