THE DARK MOLECULAR GAS

Journal Article · Sun Jun 20 04:00:00 EDT 2010 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/716/2/1191· OSTI ID:21452938

Wolfire, Mark G ^[1]; Hollenbach, David ^[2]

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1. Department of Astronomy, University of Maryland, College Park, MD 20742-2421 (United States)
2. SETI Institute, 515 N. Whisman Road, Mountain View, CA 94043 (United States)

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{sub 2} mass. However, a significant H{sub 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{sup +}. Here, H{sub 2} self-shields or is shielded by dust from UV photodissociation, whereas CO is photodissociated. This H{sub 2} gas is 'dark' in molecular transitions because of the absence of CO and other trace molecules, and because H{sub 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 this paper, we theoretically model this dark mass and find that the fraction of the molecular mass in this dark component is remarkably constant ({approx}0.3 for average visual extinction through the cloud A-bar{sub V{approx_equal}}8) and insensitive to the incident ultraviolet radiation field strength, the internal density distribution, and the mass of the molecular cloud as long as A-bar{sub V}, or equivalently, the product of the average hydrogen nucleus column and the metallicity through the cloud, is constant. We also find that the dark mass fraction increases with decreasing A-bar{sub V}, since relatively more molecular H{sub 2} material lies outside the CO region in this case.

• OSTI ID: 21452938
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 2 Vol. 716; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
ASTROPHYSICS
CARBON COMPOUNDS
CARBON MONOXIDE
CARBON OXIDES
CHALCOGENIDES
CHEMICAL REACTIONS
COSMIC RADIATION
DECOMPOSITION
ELECTROMAGNETIC RADIATION
ELEMENTS
GAMMA RADIATION
HYDROGEN
IONIZING RADIATIONS
NONMETALS
OXIDES
OXYGEN COMPOUNDS
PHOTOCHEMICAL REACTIONS
PHOTOLYSIS
PHYSICS
RADIATIONS
ULTRAVIOLET RADIATION