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Masers from the H2O, OH, and CH3OH molecules have been found in many hundreds of star forming regions. H2O masers arise in outflows from very young high- as well as low-mass. CH3OH masers, in contrast, are only found in regions of high-mass star formation. They are associated with objects in different evolutionary stages, from the very early to developed ultracompact HII regions; OH masers are only found associated with the latter. CH3OH maser pumping requires intense far-infared radiation and fairly high densities. This places these masers in the immediate vicinity of high-mass protostars and makes them effective signposts. Ongoing and future CH3OH maser surveys will in fact be a most effective means to find such objects.
Masers from other molecular species (NH3 and SiO) are rare and may highlight interesting objects in very short evolutionary phases.
Increasingly, trigonometric parallaxes of masers yield few percent distance determinations to far away regions.
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