The Dark Molecular Gas
Mark G. Wolfire, David Hollenbach, and Christopher F. McKee

TL;DR
This paper models the 'dark' molecular gas in interstellar clouds, revealing it constitutes about 30% of the total molecular mass and is largely unaffected by UV radiation or cloud properties, but increases with lower extinction.
Contribution
It provides a theoretical model quantifying the dark molecular gas fraction, showing its constancy under various conditions and its dependence on mean visual extinction.
Findings
Dark molecular gas fraction is approximately 30%.
Dark mass fraction is insensitive to UV radiation and cloud density.
Dark mass increases as mean A_V decreases.
Abstract
The mass of molecular gas in an interstellar cloud is often measured using line emission from low rotational levels of CO, which are sensitive to the CO mass, and then scaling to the assumed molecular hydrogen H_2 mass. However, a significant H_2 mass may lie outside the CO region, in the outer regions of the molecular cloud where the gas phase carbon resides in C or C+. Here, H_2 self-shields or is shielded by dust from UV photodissociation, where as CO is photodissociated. This H_2 gas is "dark" in molecular transitions because of the absence of CO and other trace molecules, and because H_2 emits so weakly at temperatures 10 K < T < 100 K typical of this molecular component. This component has been indirectly observed through other tracers of mass such as gamma rays produced in cosmic ray collisions with the gas and far-infrared/submillimeter wavelength dust continuum radiation. In…
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