Absolute Dimensions and Apsidal Motion of the Eccentric Binary V731 Cephei

Abstract

A detailed analysis of new and existing photometric, spectroscopic and spatial distribution data of the eccentric binary V731 Cep was performed. Spectroscopic orbital elements of the system were obtained by means of cross-correlation technique. According to the solution of radial velocities with UBVRcIc light curves, V731 Cep consists of two main-sequence stars with masses M${}_1$=2.577 (0.098) M${}_{\odot }$, M${}_2$=2.017 (0.084) M${}_{\odot }$, radii R${}_1$=1.823 (0.030) R${}_{\odot }$, R${}_2$=1.717 (0.025) R${}_{\odot }$, and temperatures T${}_{eff1}$=10700 (200) K, T${}_{eff2}$=9265 (220) K separated from each other by a=23.27 (0.29) R${}_{\odot }$ in an orbit with inclination of 88${}^{\circ }$.70 (0.03). Analysis of the O--C residuals yielded a rather long apsidal motion period of U=10000(2500) yr compared to the observational history of the system. The relativistic contribution to the observed rates of apsidal motion for V731 Cep is significant (76%). The combination of the absolute dimensions and the apsidal motion properties of the system yielded consistent observed internal structure parameter (log${\bar{k}}_{2,obs}$=-2.36) compared to the theory (log${\bar{k}}_{2,theo}$=-2.32). Evolutionary investigation of the binary by two methods (Bayesian and evolutionary tracks) shows that the system is t=133(26) Myr old and has a metallicity of [M/H]=-0.04(0.02) dex. The similarities in the spatial distribution and evolutionary properties of V731 Cep with the nearby ($\rho \sim 3^{\circ }$.9) open cluster NGC 7762 suggests that V731 Cep could have been evaporated from NGC 7762.

10 pages, including 8 figures and 8 tables, accepted for publication in MNRAS

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