CONTACT BINARIES IN THERMAL-EQUILIBRIUM

Contact binaries in thermal equilibrium do not satisfy the Roche equip otential condition but a more general surface condition which is based on the Roche potential and Bernoulli's equation. Internal mass motion s are important particularly in the primary's envelope. This implies t hat the primary's radius is smaller than predicted by Roche geometry. The structure equations for stationary contact binaries (with spherica lly averaged components) are derived. They involve averaged properties of the velocity field of the internal mass motions. In late-type syst ems these properties can be approximately described by few parameters. In the simplest treatment only two parameters w(1), c(1) are sufficie nt, where w(1) expresses the mean kinetic energy of the internal mass motions in the primary's envelope and the coefficient c(1) expresses t he correlation of these motions with the orbital motion. For given cor relation, w(1) is roughly determined by the contact condition. The cor relation coefficient c(1) can be determined from observations. A first test concerns the system AB And as observed by Hrivnak (1988). The re sults support the assumption of thermal equilibrium. If AB And is unev olved and in thermal equilibrium, the correlation is small. This is co mpatible with a velocity field as proposed by Webbink (1977). Observat ional support for the assumption of thermal equilibrium in late-type s ystems comes from the high level of magnetic activity and the from the W-type syndrome. Thermal equilibrium explains these facts.