A multiyear photopolarimetric study of the semi-regular variable V CVn and identification of analog sources^★

¹ Department of Physics & Physical Oceanography, Memorial University of Newfoundland & Labrador, St. John’s, NL A1C 5S7, Canada
e-mail: hneilson@mun.ca
² Sternwarte Freimann, 80939 Munich, Germany
³ Parton, Scotland, UK
⁴ Department of Physics & Astronomy, East Tennessee State University, Johnson City, TN 37614, USA
⁵ Astrophysics Research Institute, Liverpool John Moores University, Liverpool Science Park, 146 Brownlow Hill, Liverpool, L3 5RF, UK
⁶ Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721-0065, USA

Received: 5 October 2022
Accepted: 15 May 2023

Abstract

The semi-regular variable star V Canum Venaticorum (V CVn) is well known for its unusual linear polarization position angle (PA). Decades of observing V CVn reveal a nearly constant PA spanning hundreds of pulsation cycles. This phenomenon has persisted through variability that has varied by two magnitudes in optical brightness and through variability in the polarization amplitude over 0.3 and 6.9%. Additionally, the polarization fraction of V CVn varies inversely with brightness. This paper presents polarization measurements obtained over three pulsation cycles. We find that the polarization maximum does not always occur precisely at the same time as the brightness minimum. Instead, we observe a small lead or lag in relation to the brightness minimum, spanning a period of a few days up to three weeks. Furthermore, the PA sometimes exhibits a non-negligible rotation, especially at lower polarization levels. To elucidate the unusual optical behavior of V CVn, we present a list of literature sources that also exhibit polarization variability with a roughly fixed PA. We find this correlation occurs in stars with high tangential space velocities, for instance, “runaway” stars, suggesting that the long-term constant PA is related to how the circumstellar gas is shaped by the star’s high-speed motion through the interstellar medium.