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Intense stimulated (maser) emissions from molecules such as OH, H$_2$O, and CH$_3$OH are seen toward nearly all regions where massive stars and ultracompact HII regions form, as well as toward newly formed low-mass stars. Masers can be observed across the Milky Way and, thus, they serve as excellent signposts for sites of star formation. However, because the signals are so intense, they provide unique and extremely detailed information relating to locations, kinematics, distances, and magnetic fields.
Studies of molecular maser emission reveal many facets of the conditions under which stars form. They may help resolve why ultra-compact HII regions are often cometary and how long they survive. OH maser transitions are strongly split by the Zeeman effect, giving a powerful tool for mapping not only the magnetic field configuration around young massive stars, but also the nature of the magnetic field of the Milky Way. Some very intense H$_2$O and CH$_3$OH masers, while in the vicinity of continuum and IR sources, are found towards totally obscured locations that may be sites of strong shocks and/or contain extremely luminous and young objects. Finally, surprisingly strong maser emissions are observed from nearby galaxies; these so-called megamasers may provide important clues to the understanding of how masers operate.
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