Magnetic Fields in Evolved Stars: Imaging the Polarized Emission of High-Frequency SiO Masers

TL;DR

This study uses high-frequency SiO maser observations to reveal stable large-scale magnetic fields in the circumstellar envelope of evolved stars, demonstrating their potential role in stellar mass loss and magnetic field structure.

Contribution

First direct imaging of magnetic field stability from stellar radii to hundreds of AU in evolved stars using polarized SiO masers.

Findings

Magnetic fields are stable and large-scale from stellar surface to ~1400 AU.

SiO masers exhibit high linear polarization aligned with magnetic field structure.

W Hya shows evidence of bipolar outflow or disk-like rotation.

Abstract

We present Submillimeter Array observations of high frequency SiO masers around the supergiant VX Sgr and the semi-regular variable star W Hya. The J=5-4, v=1 28SiO and v=0 29SiO masers of VX Sgr are shown to be highly linearly polarized with a polarization from ~5-60%. Assuming the continuum emission peaks at the stellar position, the masers are found within ~60 mas of the star, corresponding to ~100 AU at a distance of 1.57 kpc. The linear polarization vectors are consistent with a large scale magnetic field, with position and inclination angles similar to that of the dipole magnetic field inferred in the H2O and OH maser regions at much larger distances from the star. We thus show for the first time that the magnetic field structure in a circumstellar envelope can remain stable from a few stellar radii out to ~1400 AU. This provides further evidence supporting the existence of large scale and dynamically important magnetic fields around evolved stars. Due to a lack of parallactic angle coverage, the linear polarization of masers around W Hya could not be determined. For both stars we observed the 28SiO and 29SiO isotopologues and find that they have a markedly different distribution and that they appear to avoid each other. Additionally, emission from the SO 5_5-4_4 line was imaged for both sources. Around W Hya we find a clear offset between the red- and blue-shifted SO emission. This indicates that W Hya is likely host to a slow bipolar outflow or a rotating disk-like structure.