THE FORMATION OF H-2 ON INTERSTELLAR DUST

We consider the process of H-2 formation on realistic dust grain materials. We argue that formation on amorphous H2O ice and polymeric carbonaceous materials leaves H-2 in a highly excited vibrational state, while formation on aromatic carbonaceous material gives H-2 in states of low excitation. We calculate the H-2 vibrational emission intensity arising from H-2 formation in dark clouds from which UV radiation is excluded. We show that this emission may be detectable with present technology, and should certainly be detectable at very high spectral resolution. Formation of H-2 on amorphous ice leads to local heating which may desorb local CO. We propose that the observed extinction threshold for solid CO is controlled by this process, and show that it leads to CO deposition in distinct polar and non-polar environments, as observed. Observational consequences of this prediction are described.