New Insights into the Physical Conditions and Internal Structure of a Candidate Proto-globular Cluster
New Insights into the Physical Conditions and Internal Structure of a Candidate Proto-globular Cluster
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DOI：
10.3847/1538-4357/ab0d1e
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发表时间：
2019-03
期刊：
影响因子：
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通讯作者：
Molly K. Finn;Kelsey E. Johnson;C. Brogan;C. Wilson;R. Indebetouw;W. Harris;J. Kamenetzky;A. Bemis
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文献类型：
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作者：
Molly K. Finn;Kelsey E. Johnson;C. Brogan;C. Wilson;R. Indebetouw;W. Harris;J. Kamenetzky;A. Bemis
我们目前的分辨率为100.“1（100 pc）阿塔卡玛大型毫米/亚毫米阵列观测的分子云中确定的合并的星系与潜在的形成一个球状星团，绰号为“Fioracacker”。由于该区域的星星形成还没有开始到一个可感知的水平，这片云提供了一个例子，说明在恒星形成和破坏纳塔尔物理条件之前，球状星团的出生环境可能是什么样子。利用~（12）CO（2-1），~（12）CO（3-2），~（13）CO（2-1），HCN（4-3），HCO+（4-3）等分子线的辐射，我们能够分辨出该云团的结构，发现其特征半径为22 pc，质量为（1-9）× 106 M ~（-1）。我们还对12 CO/13 CO和H2/12 CO的丰度比进行了限制。基于质量的计算，我们确定了该区域常用的CO到H2的转换因子在空间上是变化的，平均值在cm−2（K km s−1）−1范围内。我们证明，如果云是束缚的（正如其明亮、紧凑的形态所暗示的那样），则需要超过P/k > 108 K cm−3的外部压力。这与理论预期一致，即球状星团的形成需要高压环境，远高于银河系中的典型值。云及其周围物质的位置-速度图表明，这种高压可能是由细丝碰撞产生的冲压压力产生的。柱密度的径向分布可以用高斯分布和Bonnor-Ebert分布两者拟合。如果Bonnor-Ebert拟合被认为是云的物理结构的指示，它将意味着云是重力稳定和压力限制的。HCN和HCO+在该区域的相对线强度也表明，这些分子线可以用作一个集群的演化阶段的示踪剂。
We present ∼0.″1 resolution (∼10 pc) Atacama Large Millimeter/submillimeter Array observations of a molecular cloud identified in the merging Antennae galaxies with the potential to form a globular cluster, nicknamed the “Firecracker.” Since star formation has not yet begun at an appreciable level in this region, this cloud provides an example of what the birth environment of a globular cluster may have looked like before stars formed and disrupted the natal physical conditions. Using emission from 12CO(2–1), 12CO(3–2), 13CO(2–1), HCN(4–3), and HCO+(4–3) molecular lines, we are able to resolve the cloud’s structure and find that it has a characteristic radius of 22 pc and a mass of (1–9) × 106 M⊙. We also put constraints on the abundance ratios 12CO/13CO and H2/12CO. Based on the calculation of the mass, we determine that the commonly used CO-to-H2 conversion factor in this region varies spatially, with average values in the range cm−2 (K km s−1)−1. We demonstrate that if the cloud is bound (as is circumstantially suggested by its bright, compact morphology), an external pressure in excess of P/k > 108 K cm−3 is required. This would be consistent with theoretical expectations that globular cluster formation requires high-pressure environments, much higher than typical values found in the Milky Way. The position–velocity diagram of the cloud and its surrounding material suggests that this high pressure may be produced by ram pressure from the collision of filaments. The radial profile of the column density can be fit with both a Gaussian and a Bonnor–Ebert profile. If the Bonnor–Ebert fit is taken to be indicative of the cloud’s physical structure, it would imply the cloud is gravitationally stable and pressure-confined. The relative line strengths of HCN and HCO+ in this region also suggest that these molecular lines can be used as a tracer for the evolutionary stage of a cluster.