BVRcIc Observations, Third-body Orbital Study, and Analysis of the UV Leo-type, Pre-W UMA Binary V642 Virginis
Abstract
V642 Vir is a polar spotted, well-detached, UV Leo-type, low-mass, pre-WUMa (T1 similar to 4250K, similar to K6V) eclipsing binary. It was observed in 2020 April, May, and June at the Dark Sky Observatory in North Carolina, USA with the 0.81 m reflector of Appalachian State University.
A total of 88 timings were used in our 22-year period study which included 12 Transiting Exoplanet Survey Satellite (TESS) timings. The O - C plots show a low-amplitude oscillation of residuals that points to the existence of an orbiting third body, a dwarf of minimum mass, 0.15 M (circle dot) in an eccentric orbit (e = 0.41), with an orbital period of 20.07 yr.
The odd light curves of V642 Virginis indicate that it has polar spots similar to UV Leo and the recently published V1023 Per. Its present large polar spot region indicates that it must have a strong magnetic field and that it is synchronously rotating.
The BVRcIc simultaneous Wilson-Devinney Program solution gives a detached binary (primary and secondary components are underfilling their respective Roche Lobes, with 76% and 78% fill outs respectively). The cool spot region models near the pole of the primary component (centered at 10 degrees colatitude) and is angled toward the secondary component.
Its large radius (68 degrees) and T-fact (T-spot/T-surface = 0.69) also attest to the conclusion of the strength of the magnetic field. The small Delta T in the components (similar to 318 K) and mass ratio near unity (0.9542 +/- 0.0005) show that the stars are similar in spectral type (secondary similar to K9V).
The inclination is high, similar to 86.87 +/- 0.04 degrees, yet there is no time of constant light due to the two stars' essentially equal radii.