Maser polarization through anisotropic pumping

¹ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
e-mail: boy.lankhaar@chalmers.se
² Leiden Observatory, Leiden University, Post Office Box 9513, 2300 RA Leiden, Netherlands
³ INAF – Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius, Italy

Received: 29 October 2023
Accepted: 24 December 2023

Abstract

Context. Polarized emission from masers is an excellent tool to study magnetic fields in maser sources. The linear polarization of the majority of masers is understood as an interplay of maser saturation and anisotropic pumping. However, for the latter mechanism, no quantitative modeling has been presented yet.

Aims. We aim to construct a comprehensive model of maser polarization, including quantitative modeling of both anisotropic pumping and the effects of maser saturation on the polarization of masers.

Methods. We extended regular (isotropic) 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 yielded the anisotropic pumping and decay parameters, which were subsequently used in one-dimensional proper maser polarization radiative transfer modeling.

Results. We present the anisotropic pumping parameters for a variety of transitions from class I CH₃OH masers, H₂O masers, and SiO masers. SiO masers are highly anisotropically pumped due to them occurring in the vicinity of a late-type star, which irradiates the maser region with a strong directional radiation field. Class I CH₃OH masers and H₂O masers occur in association with shocks, and they are modestly anisotropically pumped due to the anisotropy of the excitation region.

Conclusions. 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 CH₃OH masers. The common 22 GHz H₂O maser has a relatively weak anisotropic pumping; in contrast, we predict that the 183 GHz H₂O 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.