[论文解读] Formation of proto-cluster: a virialized structure from gravo-turbulent collapse I. Simulation of cluster formation in collapsing molecular cloud

Yueh-Ning Lee, P. Hennebelle|arXiv (Cornell University)|Mar 25, 2016

Astrophysics and Star Formation Studies参考文献 72被引用 33

一句话总结

本文通过高分辨率磁流体动力学（MHD）模拟研究了坍缩分子云中气态原星团的形成过程，表明原星团起源于引力-湍流坍缩，并达到全局维里平衡。关键结果是原星团的质量-大小关系（$R \sim M^{0.38}$ 至 $M^{0.5}$）与观测结果一致，其能量平衡由湍流和旋转支持，证实其为稳定、自调节的恒星星团前体。

ABSTRACT

Stars are often observed to form in clusters. It is therefore important to understand how such a region of concentrated mass is assembled out of the diffuse medium and its properties eventually prescribe the important physical mechanisms and determine the characteristics of the stellar cluster. We study the formation of a gaseous proto-cluster inside a molecular cloud by performing high resolution MHD simulations and associate its internal properties to those of the parent cloud by varying the level of the initial turbulence of the cloud, with a view to better characterize the subsequent stellar cluster formation. The gaseous proto-cluster is formed out of global collapse of a molecular cloud, and has non-negligible rotation due to angular momentum conservation during the collapse of the object. Most of the star formation occurs in this region which occupies only a small volume fraction of the whole cloud. We identify such regions in simulations and compare the gas and sink particles to observations. The gaseous proto-cluster inferred from simulation results present a mass-size relation that is compatible with observations. We stress that the stellar cluster radius, although clearly correlated with the gas cluster radius, depends sensitively on its definition. Energy analysis is performed to confirm that the gaseous proto-cluster is a product of gravo-turbulent reprocessing and that the support of turbulent and rotational energy against self-gravity yields a state of global virial equilibrium although collapse is occurring at smaller scale and the cluster is forming stars actively. This object then serves as the antecedent of the stellar cluster, to which the energy properties are passed on.

研究动机与目标

• 理解在坍缩分子云中，致密气态原星团如何作为恒星星团的前体形成。
• 确定控制原星团结构与稳定性的物理机制，特别是湍流、引力与角动量之间的相互作用。
• 将原星团的性质（质量、大小、能量）与母分子云的性质联系起来，尤其是初始湍流水平。
• 验证原星团作为维里平衡结构的特性，其能量属性继承自母云，是后续恒星形成的基础。

提出的方法

• 在自引力作用下，对具有不同初始湍流水平的分子云坍缩进行高分辨率磁流体动力学（MHD）模拟。
• 通过气体动力学特征（吸积与旋转）和吸积粒子分布，利用速度与密度阈值定义椭球形区域，识别原星团。
• 采用特征半径如 $(R^2H)^{1/3}$ 和速度量度如 $W_0 = \int \vec{v} \cdot \vec{n} dm / \| \int \vec{v} \times \vec{n} dm \|$，以稳健地确定原星团的大小及吸积-旋转过渡区域。
• 通过计算气体与吸积粒子的动能、引力势能及总比能量，开展能量分析，以评估维里平衡状态。
• 采用分段幂律拟合方法，根据速度与密度梯度区分吸积包层与原星团核心。
• 将模拟结果与嵌入式星团及气态团块的观测数据在质量-大小关系与能量平衡方面进行比较。

实验结果

研究问题

• RQ1 分子云的初始湍流水平在多大程度上影响气态原星团的形成与结构？
• RQ2 湍流、引力、旋转等物理机制如何决定原星团的质量-大小关系？其与观测到的团块相比有何异同？
• RQ3 尽管存在恒星形成与小尺度坍缩，气态原星团是否仍处于维里平衡状态？
• RQ4 原星团的性质（质量、大小、能量）在多大程度上反映了母分子云的初始条件？
• RQ5 在复杂湍流环境中，如何利用运动学与动力学标准稳健地定义原星团边界？

主要发现

• 气态原星团通过分子云的全局引力坍缩形成，其显著旋转源于角动量守恒。
• 原星团表现出 $R \sim M^{0.38}$ 至 $M^{0.5}$ 的质量-大小关系，与ATLASGAL等巡天的观测数据一致。
• 能量分析证实原星团处于全局维里平衡状态，湍流能与旋转能共同平衡引力能。
• 原星团是占据母云极小体积分数的致密、正在形成恒星的独立结构，但其继承并重新处理了大尺度云体的能量属性。
• 采用不同方法定义原星团大小（如使用 $R$、$\sqrt{RH}$ 或 $(R^2H)^{1/3}$）均得到一致结果，证实了识别算法的稳健性。
• 原星团是恒星星团的前驱，其能量状态可传递至最终的恒星系统，即使模型中未包含恒星反馈效应。