2018ApJ...867...13Z

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 x 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.