The cooling of astrophysical media by H2

We have computed the rate of radiative energy loss from a gas containing H2 which is collisionally excited by H, He and H2. For this purpose, we have used the results of recent quantum mechanical calculations of the cross-sections for rovibrational transitions between all the energy levels up to approximately 20 000 K above the ground state. The temperature dependence of the rate coefficients for collisional de-excitation is found to be well represented by a simple functional form. The cooling function has been evaluated in steady state on a grid covering a wide range of values of the gas density and temperature, the atomic to molecular hydrogen density ratio, and the ortho- to para-H2 ratio. A fortran program is provided for the purpose of rapid numerical interpolation to any desired set of values of these parameters. The properties of the cooling function are discussed, as are the time-scales required for the attainment of steady state.