MASERS IN STAR-FORMING REGIONS

Maser action in the interstellar medium produces the brightest and most spectacular molecular lines that radio astronomers can study. Strong maser action was first detected in OH (at 1.6 GHz) and water (at 22 GHz) in star-forming regions, but with improvements in mm and submm-wave technology, and improved laboratory data, many new maser transitions are being identified. For methanol alone over 20 maser transitions have been identified in star-forming regions. This review summarizes recent observational developments. Masers provide the most readily detectable indicators of the formation of massive young stars, and offer our best prospect for making a complete census of star-forming regions in the Galaxy. Using radio interferometers the structure of the regions can be probed on angular scales of 1 milliarcsecond. The use of masers as probes of the physical conditions in these regions is discussed.