Catching the Birth of a Dark Molecular Cloud for the First Time

TL;DR

This study reports the first direct observation of a dark molecular cloud in formation, measuring the atomic to molecular hydrogen transition and providing insights into cloud formation timescales and H$_2$ formation rates.

Contribution

It presents the first direct measurement of HI/H$_2$ abundance variation during cloud formation, challenging existing rapid formation models and supporting classical star formation timescales.

Findings

Measured HI/H$_2$ ratio from 2% to 0.2% in a forming cloud.

Derived a cloud formation timescale of approximately 6 million years.

Provided constraints on H$_2$ formation rates under different ISM conditions.

Abstract

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 (HI) 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 HI self-absorption. Through a combined analysis of HI narrow self-absorption, CO emission, dust emission, and extinction, we directly measured, for the first time, the [HI]/[H$_2$] abundance varying from 2% to 0.2%, within one region. These measured HI 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$\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.