[论文解读] Stellar Collisions in Young Clusters: Formation of (Very) Massive Stars?

Marc Freitag|ArXiv.org|Nov 26, 2007

Stellar, planetary, and galactic studies参考文献 2被引用 4

一句话总结

本文研究了年轻致密星团中恒星碰撞作为形成（极）大质量恒星（VMS，M* > 150 M☉）的机制，通过碰撞暴增过程实现。该研究结合恒星动力学与流体动力学，表明高密度（>10⁷ pc⁻³）和引力聚焦效应至关重要，碰撞率受双星相互作用与气体吸积增强；关键结果为，仅在极为丰裕的星团中，持续的碰撞增长才能形成VMS，而气体驱动的核心坍缩可能使这一过程在气体消散前的嵌入阶段即得以实现。

ABSTRACT

In young star clusters, the density can be high enough and the velocity dispersion low enough for stars to collide and merge with a significant probability. This has been suggested as a possible way to build up the high-mass portion of the stellar mass function and as a mechanism leading to the formation of one or two very massive stars (M > 150 Msun) through a collisional runaway. I quickly review the standard theory of stellar collisions, covering both the stellar dynamics of dense clusters and the hydrodynamics of encounters between stars. The conditions for collisions to take place at a significant rate are relatively well understood for idealised spherical cluster models without initial mass segregation, devoid of gas and composed of main-sequence (MS) stars. In this simplified situation, 2-body relaxation drives core collapse through mass segregation and a collisional phase ensues if the core collapse time is shorter than the MS lifetime of the most massive stars initially present. The outcome of this phase is still highly uncertain. A more realistic situation is that of a cluster still containing large amounts of interstellar gas from which stars are accreting. As stellar masses increase, the central regions of the cluster contracts. This little-explored mechanism can potentially lead to very high stellar densities but it is likely that, except for very rich systems, the contraction is halted by few-body interactions before collisions set in. A complete picture, combining both scenarios, will need to address many uncertainties, including the role of cluster sub-structure, the dynamical effect of interstellar gas, non-MS stars and the structure and evolution of merged stars.

研究动机与目标

• 评估年轻致密星团中的恒星碰撞是否能解释（极）大质量恒星（M* > 150 M☉）的形成。
• 评估引力聚焦、双星相互作用和潮汐捕获在增强碰撞率方面的作用，使其超越几何截面。
• 研究残余星际气体和吸积驱动的核心收缩如何在气体消散前将恒星密度提升至碰撞水平。
• 识别碰撞暴增能够形成单个或少数VMS而非连续质量函数的条件。

提出的方法

• 使用双体弛豫 timescale 和碰撞 timescale 公式（t_coll ∝ 1/(nσ_rel(r₁+r₂)²)）估算理想球形星团中的碰撞率。
• 应用引力聚焦修正项（1 + G(m₁+m₂)/(σ_rel²(r₁+r₂)))，以考虑低速度弥散下截面增大的效应。
• 利用相互作用 timescale（t_bin,inter ∝ 1/(nσ_rel a m_bin)）建模双星相互作用，评估共振与非共振相遇的影响。
• 评估潮汐捕获和共包层阶段在形成快速合并的紧密双星中的作用。
• 通过 R_sys ∝ M_sys⁻³ 分析吸积驱动的核心收缩，其中气体吸积导致系统收缩。
• 使用蒙特卡洛模拟和SPH模型研究合并结果及碰撞增长中的自限效应。

实验结果

研究问题

• RQ1 在年轻星团中，何种条件下恒星碰撞以显著速率发生，特别是对大质量恒星而言？
• RQ2 双星相互作用和潮汐捕获如何影响有效碰撞率，使其超越简单的几何截面？
• RQ3 嵌入星团中星际气体的吸积是否能驱动核心收缩至足以触发碰撞暴增的密度？
• RQ4 在碰撞情景下，什么限制了极大质量恒星（VMS）的生长？为何连续质量函数不太可能形成？
• RQ5 残余气体的存在如何影响核心坍缩和碰撞形成的时间与效率？

主要发现

• 恒星碰撞率仅在密度超过10⁷ pc⁻³（对10 M☉恒星）和10⁶ pc⁻³（对120 M☉恒星）时才变得显著，要求极端核心条件。
• 在典型星团速度弥散（<1000 km s⁻¹）下，引力聚焦主导于几何截面，显著提高碰撞概率。
• 双星相互作用可使有效碰撞率比单星相遇提高3–5倍，尤其通过共振相互作用。
• 吸积驱动的核心收缩可在丰裕系统（N_sys ≫ 100）中将恒星密度提升至碰撞水平，可能在气体消散前实现VMS形成。
• 若合并 timescale 低于VMS的热 timescale，碰撞暴增将自限，阻止进一步收缩与生长。
• 仅可能通过此机制形成一个或两个VMS，且在初始质量函数中m_max与m_ra之间存在显著间隙，原因在于合并机会有限。