Polarization morphology of SiO masers in the circumstellar envelope of the AGB star R Cassiopeiae

TL;DR

This study maps and analyzes the polarization of SiO masers around the AGB star R Cassiopeiae, revealing magnetic field structures and polarization behaviors close to the star.

Contribution

It provides detailed polarization measurements of SiO masers in R Cas, applying the Goldreich et al. model to interpret magnetic field orientations in the circumstellar envelope.

Findings

Linear polarization reaches tens of percent

Circular polarization is a few percent

Polarization angles tend to be radial or perpendicular to the star

Abstract

Silicon monoxide maser emission has been detected in the circumstellar envelopes of many evolved stars in various vibrationally-excited rotational transitions. It is considered a good tracer of the wind dynamics close to the photosphere of the star. We have investigated the polarization morphology in the circumstellar envelope of an AGB star, R Cas. We mapped the linear and circular polarization of SiO masers in the v=1, J=1-0 transition. The linear polarization is typically a few tens of percent while the circular polarization is a few percent. The fractional polarization tends to be higher for emission of lower total intensity. We found that, in some isolated features the fractional linear polarization appears to exceed 100%. We found the Faraday rotation is not negligible but is ~15 deg., which could produce small scale structure in polarized emission whilst total intensity is smoother and partly resolved out. The polarization angles vary considerably from feature to feature but there is a tendency to favour the directions parallel or perpendicular to the radial direction with respect to the star. In some features, the polarization angle abruptly flips 90 deg. We found that our data are in the regime where the model of Goldreich et al (1973) can be applied and the polarization angle flip is caused when the magnetic field is at close to 55 deg. to the line of sight. The polarization angle configuration is consistent with a radial magnetic field although other configurations are not excluded.