On the ortho-H2/para-H2 ratio in molecular clouds

Details On the ortho-H2/para-H2 ratio in molecular clouds On the ortho-H2/para-H2 ratio in molecular clouds

Jul 1984

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1984mnras.209...25f&link_type=abstract

Monthly Notices of the Royal Astronomical Society (ISSN 0035-8711), vol. 209, July 1, 1984, p. 25-31.

Astronomy and Astrophysics

Astronomy

56

Abundance, Interstellar Gas, Molecular Clouds, Ortho Hydrogen, Para Hydrogen, Proton Density (Concentration), Time Dependence

Scientific paper

The relative abundance of ortho and para modifications of H2 in interstellar clouds is considered. It is shown that, due to a derived proton exchange reaction, the relative abundances may be expected to vary significantly over the lifetime of a molecular cloud. The conversion of the ortho to the para form is more rapid when the proton density in the cloud is higher. In dense clouds subject to a cosmic ray primary ionization rate of 10 to the -17th/s, thermal equilibrium is established over a time scale of roughly ten million yr, and the time scale is proportional to the primary ionization rate. In more diffuse clouds, where photoprocesses are important, the time scale is about a million yr and the ionization rate is 10 to the -17th/s. It is concluded that the ortho/para abundance ratio in a given cloud depends on its chemical history; neither a 'normal' nor a thermal equilibrium ratio may be safely assumed.

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