Constraining Theories of SiO Maser Polarization: Analysis of a $pi$/2 EVPA Change

TL;DR

This study analyzes SiO maser polarization features with EVPA rotations to test and constrain different theoretical models of maser polarization mechanisms in the environments of AGB stars.

Contribution

It provides observational constraints on maser polarization theories by analyzing EVPA rotations and polarization dependence across multiple epochs of TX Cam.

Findings

Observed EVPA rotation consistent with specific theoretical models.

Identified dependencies of polarization fraction on magnetic field angle.

Discussed potential causes for differences between observations and models.

Abstract

The full theory of polarized SiO maser emission from the near-circumstellar environment of Asymptotic Giant Branch stars has been the subject of debate, with theories ranging from classical Zeeman origins to predominantly non-Zeeman anisotropic excitation or propagation effects. Features with an internal electric vector position angle (EVPA) rotation of $\sim \pi /2$ offer unique constraints on theoretical models. In this work, results are presented for one such feature that persisted across five epochs of SiO $\nu=1, J=1-0$ VLBA observations of TX Cam. We examine the fit to the predicted dependence of linear polarization and EVPA on angle ($\theta$) between the line of sight and the magnetic field against theoretical models. We also present results on the dependence of $m_c$ on $\theta$ and their theoretical implications. Finally, we discuss potential causes of the observed differences, and continuing work.