Astrophysical masers are both a fascinating phenomenon and a powerful tool for probing a wide range of different astrophysical environments. Following the initial discovery of ‘mysterium’ in the early 1960s, later identified as maser emission from the OH molecule, maser emission has been identified from many different molecules. Strong emission is detected at centimeter and millimeter wavelengths from OH, H2O, SiO and CH3OH molecules, with weaker emission from higher frequency transitions of these molecules and from other molecules such as CH, NH3 and CHOH.
Considerable progress was made during the 1990s in understanding and observing the complexities of the regions in which the masers are found. Masers are observed in the interstellar medium and star-formation regions, in the expanding winds from evolved stars, in the compressed shells of supernovae remnants, in comets and in or near the nuclei of galaxies. Systematic surveys have led to the detections of many hundreds of maser sources. Much progress has been made in understanding the pumping mechanisms of the different maser transitions. For some masers, such as OH masers and the ‘class II’ methanol masers (associated with ultra-compact HII regions), the population inversion requires radiative pumping from far-infrared photons and the masers are closely associated with cool dust – either from molecular clouds, or in the atmospheres of evolved stars. Other masers, including ‘class I’ methanol masers (not associated with ultra-compact HII regions) are likely to be collisionally pumped.
In the interstellar medium, OH, H2O and methanol masers are associated with ultra-compact HII regions, embedded infrared objects, hot molecular cores, Herbig-Haro objects and outflows from young stellar objects. High resolution observations from long baseline arrays, such as MERLIN in the UK, the Very Long Baseline Array in the US, and the European VLBI Network (EVN), have shown highly complex and varied structures with spherical expansion, bipolar outflows, disks and rings, bow-shocks and infall. By combining the spatial imaging capabilities with high velocity resolution dynamical information it is possible to image and interpret the three-dimensional distribution of masers in great detail, and in some cases to obtain proper motions and accurate distance estimates.
Since the 1980s masers have been used to study the outflowing winds from evolved stars. These winds play a critical role in stellar evolution as they return more than 40 per cent of a star’s mass back to the interstellar medium for recycling. The classical winds of the so-called OH/IR stars have a spherical structure and the masers probe the winds at different radii providing information on the densities, temperatures and mass-loss rates in the outflows. These compact and bright sources also provide ideal samples for studying stellar dynamics and Galactic structure. Much more recently, masers have been discovered in post-AGB stars. These objects are in a very short-lived evolutionary stage during which the stars evolve from the AGB to become planetary nebulae. The maser and infrared properties can be used to identify post-AGB stars. Planetary nebulae can reveal many different forms. While some appear circular, others have the shapes of butterfly wings, or show bipolar or elliptical structures. The onset of these structures appears to begin in the post-AGB phase and masers provide an excellent means for studying the rapid changes in wind morphologies and dynamics. Of particular interest has been the recent detection of high-velocity water jets in several post-AGB stars. Such jets may indicate a very early stage in the transition away from the AGB when a hot fast wind from the central star switches on.