Astronomy and Astrophysics – Astrophysics
Scientific paper
Feb 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995a%26a...294..497k&link_type=abstract
Astronomy and Astrophysics (ISSN 0004-6361), vol. 294, no. 2, p. 497-508
Astronomy and Astrophysics
Astrophysics
12
Binary Stars, Roche Limit, Thermal Stability, Thermodynamic Equilibrium, Angular Velocity, Equipotentials, Velocity Distribution
Scientific paper
Contact binaries in thermal equilibrium do not satisfy the Roche equipotential condition but a more general surface condition which is based on the Roche potential and Bernoulli's equation. Internal mass motions are important particularly in the primary's envelope. This implies that the primary's radius is smaller than predicted by Roche geometry. The structure equations for stationary contact binaries (with spherically averaged components) are derived. They involve averaged properties of the velocity field of the internal mass motions. In late-type systems these properties can be approximately described by few parameters. In the simplest treatment only two parameters w1, c1 are sufficient, where w1 expresses the mean kinetic energy of the internal mass motions in the primary's envelope and the coefficient c1 expresses the correlation of these motions with the orbital motion. For given correlation, w1 is roughly determined by the contact condition. The correlation coefficient c1 can be determined from observations. A first test concerns the system AB And as observed by Hrivnak (1988). The results support the assumption of thermal equilibrium. If AB And is unevolved and in thermal equilibrium, the correlation is small. This is compatible with a velocity field as proposed by Webbink (1977). Observational support for the assumption of thermal equilibrium in late-type systems comes from the level of magnetic activity and from the W-type syndrome. Thermal equilibrium explains these facts.
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