The evolution of low-mass close binary systems. III - 1.50 + 0.50 solar masses: Unsteady mass loss and shrinking secondaries

Abstract

The evolution of a binary system of 1.50 M/sub sun/ and 0.50 M/sub sun/ components with initial separation 3.00 R/sub sun/ is computed. In the computation, both components are followed simultaneously.The more massive component (primary) fills its Roche lobe during core hydrogen burning (case A evolution), and mass transfer quickly grows to a typical thermal time scale rate (approx.1.1 x 10/sup -7/ M/sub sun/ yr/sup -1/). The primary's rapid decrease in mass and its underluminosity lead to the growth of an abnormally deep surface convection zone, preventing stabilization of the mass loss rate. Runaway (dynamical time scale) mass loss develops, reaching 1.5 x 10/sup -4/ M/sub sun/ yr/sup -1/. This rate is itself unsteady because of a Bath-type mechanism. Two runaway episodes occur, leaving a 0.67 M/sub sun/+1.33 M/sub sun/ binary still in a semidetached state.The deep convective envelope of the secondary dominates its evolution, leading to contraction in response to accretion, and avoidance of contact. The rapid compression of the secondary's core during dynamical time scale mass transfer causes runaway hydrogen burning, driving large-scale convection which ultimately mixes the star completely.The behavior of the system modeled, and the absence of observational counterparts to it, are interpreted as supporting themore » fission theory of the origin of close binaries, and as indicating that most W UMa systems have always been contact binaries.« less