[论文解读] Star cluster progenitors are dynamically decoupled from their parent molecular clouds

N. Peretto, A. J. Rigby+4 more|arXiv (Cornell University)|May 4, 2023

Spectroscopy and Laser ApplicationsChemistry被引用 3

一句话总结

本研究揭示，星团前体——分子云中密集的、数秒差距量级的团块——在动力学上与周围环境相分离，其密度剖面更陡峭（ρ ∝ r⁻²），速度 dispersion 剖面更平坦（σ ∝ r⁰），均不同于周围气体。利用赫歇尔、GRS 和 IRAM 的多波段数据，作者发现这些团块具有自引力性并正在进行整体引力坍缩，表明星团形成发生在原本稳定的分子云中一个独立的动力学状态内。

ABSTRACT

The formation of stellar clusters dictates the pace at which galaxies evolve, and solving the question of their formation will undoubtedly lead to a better understanding of the Universe as a whole. While it is well known that star clusters form within parsec-scale over-densities of interstellar molecular gas called clumps, it is, however, unclear whether these clumps represent the high-density tip of a continuous gaseous flow that gradually leads towards the formation of stars, or a transition within the gas physical properties. Here, we present a unique analysis of a sample of 27 infrared dark clouds embedded within 24 individual molecular clouds that combine a large set of observations, allowing us to compute the mass and velocity dispersion profiles of each, from the scale of tens of parsecs down to the scale of tenths of a parsec. These profiles reveal that the vast majority of the clouds, if not all, are consistent with being self-gravitating on all scales, and that the clumps, on parsec-scale, are often dynamically decoupled from their surrounding molecular clouds, exhibiting steeper density profiles ($ρ\propto r^{-2}$) and flat velocity dispersion profiles ($σ\propto r^0$), clearly departing from Larson's relations. These findings suggest that the formation of star clusters correspond to a transition regime within the properties of the self-gravitating molecular gas. We propose that this transition regime is one that corresponds to the gravitational collapse of parsec-scale clumps within otherwise stable molecular clouds.

研究动机与目标

• 确定星团前体团块在其母分子云中的动力学状态。
• 研究团块是否代表气体的连续流动，或分子云内物理性质的转变。
• 评估分子云中低恒星形成效率是否源于气体物理性质的动力学转变。
• 检验团块是否为引力束缚并独立于其宿主云发生坍缩。
• 阐明整体云结构与运动学在调控恒星形成效率中的作用。

提出的方法

• 为 27 个红外暗云（IRDCs）构建质量与速度 dispersion 剖面，覆盖 24 个分子云，尺度范围从 ~0.2 pc 至 ~30 pc。
• 结合赫歇尔的 H₂ 柱密度图、GRS 的 13 CO(1-0) 数据立方体和 IRAM 的 N₂H⁺(1-0) 数据立方体，以探测稀薄与致密气体组分。
• 对质量面密度（γ）和速度 dispersion（β）的径向剖面拟合一维幂律模型，以量化结构与动力学特性。
• 计算维里比值，以评估各云内不同半径处的引力束缚性与稳定性。
• 利用空间分辨光谱与柱密度等高线，定义剖面分析的感兴趣区域。
• 应用统计建模，将观测剖面与拉尔森关系及自引力模型的理论预期进行比较。

Figure 1: Images of SDC18.888-0.476. (a): top - Spitzer 8 $\mu$ m ; middle- H 2 column density from Herschel observations; bottom - N 2 H + (1-0) integrated emission. The contours are identical in all panels, and are those of the H 2 column density image. The yellow contour corresponds to $N_{\rm{N_

实验结果

研究问题

• RQ1 星团前体团块是否与母分子云动力学耦合，还是代表一种独立的动力学状态？
• RQ2 观测到的团块密度与速度 dispersion 剖面是否偏离拉尔森关系，表明物理条件发生转变？
• RQ3 分子云中低恒星形成效率是否与致密团块与大尺度云结构的动力学解耦有关？
• RQ4 在稀薄与致密区域之间，速度 dispersion 剖面（β 从 ~0.5 到 ~0）发生转变的起源是什么？
• RQ5 观测剖面是否可由嵌入稳定云中的团块整体引力坍缩来解释，而非整个云的坍缩？

主要发现

• 绝大多数分子云在从 ~0.2 pc 至 ~30 pc 的所有尺度上均表现出自引力性。
• 致密团块的质量面密度剖面更陡峭（γ ≈ 2，ρ ∝ r⁻²），而稀薄云区则较平缓（γ ≈ 1）。
• 速度 dispersion 剖面显示明显转变：稀薄区域 β ≈ 0.5，致密团块 β ≈ 0，表明速度 dispersion 剖面平坦。
• 团块中观测到的对拉尔森关系（ρ ∝ r⁻²，σ ∝ r⁰.5）的偏离，证实其与周围气体的动力学解耦。
• 数据最支持一种情景：数秒差距量级的团块经历整体引力坍缩，而其宿主云保持稳定。
• 在 >2 pc 尺度上速度 dispersion 剖面的广泛差异，表明云演化存在多样的演化状态或外部影响。

Figure 2: Sketch of molecular cloud configuration and relevant tracers. Diffuse gas (represented in green) is traced by 13 CO(1-0) and dust continuum. However, only the former is able to disentangle the emission of multiple clouds along the line of sight by segmenting them in velocity space. In this