Pumping of H II/OH masers - IR line overlaps and collisional excitation by H2

The first quantitative model of H II/OH maser sources in which both radiative and collisional processes are taken into account in a coherent manner is presented. The model incorporates recently computed rates of rotational excitation of OH by para-H2 and the effects of 'thermal' overlap of the far infrared lines of OH. First the characteristics of collisional pumping in the absence of line overlap, then the global effects when overlap is included are illustrated. When both mechanisms operate, the observed properties of ground and excited state maser emission of H II/OH sources can be reproduced, whereas this is not the case when only one of the mechanisms is included. It is shown that thermal far infrared radiation from local dust effectively quenches the masers if the temperature of the dust is comparable to or greater than the kinetic temperature of the gas.