Cooling of Interstellar Clouds of Neutral Hydrogen and the Abundance of Molecular Hydrogen

The structure of the radiative cooling layer behind the shock front due to interstellar cloud-collision is calculated for a collision velocity of Mach number 20 ( 16 km!sec). The magnetic field is assumed to be 0 or 10- gauss parallel to the shock front. The cooling mechanisms involved are the collision of H atoms with 0 atoms, C+, Si+ and Fe+ ions, and H2 molecules (both ortho- and para-H2) as well as electron collision with ions. Cooling times for the gas to return to its pre-shock temperature ( ) range from 6 x 10 to 6 x 10 years, depending on the various assumptions in the calculation. The variation of the abundance of molecular hydrogen in the course of cooling is investigated by taking into consideration molecular formation on the surface of graphite grains and the collisional and radiative destruction of H2 molecules. If the radiative destruction is not efficient, the abundance ratio of the molecular to atomic hydrogen is in the range 10-' 10- between cloud-cloud collisions. On the other hand, the efficient photo-destruction will reduce the ratio greatly, and an appreciable number of molecules exists only in colliding clouds. The intensity of infrared lines emitted from the shocked cloud is also calculated, and the strongest rotational line of H2 is found to be either at 6.9 (J= ) or at 9.7;i(J= ).