RZ Tauri: An Unstable W Ursae Majoris Binary with a Magnetically Active Component

Abstract

New BV light curves and photometric solutions for the W Ursae Majoris binary RZ Tauri (spectral type F0) are presented. The light curves appear to exhibit a typical O'Connell effect, with primary maximum being 0.023 mag (V) and 0.030 mag (B) brighter than secondary maximum. From the present times of minimum light and those collected from the literature, the changes in the orbital period of the system are analyzed. The results suggest that the orbital period of RZ Tau continuously increases at a rate of dp/p = 1.10 x 10(-10) and that a short-term wavelike variation with an amplitude of DeltaP = 3.8 x 10(-6) days and a period of P-mod = 60 yr are superposed on the secular increase. The orbital period increase may be explained by mass transfer from the less massive to the more massive component at a rate of Deltam/Deltat = 8.81 x 10(-8) M. yr(-1), and the short-term oscillations may be due to magnetic activity on the secondary component. The present light curves and the radial velocity curves obtained by Struve et al. in 1950 were analyzed by means of the latest version of the Wilson-Devinney code. The results suggest that RZ Tau is an A-subtype W UMa contact binary with a mass ratio of 0.379. The asymmetry of the light curves may be explained by a cool spot on the secondary component. The basic parameters of the system are be found to be M-1 = 1.70 +/- 0.16 M., M-2 = 0.64 +/- 0.06 M., R-1 = 1.56 +/- 0.07 R., R-2 = 1.04 +/- 0.05 R., L-1 = 6.19 +/- 0.61 L., and L-2 = 2.60 +/- 0.27 L..