Constraining Theories of Polarized SiO Maser Transport: Multi-epoch Analysis of a π/2 Electric Vector Rotation Feature

Abstract

The detailed polarization mechanisms of SiO masers originating from the near circumstellar environment of Asymptotic Giant Branch stars are not yet definitively known. Prevailing theories are broadly classified as either Zeeman or non-Zeeman in origin, the latter including effects such as anisotropic pumping or anisotropic resonant scattering. The predicted behavior of the linear and circular polarization fractions and electric vector position angle vary by theory. In particular, individual maser features that exhibit a rotation in linear polarization of ∼ π/2 as a function of frequency over their extent can be utilized as a test of several maser polarization transport theories. In this paper, we analyze one SiO (ν = 1, J = 1 − 0) maser feature toward the Mira variable, TX Cam that exhibits this internal polarization rotation and persists across five epochs (spanning ∼ 3 months). We compare our results to the predictions by several maser polarization theories and find that the linear polarization across the feature is consistent with a geometric effect for a saturated maser originating when the angle between the projected magnetic field and the line of sight (θ) crosses the Van Vleck angle θ F ∼ 55 • . However, the electric vector position angle (EVPA) exhibits a smooth rotation across the spatial extent of the feature rather than the expected abrupt π/2 flip. We discuss possible explanations for this discrepancy and alternative theories. Circular polarization across the feature is also analyzed and it is the most accurately described by Zeeman effects giving rise to a circular polarization fraction of the form m c ∝ ∼ cos θ.

Characterizing maser polarization: effects of saturation, anisotropic pumping, and hyperfine structure

“…2), a blunt 90 • flip around the magic angle is characteristic of masers where the rate of stimulated emission is on the same order as the magnetic precession rate. Tobin et al (2019) estimate log(R/gΩ) ∼ −1, and our simulations of a magic angle flip at these conditions (Fig. 2, log(R/gΩ) = −1) show a similar blunted magic angle flip in the polarization angle.…”

“…For stronger masers, where 100 gΩ/R 0, we also observed a flip in the polarization angle, but this flip is gradual (over ∼10 • ), and does not predict zero polarization at the magic angle. The 90 • flip feature of J = 1−0 SiO masers has recently be investigated by Tobin et al (2019). Tobin et al (2019) analyze the changing polarization fraction and angle of SiO maser spots across a clump.…”

“…The 90 • flip feature of J = 1−0 SiO masers has recently be investigated by Tobin et al (2019). Tobin et al (2019) analyze the changing polarization fraction and angle of SiO maser spots across a clump. They assume a gradually changing propagation angle with the projected angular distance.…”

SiO bands in infrared spectra of late spectral stars

“…Çäåñü óìåñòíî îò ìå òèòü, ÷òî äî ñèõ ïîð îñíîâ íàÿ èç âåñ òíîñòü ýòîé ìî ëå êó ëû îïðå äå ëÿ ëàñü âñå aeå åå ñïî ñîá íîñ òüþ ôîð ìè ðî âàòü ìà çåð íûå ëè íèè, êî òî ðûå ïðî äîë aeà þò àê òèâ íî èñ ïîëü çî âàòü ñÿ â àñ òðî ôè çè ÷åñ êèõ èññëå äî âà íè ÿõ (ñì. ðà áî òû [34,36,40] è ññûë êè â íèõ).…”

Analysis of the first overtone bands of isotopologues of CO and SiO in stellar spectra

“…Silicon monoxide, SiO, is an important constituent of these astrophysical environments. The millemetrerange maser lines of SiO have been extensively used in the astrophysical applications, see Tobin et al (2019), Vlemmings et al (2011), and Yoon et al (2014 for more details. Thermal lines of SiO in the sub-mm and mm spectral ranges have also been studied extensively as a probe of circumstellar dust formation, conspicuous axial symmetry and bipolar dynamics, see González Delgado et al (2003) for AGB stars and de Vicente et al (2016) for a variety of evolved stars, as well as references therein.…”