Photometric and spectroscopic properties of the eclipsing system V864 Monocerotis

Abstract

Abstract We present the orbital period variability and evolutionary status of the W UMa-type binary system V864 Mon from accurately measured fundamental parameters. New BV photometric observations of this system were performed in 2019 January and 2022 January, and the first high-resolution spectroscopic observations were carried out on three nights between 2019 January and March. A total of 29 times of minimum light were collected to examine the behavior of the orbital period. Our analysis of these timings indicates a continuous period increase at a rate of +2.62 × 10−7 d yr−1 over the past 20 years, which can be interpreted as a mass transfer from the less massive primary to the secondary component with a rate of 1.22 × 10−7 M⊙ yr−1. We measured the radial velocities (RVs) for both components, and determined the effective temperature and projected rotational velocity of the more massive secondary star to be Teff,2 = 5450 ± 94 K and v2sin i = 192 ± 40 km s−1, respectively, from the comparison of the observed spectrum at the primary minimum and the theoretical models. The individual masses and radii of both components were determined from a simultaneous analysis of the light and RV curves, which are M1 = 0.34 ± 0.02 M⊙, R1 = 0.69 ± 0.01 R⊙, and M2 = 1.06 ± 0.04 M⊙, R2 = 1.16 ± 0.02 R⊙, respectively. Our results indicate that V864 Mon is a W-subtype of W UMa stars with time-varying spot activity. The positions in the mass-luminosity and mass-radius diagrams indicate that the secondary star belongs to the main-sequence region, while the hotter primary is located beyond the terminal-age main sequence.