The detailed mechanisms responsible for the polarization of SiO masers toward late-type evolved stars continues to be the subject of debate. Possible primary sources include the local magnetic field or anisotropic pumping, while additional polarization may arise due to conversion from linear to circular modes through scattering or Faraday rotation. Some maser features display an internal rotation in their electric vector position angle (EVPA) by 𝚫χ∼π/2 across their extent; such features can provide robust constraints on SiO maser polarization theories as they allow inference of the angle, θ, between the magnetic field and the line of sight. Masers remain an important high-resolution probe of the near-circumstellar environment of these objects and reducing uncertainties in maser polarization theory is critical to our understanding of the astrophysics of these regions. In this thesis defense, I will describe two research studies of SiO EVPA reversal features. The first of these (Tobin et al., 2018; 2019) concerns the analysis of a single SiO ν = 1, J = 1−0 maser feature displaying an EVPA reversal of 𝚫χ > π/2 toward the Mira variable, TX Cam, as observed by the Very Long Baseline Array in five epochs forming part of a prior larger imaging sequence. While we find that the fractional linear polarization profile, ml(θ), across the EVPA rotation feature is consistent with the asymptotic theoretical maser polarization solution derived by Goldreich et al. (1973) for saturated masers in the limit of small Zeeman splitting, with θ passing through the critical Van Vleck angle, the corresponding EVPA profile χ(θ) rotates too smoothly to arise from this mechanism alone. I will discuss this and other results, as well as our investigation of possible sources of this discrepancy. The second research study concerns development of a new theoretical formalism for maser polarization radiative transport more general than several previous approaches and specifically including optional Faraday rotation. This formalism uses the radiative transfer solution method of Landi Degl’Innocenti (1987) to integrate the Stokes {I,Q,U,V} parameters. This new formalism allows a more complete analysis of the observational results discussed above. Initial results will be presented from a one-dimensional numerical implementation of this maser polarization radiative transfer code. The two studies described here provide important new constraints on maser polarization theory and open new observational and theoretical avenues for further exploration of this area of research.
