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 time scale of 6 \times 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 .