Photometric Variability of the mCP Star CS Vir: Evolution of the Rotation Period

Abstract

The aim of this study is to accurately calculate the rotational period of CS\,Vir by using {\sl STEREO} observations and investigate a possible period variation of the star with the help of all accessible data. The {\sl STEREO} data that cover five-year time interval between 2007 and 2011 are analyzed by means of the Lomb-Scargle and Phase Dispersion Minimization methods. In order to obtain a reliable rotation period and its error value, computational algorithms such as the Levenberg-Marquardt and Monte-Carlo simulation algorithms are applied to the data sets. Thus, the rotation period of CS\,Vir is improved to be 9.29572(12) days by using the five-year of combined data set. Also, the light elements are calculated as $HJD_\mathrm{max} = 2\,454\,715.975(11) + 9_{\cdot}^\mathrm{d}29572(12) \times E + 9_{\cdot}^\mathrm{d}78(1.13) \times 10^{-8} \times E^2$ by means of the extremum times derived from the {\sl STEREO} light curves and archives. Moreover, with this study, a period variation is revealed for the first time, and it is found that the period has lengthened by 0.66(8) s y$^{-1}$, equivalent to 66 seconds per century. Additionally, a time-scale for a possible spin-down is calculated around $\tau_\mathrm{SD} \sim 10^6$ yr. The differential rotation and magnetic braking are thought to be responsible of the mentioned rotational deceleration. It is deduced that the spin-down time-scale of the star is nearly three orders of magnitude shorter than its main-sequence lifetime ($\tau_\mathrm{MS} \sim 10^9$ yr). It is, in return, suggested that the process of increase in the period might be reversible.