A circumstellar maser is maser emission from molecules in the circumstellar envelopes of red giants, and also of some young stars, of spectral types M, S, and C. The kinds of masers found in circumstellar space correlate closely with the spectral type of the associated stars.
In M stars, where oxygen is more plentiful than carbon, masers from oxygen-bearing molecules, such as OH (hydroxyl), H2O (water), and SiO (silicon dioxide), are found. OH 1612-MHz masers occur at the outer edge of circumstellar envelopes, where interstellar ultraviolet radiation produces OH from the dissociation of H2O. The line profile of OH 1612-MHz masers has a characteristic double peak, interpreted as emission from the approaching and receding parts of the expanding shell. H2O and SiO masers occur closer to the star and show intensity variations on a timescale of a month to a year. In carbon stars, where carbon is more abundant than oxygen, masers from HCN (hydrogen cyanide) and SiS (silicon sulfide) molecules are found. HCN masers serve as a useful probe of the inner region of the envelope in carbon stars, whereas SiS masers provide data on the outer envelope. In evolved (red giant) S stars, where oxygen is as abundant as carbon, SiO masers are frequently found. They have also been discovered, however, in several star-forming regions, including the Orion Complex, and the W 51 and Sagittarius B2 molecular clouds.