Physics of Cosmic Masers

Abstract

Natural masers, which emit powerful narrow-band radiation in molecular microwave lines, were discovered about 20 years ago. They draw the astrophysicist's attention in two ways: 1) as a unique physical phenomenon and 2) as very informative probes of physical conditions in those astrophysical objects (very young and very old stars) with which they are intimately connected. In this paper, the first aspect of masers will be stressed, though some consideration will be given to the astrophysical aspect as well. The limited space here will not allow a complete discussion. For a more systematic account, refer to reviews on astrophysical masers by Litvak 1974; Strelnitskij 1974; Kegel 1975; Cook 1977; Moran 1976; Reid and Moran 1981; Elitzur 1982. I shall limit myself only to several topics of general character, which, to me personally, seem to be particularly important for understanding the fundamental principles of cosmic maser physics. I shall then try to illustrate these general principles by the example of H2O masers in star formation regions. These masers, in my opinion, are the most intriguing of all the cosmic masers known at present. And (to make the subjectivity of the report complete!) I shall lay special stress on the results obtained by our group in the last few years.