The Photometric Investigation of V921 Her using the Lunar-based Ultraviolet Telescope of Chang'e-3 mission

TL;DR

This study analyzes ultraviolet light curves of V921 Her, revealing it as a marginal contact binary with a third component, and discusses its evolutionary status and period changes based on combined photometric and radial velocity data.

Contribution

First combined photometric and radial velocity analysis of V921 Her in ultraviolet, revealing its contact binary nature, third component, and period evolution.

Findings

V921 Her is an early type marginal contact binary with a third component.

The system has a poor thermal contact with a 700K temperature difference.

The orbital period is increasing at a rate of 2.79×10^{-7} day/year.

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 $700K$ 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 are calculated to be $M_1 = 1.784(\pm0.055)M_\odot$, $M_2 = 0.403(\pm0.012)M_\odot$. 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 $dP/dt=+2.79\times{10^{-7}}day\cdot year^{-1}$, which may due to mass transfer from the less massive component to the more massive one.