Compact maser sources

The past decade has seen tremendous growth in the study of cosmic maser sources. Radio interferometers have fully resolved the structure of the sources on all angular scales, and have enabled large numbers of sources to be studied in detail for the first time. Astronomers now have a far clearer picture of the disposition of hydroxyl, water and silicon monoxide masers in star-forming regions and in circumstellar envelopes than was possible ten years ago. There have also been a major new developments, including the discovery of extremely powerful megamasers in the nuclei of distant galaxies, the discovery of strong 12 GHz methanol masers in star-forming regions and the discovery of widespread 89 GHz hydrogen cyanide masers in circumstellar envelopes. Large numbers of new maser sources have been fond using the all-sky infrared survey by the IRAS satellite to provide candidates. Maser sources of different types can be identified by their characteristic infrared emission. IRAS follow-up surveys have also revealed new types of circumstellar maser associated with stars in unusual stages of their evolution. On the theoretical front there have been major advances in our understanding of saturation and competitive gain. New mechanisms for producing polarisation in saturated masers have been found, and new pump mechanisms have been proposed for several maser lines, including the first satisfactory pump for strong water masers. This review summarises these and other important developments, concentrating mainly on the new observations and the new types of astrophysical investigation which these have made possible. The use of masers for astronomical distance measurements is also discussed.