VLBA SiO maser observations of the OH/IR star OH 44.8-2.3: magnetic field and morphology

TL;DR

This study uses high-resolution VLBI observations to analyze SiO masers in the OH/IR star OH 44.8-2.3, revealing magnetic field strength, polarization properties, and maser morphology, advancing understanding of magnetic fields in evolved stars.

Contribution

First high-resolution VLBI polarization study of SiO masers in a high mass-loss OH/IR star, providing insights into magnetic field structure and maser polarization mechanisms.

Findings

SiO masers form a ring at ~5.4 AU from the star

Masers exhibit up to 100% linear polarization

Detected magnetic field strength of approximately 1.5 G

Abstract

SiO maser emission occurs in the extended atmosphere of evolved stars and can be studied at high angular resolution. Very long baseline interferometry (VLBI) observations of Mira variables indicate that SiO masers are significantly linearly polarized with linear polarization fraction up to 100%. However, no information is available at high angular resolution for SiO masers in higher mass loss OH/IR stars. We extend the VLBI SiO maser studies to OH/IR stars. The observations enable us to understand the SiO pumping mechanisms in higher mass loss evolved objects and compare those with Mira variables. Additionally, polarimetric observations of SiO masers help us to understand the magnetic field strength and morphology and to distinguish between conflicting polarization theories. The 43 GHz SiO maser observations of the OH/IR star OH 44.8-2.3 were performed with the VLBA in full polarization spectral line mode. Auxiliary EVLA observations were performed to allow for the absolute calibration of the polarization angle. The Zeeman splitting was measured by cross correlating the right and left circular polarization spectra as well as the S-curve fitting. Additionally, we analyzed the 1612 MHz OH maser observations of OH 44.8-2.3 from the VLA archive. The SiO masers of OH 44.8-2.2 form a ring located at ~5.4 AU around the star. The masers appear to be highly linearly polarized with fractional linear polarization up to 100%. The linear polarization vectors are consistent with a dipole field morphology in this star. We report a tentative detection of circular polarization of ~0.7% for the brightest maser feature. The magnetic field measured for this feature corresponds to 1.5\pm0.3 G. Additionally, the distribution of the 1612 MHz OH maser emission could indicate an elongated morphology.