Catching the Birth of a Dark Molecular Cloud for the First Time
Catching the Birth of a Dark Molecular Cloud for the First Time
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DOI：
10.3847/1538-4357/aad571
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发表时间：
2018-10
期刊：
影响因子：
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通讯作者：
P. Zuo;Di Li;J. Peek;J. Peek;Q. Chang;Xia Zhang;N. Chapman;P. Goldsmith;Zhi-Yu Zhang
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文献类型：
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作者：
P. Zuo;Di Li;J. Peek;J. Peek;Q. Chang;Xia Zhang;N. Chapman;P. Goldsmith;Zhi-Yu Zhang
星际介质（ISM）中的大部分氢是以原子形式存在的。从原子到分子的转变，特别是H2分子的形成，是宇宙结构形成过程中通往恒星的关键一步。由于原子氢（H i）的发射混乱，并且缺乏来自冷ISM的H2信号，因此在空间中量化H2的形成是困难的。在这里，我们发现了一个罕见的，孤立的黑云，目前正在形成H2，由一个突出的氢自吸收“环”证明。通过对H i窄自吸收、CO排放、粉尘排放和消光的综合分析，我们首次在一个区域内直接测量到[H i]/[H2]丰度在2% ~ 0.2%之间变化。这些测量到的氢丰度比通常假设的原行星盘模型的初始条件要高几个数量级。任何快速云形成模型都不可能产生如此低的氢原子丰度。我们得出的云形成时间尺度为~ 6 × 106年，与全球银河系恒星形成速率一致，并且比快速恒星形成模型更倾向于经典恒星形成图像。我们的测量也有助于在各种ISM条件下限制H2的形成速率。
The majority of hydrogen in the interstellar medium (ISM) is in atomic form. The transition from atoms to molecules and, in particular, the formation of the H2 molecule, is a key step in cosmic structure formation en route to stars. Quantifying H2 formation in space is difficult, due to the confusion in the emission of atomic hydrogen (H i) and the lack of a H2 signal from the cold ISM. Here we present the discovery of a rare, isolated dark cloud currently undergoing H2 formation, as evidenced by a prominent “ring” of H i self-absorption. Through a combined analysis of H i narrow self-absorption, CO emission, dust emission, and extinction, we directly measured, for the first time, the [H i]/[H2] abundance varying from 2% to 0.2%, within one region. These measured H i abundances are orders of magnitude higher than usually assumed initial conditions for protoplanetary disk models. None of the fast cloud formation model could produce such low atomic hydrogen abundance. We derived a cloud formation timescale of ∼6 × 106 years, consistent with the global Galactic star formation rate, and favoring the classical star formation picture over fast star formation models. Our measurements also help constrain the H2 formation rate, under various ISM conditions.