期刊
ASTROPHYSICAL JOURNAL
卷 867, 期 1, 页码 -出版社
IOP PUBLISHING LTD
DOI: 10.3847/1538-4357/aad571
关键词
ISM: atoms; ISM: clouds; ISM: molecules
资金
- National Key R&D Program of China [2017YFA0402600]
- National Natural Science Foundation of China [11725313, 11690024]
- International Partnership Program of Chinese Academy of Sciences [114A11KYSB20160008]
- National Science Foundation [AST-1100968]
- CAS Interdisciplinary Innovation Team program
- ERC [321302]
- European Research Council (ERC) [321302] Funding Source: European Research Council (ERC)
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 H-2 molecule, is a key step in cosmic structure formation en route to stars. Quantifying H-2 formation in space is difficult, due to the confusion in the emission of atomic hydrogen (H I) and the lack of a H-2 signal from the cold ISM. Here we present the discovery of a rare, isolated dark cloud currently undergoing H-2 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]/[H-2] 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 similar to 6 x 10(6) 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 H-2 formation rate, under various ISM conditions.
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