Masers and Star Formation

Abstract

The physics of astronomical masers and their connection with star-forming regions are reviewed. For a better understanding of the subject, a qualitative discussion is given of the basic concepts about masers. These are: Amplification, saturation, thermalization, beaming, apparent size, variability, line width and polarization. The difference between laboratory and astronomical masers is discussed and a few examples of the usefulness of astronomical masers are given. The basic requirements for the construction of a maser model are presented and the accuracy with which the various inputs are known is commented on. A qualitative discussion is given of the most common models, which are collisional and radiative. Specific pumping mechanisms for OH and H2O masers in star-forming regions are presented and criticized. The current status of the observations of these masers is reviewed and the implications on the theoretical models is discussed.