Maser polarization through anisotropic pumping

(Abridged) Polarized emission from masers is an excellent tool to study magnetic fields in maser sources. The linear polarization of most masers is understood as an interplay of maser saturation and anisotropic pumping. However, for the latter mechanism, no quantitative modeling has been presented yet. We present a comprehensive model of maser polarization, including quantitative modeling of both anisotropic pumping and the effects of maser saturation on the polarization of masers. We extend regular maser excitation modeling with a dimension that describes the molecular population alignments, as well as including the linear polarization dimension to the radiative transfer. The results of the excitation analysis yield the anisotropic pumping and decay parameters, that are subsequently used in one-dimensional proper maser polarization radiative transfer modeling. We present the anisotropic pumping parameters for a variety of transitions from class I CH$_3$OH masers, H$_2$O masers and SiO masers. SiO masers are highly anisotropically pumped due to them occurring in the vicinity of a late-type star, that irradiates the maser region with a strong directional radiation field. Class I CH$_3$OH masers and H$_2$O masers occur in association with shocks, and they are modestly anisotropically pumped due to the anisotropy of the excitation region. Our modeling constitutes the first quantitative constraints on the anisotropic pumping of masers. We find that anisotropic pumping can explain the high polarization yields of SiO masers, as well as the modest polarization of unsaturated class I CH$_3$OH masers. We predict that the $183$ GHz H$_2$O maser is strongly anisotropically pumped. Finally, we outline a mechanism through which non-Zeeman circular polarization is produced, when the magnetic field changes direction along the propagation through an anisotropically pumped maser.
Comment: 22 pages, 10 figures, accepted to A&A