Abstract
HYDROGEN molecules are expected to have very short lifetimes (∼ 1,000 yr) in the interstellar radiation field so the overall interstellar density of H2 must be very low1. This view is supported by the observational evidence2 that the upper limit of the H2 density is one-tenth that of atomic hydrogen, and also by the overall correlation3 found between optical extinction and measurements of hydrogen atoms at 21 cm wavelength. Nevertheless, localized regions of the sky have been observed in which there is a negative correlation of H atom density and optical extinction3–6. Moreover, Heiles7 has reported normal OH emission from regions which show no H atom emission at all at 21 cm. These results have been interpreted to mean that the H atoms have been converted into H2 molecules in the presence of the dust. Observations8 of OB clusters and associations which involve large amounts of extinction and from which there is little Hα emission have been interpreted9 to mean that hydrogen molecules have been formed in the gas by surface reactions on grains and that these gaseous molecules have condensed on to the grain surfaces, so becoming stable against photodissociation. We discuss here the formation of hydrogen molecules in situations relevant to these observations.
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STECHER, T., WILLIAMS, D. Localized Interstellar Molecular Hydrogen. Nature 219, 1349–1350 (1968). https://doi.org/10.1038/2191349a0
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DOI: https://doi.org/10.1038/2191349a0
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