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

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.

Automated summary

The semi-regular variable star V CVn is known for its unusual linear polarization position angle (PA). Despite variations in optical brightness and polarization amplitude, the constant PA has persisted over decades of observations. Our study reveals that the polarization maximum does not always coincide precisely with the brightness minimum, but instead exhibits a small lead or lag. Furthermore, the polarization position angle sometimes undergoes noticeable rotation, particularly at lower polarization levels. We find a correlation between this unusual optical behavior and high tangential space velocities, suggesting a relationship between the long-term constant PA and the shaping of circumstellar gas by the star's high-speed motion through the interstellar medium.