The transfer of polarized radiation in astrophysical masers in the presence of magnetic fields

Abstract

Polarized emission from astrophysical masers can provide important information about physical conditions in both star forming and circumstellar environments. Calculations are performed which give the spectra of the emergent maser radiation for different magnetic fields, radiative intensities, decay rates, and pumping rates thought to be appropriate in astrophysical masing regions. Results are obtained which for the first time make it possible to study the linear polarization of radiatively saturated masers when the stimulated emission rate and the Zeeman frequency are comparable, a regime which is of importance for nonparamagnetic masing molecules (i.e. $\rm H\sb2 O$, SiO, $\rm CH\sb3 OH$, $\rm H\sb2 CO$) in astrophysics. The change in both the amount of linear polarization and its position angle as a function of the stimulated emission rate is followed through this regime.

A striking feature of maser emission from OH molecules is the tendency for one of the circular polarizations to dominate the emission spectrum. The two mechanisms which may explain these observations both involve a masing gas in a velocity gradient. These mechanisms are studied by calculating the emergent spectra of the radiation without making the "large velocity gradient" approximation which has always been made in previous calculations of masing in a velocity gradient. Eliminating this approximation leads to previously unsuspected narrow line profiles even in regions where large velocity gradients are present, as well as to polarized spectra that resemble those of the observations.