Abstract
When OH and H2O sources are found in Hii regions, a compact source of radio continuum and infrared emission is usually, perhaps always, found nearby, < 1017 cm away (approximately the conjectured size of a prestellar condensation). Whether the masers are self-excited, are amplifying the background radiation, or are themselves associated with an exciting star is clearly model-dependent, and observations have not yet given the answer. H2O masers occur less frequently than OH masers. When they occur together they appear as a cluster of compact sources of line radiation, exhibiting the same range of radial velocities, The OH sources nearly always are strongly polarized, while the H2O maser is not. Both classes of maser are probably saturated, and some care is needed in inferring the maser geometry from the observations. The period of stability of neither OH nor H2O can be much more than 103 yr, and this fact can be used as an estimator of stellar formation rate.
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