Masers as chemical probes of star-forming regions

Maser emission from the OH, H2O, and CH3OH molecules is found toward hundreds of molecular cloud cores with recent and on-going star formation. The often very high intensities of the observed emission imply substantial abundances of the maser molecules, which are difficult or impossible to explain with ion-molecule gas phase chemistry. We summarize relevant observations which show that OH and Class II CH3OH masers form in the envelopes of ultracompact HII regions in which the high abundances of these molecules are produced by evaporation of methanol and water from icy dust grain mantles. In contrast, H2O masers form in outflows from protostellar objects, where high H2O abundances are produced by endothermic neutral-neutral reactions in hot postshock gas. Finally, we briefly discuss Class I CH3OH masers, which also arise in interstellar outflows.