Astrophysical Masers. 111. Trapped Infrared Lines and Cross-Relaxation

Abstract

Infrared line radiation trapped between a maser level and other rotational levels produces a rapid relaxation among the degenerate substates of the maser level. The rate of this relaxation is comparable to the spontaneous decay rates of the infrared transitions. This cross-relaxation has important effects on the apparent source sizes and the polarization properties of interstellar masers. It also affects the relative amplification of the components of a hyperfine-split maser line. The effect on apparent source size as pronounced when the cross-relaxation rate gamma exceeds the decay rate of the maser levels. In this limit, cross-relaxation enables maser radiation directed in a narrow solid angle to saturate the population excess in all magnetic sublevels. This property is essential to the arguments which suggest that the apparent sizes of interstellar OH and H/sub 2/O masers are much smaller than their physical sizes. Cross-relaxation has an important effect on the polarization of radiation emitted by saturated masers if the relaxation rate gamma is greater than the stimulated emission rate R. for cases in which the Zeeman splitting g OMEGA is greater than the maser line width DELTA omega the maser amplifies the Zeeman pattern. In the presence of rapid cross-relaxation ( gammamore » > R), the rates of amplification of the theta - and pi -components of the Zeeman pattern are unequal and depend upon the angle between the propagation direction and the magnetic field. For R < g OMEGA < DELTA omega , the limiting maser polarization is linear. However, cross-relaxation suppresses the growth of linear polarization until and unless the stimulated emission rate becomes as large as the relaxation rate. (auth)« less