The Photometric Investigation of V921 Her Using the Lunar-Based Ultraviolet Telescope of Chang’e-3 Mission

Abstract

The light curve of V921 Her in ultraviolet band observed by the Lunar-based Ultraviolet Telescope (LUT) is analyzed by the Wilson-Devinney code. Our solutions conclude that V921 Her is an early type marginal contact binary system with an additional close-in component. The binary system is under poor thermal contact with a temperature difference of nearly 700 K between the two components. The close-in component contributes about 19% of the total luminosity in the triple system. Combining the radial velocity study together with our photometric solutions, the mass of the primary star and secondary one is calculated to be $M_{\mathrm{1}}=\mathrm{1.784}(\pm \mathrm{0.055})M_{\odot }$, $M_{\mathrm{2}}=\mathrm{0.403}(\pm \mathrm{0.012})M_{\odot }$. The evolutionary scenario of V921 Her is discussed. All times of light minimum of V921 Her available in the bibliography are taken into account and the $O-C$curve is analyzed for the first time. The most probable fitting results are discussed in the paper, which also confirm the existence of a third component ( $P_3=10.2$year) around the binary system. The period of V921 Her is also undergoing a continuously rapid increase at a rate of $dP/dt=+\mathrm{2.79}\times \mathrm{1}{\mathrm{0}}^{-\mathrm{7}}\mathrm{d}\mathrm{a}\mathrm{y}·\mathrm{y}\mathrm{e}\mathrm{a}{\mathrm{r}}^{-\mathrm{1}}$, which may be due to mass transfer from the less massive component to the more massive one.