Mapping circumstellar magnetic fields of late-type evolved stars with the Goldreich-Kylafis effect: CARMA observations at $\lambda 1.3$ mm of R Crt and R Leo

TL;DR

This study detects and models linearly polarized emission from molecular lines in two evolved stars, revealing insights into their circumstellar magnetic fields through the Goldreich-Kylafis effect, with results aligning with theoretical predictions for one star.

Contribution

First detection of Goldreich-Kylafis effect polarization in thermal lines of evolved stars R Crt and R Leo, providing new observational constraints on circumstellar magnetic field geometry.

Findings

Polarization detected in CO and SiO lines of R Crt and R Leo.

Model predictions agree with observations for R Crt, but not fully for R Leo.

Fractional linear polarization up to 9.7% observed in R Leo.

Abstract

Mapping magnetic fields is the key to resolving what remains an unclear physical picture of circumstellar magnetic fields in late-type evolved stars. Observations of linearly polarized emission from thermal molecular line transitions due to the Goldreich-Kylafis (G-K) effect provides valuable insight into the magnetic field geometry in these sources that is complementary to other key studies. In this paper, we present the detection of spectral-line polarization from both the thermal $J=2-1$ CO line and the $v=1, J=5-4$ SiO maser line toward two thermal-pulsating (TP-) AGB stars, R Crt and R Leo. The observed fractional linear polarization in the CO emission is measured as $m_l\sim 3.1\%$ and $m_l\sim9.7\%$ for R Crt and R Leo respectively. A circumstellar envelope (CSE) model profile and the associated parameters are estimated and used as input to a more detailed modeling of the predicted linear polarization expected from the G-K effect. The observed thermal line polarization level is consistent with the predicted results from the G-K model for R Crt; additional effects need to be considered for R Leo.