GW Tau and II Per: Two Poor Thermal Contact Binaries at a Key Evolutionary Stage of the TRO Cycle

Abstract

Near-contact binaries lie in a key evolutionary stage and are important targets for studying the poor thermal-contact binaries (PTCs) predicted by thermal relaxation oscillations (TRO) theory. Until now, only seven PTCs with decreasing periods have been reported without an accurate measurement of the absolute parameters. In this paper, we derived the absolute parameters of GW Tau and II Per on the basis of simultaneous analysis of the light curves and radial velocity curve. Photometric solutions suggest that GW Tau and II Per are low mass ratio, marginal-contact binaries in poor thermal contact with small fill-out factor $f= 13.7 {{\ \rm per\ cent}}$ and $f= 0.2 {{\ \rm per\ cent}}$, respectively. In addition, large temperature differences between two components of about 2200 K (GW Tau) and 1600 K (II Per) are also obtained, indicating that they are PTCs in the TRO cycle with a common envelope forming. Eclipse timing variation analysis reveals that the orbital periods of GW Tau and II Per are decreasing at a rate of $\dot{P}$ = -2.466 × 10−7 d yr−1 and $\dot{P}$ = -6.201 × 10−8 d yr−1, respectively. After calculating, we found that the secular period decrease of GW Tau is caused by mass transfer from the primary component to the secondary one. However, angular momentum loss (AML) via magnetic braking is the main reason for the decrease in the period of II Per, and the AML rate is enhanced by increasing contact depth. With the orbital period decreasing, GW Tau and II Per will evolve into overcontact binaries with true thermal contact.