HOT HYDROGEN IN DIFFUSE CLOUDS

Laboratory evidence suggests that recombination of adsorbed radicals may cause an abrupt temperature excursion of a dust grain to about 1000 K. One consequence of this is the rapid desorption of adsorbed H2 molecules with excitation temperatures of this magnitude. We compute the consequences of injection of hot H2 into cold diffuse interstellar gas at a rate of 1% of the canonical H2 formation rate. We find that the level populations of H2 in J = 3, 4, and 5 are close to observed values, and that the abundances of CH+ and OH formed in reactions with hot hydrogen are close to the values obtained from observations of diffuse clouds.