WZ Cephei: A Dynamically Active W UMa-Type Binary Star

Abstract

An intensive analysis of 185 timings of WZ Cep, including our new three timings, was made to understand the dynamical picture of this active W UMa-type binary. It was found that the orbital period of the system has complexly varied in two cyclical components superposed on a secularly downward parabola over about 80y. The downward parabola, corresponding to a secular period decrease of <TEX>$-9.{^d}97{\times}10^{-8}y^{-1}$</TEX>, is most probably produced by the action of both angular momentum loss (AML) due to magnetic braking and mass-transfer from the massive primary component to the secondary. The period decrease rate of <TEX>$-6.^{d}72{\times}10^{-8}y^{-1}$</TEX> due to AML contributes about 67% to the observed period decrease. The mass flow of about <TEX>$5.16{\times}10^{-8}M_{\odot}y^{-1}$</TEX> from the primary to the secondary results the remaining 33% period decrease. Two cyclical components have an <TEX>$11.^{y}8$</TEX> period with amplitude of <TEX>$0.^{d}0054$</TEX> and a <TEX>$41.^{y}3$</TEX> period with amplitude of <TEX>$0.^{d}0178$</TEX>. It is very interesting that there seems to be exactly in a commensurable 7:2 relation between their mean motions. As the possible causes, two rival interpretations (i.e., light-time effects (LTE) by additional bodies and the Applegate model) were considered. In the LTE interpretation, the minimum masses of <TEX>$0.30M_{\odot}$</TEX> for the shorter period and <TEX>$0.49M_{\odot}$</TEX> for the longer one were calculated. Their contributions to the total light were at most within 2%, if they were assumed to be main-sequence stars. If the LTE explanation is true for the WZ Cep system, the 7:2 relation found between their mean motions would be interpreted as a stable 7:2 orbit resonance produced by a long-term gravitational interaction between two tertiary bodies. In the Applegate model interpretation, the deduced model parameters indicate that the mechanism could work only in the primary star for both of the two period modulations, but could not in the secondary. However, we couldn't find any meaningful relation between the light variation and the period variability from the historical light curve data. At present, we prefer the interpretation of the mechanical perturbation from the third and fourth stars as the possible cause of two cycling period changes.