Dynamical evolution of active detached binaries on log Jo - log M diagram and contact binary formation

Abstract

Orbital angular momentum (Jo), systemic mass (M) and orbital period (P) distributions of chromospherically active binaries (CAB) and W Ursae Majoris (W UMa) systems were investigated. The diagrams of log Jo - log P, log M - log P and log Jo-log M were formed from 119 CAB and 102 W UMa stars. The log Jo-log M diagram is found to be most meaningful in demonstrating dynamical evolution of binary star orbits. A slightly curved borderline (contact border) separating the detached and the contact systems was discovered on the log Jo - log M diagram. Since orbital size (a) and period (P) of binaries are determined by their current Jo, M and mass ratio q, the rates of orbital angular momentum loss (dlog Jo/dt) and mass loss (dlog M/dt) are primary parameters to determine the direction and the speed of the dynamical evolution. A detached system becomes a contact system if its own dynamical evolution enables it to pass the contact border on the log Jo - log M diagram. Evolution of q for a mass loosing detached system is unknown unless mass loss rate for each component is known. Assuming q is constant in the first approximation and using the mean decreasing rates of Jo and M from the kinematical ages of CAB stars, it has been predicted that 11, 23 and 39 cent of current CAB stars would transform to W UMa systems if their nuclear evolution permits them to live 2, 4 and 6 Gyrs respectively.