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Molecular Hydrogen Emission from Photodissociation Regions

Published online by Cambridge University Press:  12 April 2016

Tetsuo Hasegawa
Tetsuo Hasegawa*
Affiliation:
Institute of Astronomy, The University of Tokyo2-21-1 Osawa, Mitaka, Tokyo 181, Japan

Abstract

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We review new observational and theoretical developments of the understanding of the H2 infrared emission in the last 5 years since the discovery of the fluorescent emission in NGC 2023. An excitation analysis of H2 in a variety of Galactic sources has revealed that in many sources the excitation is expressed as a mixture of thermal and fluorescent components. This finding is in good agreement with theories of photodissociation regions, in which the population of H2 changes its character from pure fluorescence to thermal as the density of the region increases. The ortho/para abundance ratio of the fluorescent H2 is observed to lie within a limited range of 1.1 — 1.8 which is well reproduced by depth-dependent model calculations of the ultraviolet excitation and dissociation of H2 molecules. This may be understood as due to the independent self shielding of each of the ortho- and para-H2, rather than the ortho/para abundance ratio of the predissociated H2, a low formation temperature of H2 on grains, or gas phase interchange reactions. A laser emission of molecular hydrogen discovered in the planetary nebula NGC7027 further demonstrates the nonthermal nature of the H2 emission in photodissociation regions.

Type
I. Molecular Clouds, Star Formation And HII Regions
Information
International Astronomical Union Colloquium , Volume 120: Structure and Dynamics of the Interstellar Medium , 1989 , pp. 24 - 31
Copyright
Copyright © Springer-Verlag 1989

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