Evidence of a Mira-like tail and bow shock about the semi-regular variable V CVn from four decades of polarization measurements

TL;DR

This study presents forty years of polarization data for V CVn, revealing large, stable polarization and suggesting a long-term asymmetric structure caused by a bow shock and tail from stellar wind.

Contribution

It provides the first long-term polarization analysis of V CVn, indicating a stable asymmetry likely due to a bow shock, which is a novel insight into semi-regular variable stars.

Findings

V CVn shows polarization variations from 1% to 6%.

The position angle has remained nearly constant over forty years.

A bow shock and tail are likely responsible for the observed asymmetry.

Abstract

Polarization is a powerful tool for understanding stellar atmospheres and circumstellar environments. Mira and semi-regular variable stars have been observed for decades and some are known to be polarimetrically variable, however, the semi-regular variable V Canes Venatici displays an unusually large, unexplained amount of polarization. We present ten years of optical polarization observations obtained with the HPOL instrument, supplemented by published observations spanning a total interval of about forty years for V CVn. We find that V CVn shows large polarization variations ranging from 1 - 6%. We also find that for the past forty years the position angle measured for V CVn has been virtually constant suggesting a long-term, stable, asymmetric structure about the star. We suggest that this asymmetry is caused by the presence of a stellar wind bow shock and tail, consistent with the star's large space velocity.