Formation and Excitation of Molecular Hydrogen

  • A. Dalgarno
Part of the Astrophysics and Space Science Library book series (ASSL, volume 70)


The observational data from the Copernicus satellite on the relative abundances of atomic and molecular hydrogen are generally consistent with a theory that postulates an equilibrium between formation of H2 on grain surfaces and destruction by fluorescent dissociation induced by the interstellar radiation field.

H2 is detected in excited rotational levels. The rotational populations can be explained by a combination of ultraviolet pumping and excitation during the formation process. The derived densities range from 10 cm−3 to 1000 cm−3 and the gas pressures from 103 cm −3 K to well over 104 cm −3K and there is little evidence for a uniform cloud pressure supported by an intercloud medium. In some of the clouds the derived radiation field is unusually large suggesting that the cloud is close to the parent star and presumably physically associated with it. There is also observational evidence for clouds that are sheets 0.01 pc thick with densities between 100 cm−3 and 1000 cm−3, produced presumably by shock waves associated with expanding HII regions or old supernova remnants.

The Copernicus data also reveal the presence of HD in amounts which show that there must be a source of HD in addition to grain formation, which is probably the reaction sequence H+ + D → H + D+, D+ + H2 → H+ + HD. From the measured abundance of HD, the proton density can be derived and from it the ionizing flux within the cloud. Ionizing fluxes can also be derived from the observed abundances of OH. For ξ Oph the value is 1.2×l0−l7 sec−l which if correct excludes the possibility of low energy cosmic ray ionization in the cloud.

Emission lines of the 1–0 band of H2 have been detected recently in Orion and in NGC 7027. Emission from higher vibrational levels was not detected and the origin of the excitation is uncertain. Whether it is ultraviolet pumping or collision excitation, densities of order at least 106 cm−3 appear to be required in the case of Orion, suggesting the occurrence of a shock. The H2 in NGC 7027 may be formed by negative ion reactions and not by grain catalysis. The effects of H2 formation in collapsing clouds are mentioned briefly.


Molecular Hydrogen Supernova Remnant Interstellar Cloud Rotational Quantum Number Rotational Population 
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Copyright information

© D. Reidel Publishing Company, Dordrecht, Holland 1977

Authors and Affiliations

  • A. Dalgarno
    • 1
  1. 1.Center for AstrophysicsHarvard College Observatory, Smithsonian Astrophysical ObservatoryCambridgeUSA

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