16 - Cosmic ray induced photodissociation and photoionization of interstellar molecules

Summary

Introduction

The interior of a dense molecular cloud is efficiently shielded from the ultraviolet photons of the interstellar radiation field by the grains. Accordingly, the effects of the ultraviolet photons on the physical and chemical states of dense clouds are in general, neglected. However, several internal sources of ultraviolet photons may be present, including the emission from embedded stars and internal shocks driven by mass loss from young stars. Here the discussion is focused on a diffuse source of internal ultraviolet photons arising from energetic cosmic rays capable of penetrating dense clouds. Cosmic ray particles with energies between 10 and 100 MeV ionize molecular hydrogen in the interior of the clouds and generate secondary electrons with a mean energy of around 30 eV (Cravens and Dalgarno 1978). Because the fractional ionization is generally low, the secondary electrons degrade mainly through excitations of various electronic states of H₂. Typical excitation energies are 10–15 eV. Ionization of H₂ by the secondary electrons may also occur. The subsequent decays of the electronically excited states of H₂ produce ultraviolet photons within the clouds.

The idea of molecular hydrogen emission inside dense clouds was invoked by Prasad and Tarafdar (1983) to explain the large abundance of atomic carbon which exists in several molecular clouds. Observations of emission in the ³P₁–³P₀ and ³P₂–³P₁ fine-structure lines of neutral atomic carbon in dense interstellar clouds (Phillips and Huggins 1981, Keene et al. 1985, Zmuidzinas, Betz and Goldhaber 1986) have demonstrated the existence of C with abundances relative to CO exceeding 0.1.