The Cooling of Astrophysical Media by H₂ and HD

Summary

We summarize the results of recent quantum mechanical calculations of cross sections and rate coefficients for the rovibrational excitation of H₂ and HD by the principal perturbers, H, He, and H₂. These results have been used to evaluate the rate of cooling of astrophysical media by H₂ and HD molecules; these calculations are also described. The cooling of the primordial gas by rotational transitions of H₂ is considered as a special case.

All the numerical results and related software are available from http://ccp7.dur.ac.uk/.

Introduction

Molecular hydrogen is recognized as a major contributor to the cooling of astrophysical media. Its role is all the more significant under conditions, such as those which prevailed in the primordial gas, where few other coolants were present; but H₂ is also an important, sometimes the dominant coolant of low density interstellar gas, for kinetic temperatures T > 100 K. Interstellar gas can be heated to such temperatures by shock waves, by the dissipation of turbulence, or by absorbing energy from the local ultraviolet radiation field, as in photon-dominated regions.

Although the elemental abundance of deuterium is approximately 5 orders of magnitude less than that of hydrogen, it turns out that cooling by HD must often be taken into account, essentially for two reasons. First, chemical fractionation can, in media which are only partially molecular, enhance the abundance of HD, relative to that of H₂.