Spectroscopic and Decade-long Photometric Observations of the Contact Binary V2790 Ori: Evidence for a Brown Dwarf Companion and a Solar-like Magnetic Activity Cycle

Abstract

We present 22 sets of light curves and one radial velocity curve for the W UMa-type total eclipse contact binary system V2790 Ori, derived by combining all available public photometric data, the photometric data in previous studies, and our own spectroscopic and decade-long photometric observations. Our simultaneous analysis of the light curves and radial velocity curve shows that V2790 Ori is a W-subtype contact binary with a mass ratio of $q=0.322(\pm 0.001)$ and a shallow contact degree of $14.8(\pm 0.6)\%$. The orbital period analysis based on 445 eclipsing minima reveals a secular decrease at a rate of $\dot{P}=-3.18(\pm 0.75)\times 10^{-8}\mathrm{d}\mathrm{y}{\mathrm{r}}^{-1}$, superimposed with a cyclic variation with an amplitude of $A=8.98(\pm 2.19)\times 10^{-4}\mathrm{d}$ and a period of $P_3=7.44(\pm 0.52)\mathrm{yr}$. The secular decrease is caused by AML via magnetic braking, while the cyclic period variation is explained by the light-travel time effect due to a third body, which is likely to be a brown dwarf. Furthermore, our analysis indicates a mass transfer from the more massive component to the less massive one at a rate of $1.22(\pm 0.29)\times 10^{-8}{\mathrm{M}}_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$. A model with a cool spot on each component was adopted to fit the O'Connell effect observed in the light curves. Since the O'Connell effect varies over time, we identified a solar-like magnetic activity cycle with a period of approximately 5.4 yr by analyzing the magnitude difference ($\Delta m$) at the two light maxima and the O'Connell effect ratio. In addition, evolutionary analysis suggests that V2790 Ori is a newly formed contact binary that evolved from a detached phase into the present contact configuration.

22 pages, 7 figures, 9 tables, accepted by ApJ