Formation and Evolution of W Ursae Majoris Contact Binaries

Abstract

The origin and evolution of WUMa systems are discussed based on All Sky Automated Survey (ASAS) data and the mean kinematic ages of four subgroups of 97 field contact binaries (FCBs). The period distribution of eclipsing binaries discovered by ASAS suggests that a period limit to tidal locking for the close binaries is about 2.24 days, so that most W UMa systems might be formed from detached binaries with periods PP2.24 days, and a maximum advanced time from a detached system to a W UMa is about 3.23 Gyr. Moreover, the secular evolution of the angular momentum (AM), the system mass, and the orbital period of 97 FCBs were investigated according to the mean kinematic ages, which were set according to AM bins. AMs, systemic masses, and orbital periods were shown to be decreasing with kinematic age. Their first-order decreasing rates have been determined as. J /J = 1.86; 10(-10) 10 yr(-1), M/M = 0.95; 10-10 yr(-1), and. P/P = 1.24; 10(-10) 10 yr(-1), and the average amplification (A A = d ln J /d ln M) is derived to be 1.96. These are lower than those derived from detached chromospherically active binaries (CABs). This suggests that the magnetic activity level of FCBs is indeed weaker than that of CABs. Meanwhile, the decreasing rate of AM of FCBs is found to be equal to an average value in a cycle of a cyclic model of contact binaries. This might suggest that the evolution of FCBs undergoes thermal relaxation oscillation (TRO) and that the coalescence of W UMa systems is a very long process, which is also indicated by the dynamical evolution of FCBs.