The curious case of V CVn

Abstract

From many decades of observations, the star V Canum Venaticorum (V CVn) has strange behaviour regarding the maxima and minima of its light curve and linear polarisation, which have an approximately inverse relationship (sometimes with a lead/lag time between them) and an almost constant polarisation position angle. One theory proposed to explain this strange behaviour is the existence of a bow shock driven by a spherically symmetric dusty wind from the star. In this theory, the wind is assumed to vary with time due to radial pulsations. This work uses a new framework developed in Zeus3D, a multi-physics magnetohydrodynamics code, to test this theory. The results of this work show that when a time-varying stellar wind is at its maximum brightness the polarisation signal is at a minimum due to a dense, symmetric shell which forms around the star. Conversely, when the brightness is at a minimum, the symmetric shell around the star is much less dense, and the polarisation is instead dominated by the asymmetric bow shock structure, causing the polarisation signal to attain a maximum value. Numerically reproducing the observed inverse relationship between the polarisation and light curve provides a strong theoretical argument that a variable stellar wind bow shock is the solution to the curious case of V CVn.