Molecular masers as tracers of early stellar evolution

The contemporary state of the problem of molecular masers observed in regions of active star formation is briefly reviewed. The major landmarks of the history of their discovery, observations, and interpretations are mentioned. OH masers observed in these regions are believed to be a product of disintegration of dense molecular envelopes (''cocoons'') surrounding compact HII regions of young OB-stars. They are localized at, typically, ≅1017 cm from their parent star and have there densities ≅106 cm-3, temperatures ≅100 K, magnetic fields of the order of several mG. At least in some cases they seem to be falling on the star. H2O (and, as an exception, SiO) masers are connected with still earlier stages of stellar evolution, when a very strong mass loss from the star (up to ≅10-3 - 10-2 MSun/yr) takes place. H2O maser emission is produced by gas-dynamical (possibly MHD) interaction between the stellar wind and the surrounding gas. The H2O sources may be localized much closer to their stars than the OH sources, up to ≅1014 - ≅1015 cm, and they indicate much higher densities (≅1011 cm-3) and magnetic fields (up to several G), and higher temperatures (≅100 - 1000 k). They reveal also rather lively kinematics of the circumstellar gas (up to several hundred km/s). CH3OH 1.3 cm low-gain masers in Orion with their observed dimensions and derived densities and temperatures may be gravitationally unstable and with their position around a pre-Trapezium young star cluster may perferctly be a group of protostars. Much stronger CH3OH 2.5 cm masers, recently discovered in the direction of many compact HII regions, are closely associated with the OH masers. (author). 3 figs., 47 refs