H2 in expanding circumstellar shells

Abstract

Hydrogen molecules are formed in the thin dense shell of interstellar gas swept up by the expanding interstellar bubble around an early-type star with a strong stellar wind. The formation of molecules on grains is not in equilibrium with photodestruction. Theoretical calculations of the column densities of H2 in rotational levels j = 0-6 agree reasonably well with Copernicus ultraviolet observations of some early-type stars. The model explains why no H2 features with column densities in the range from 10 to the 15th to 10 to the 18th power per sq cm have been observed.