2009/02/28 by K. Stepien, K. Stȩpień · 55 citations
Earth and Planetary Sciences · Physics and Astronomy · #Astrophysics #Astrophysics and Star Formation Studies #Baroclinity #Binary star #Classical mechanics #Convection #High-pressure geophysics and materials #Kinetic energy #Mechanics #Physics #Radiative transfer #Roche lobe #Secondary circulation #Stars #Stellar, planetary, and galactic studies #astro-ph.SR
paper · pdf · doi:10.1111/j.1365-2966.2009.14981.x
published in Monthly Notices of the Royal Astronomical Society 397(2), 857-867 (Oxford University Press) · 12 pages, 7 figures, accepted to MNRAS
arxiv created 2009/05/04 · openalex publication_date 2009/07/03 · arxiv updated 2015/05/12 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/05
A hydrodynamic model for the energy transport between the components of a contact binary is presented. Energy is transported by a large-scale, steady circulation carrying high entropy matter from the primary to secondary component. The circulation is driven by the baroclinic structure of the common envelope, which is a direct consequence of the non-uniform heating at the inner critical Roche lobes due to unequal emergent energy fluxes of the components. The mass stream flowing around the secondary is bound to the equatorial region by the Coriolis force and its width is determined primarily by the flow velocity. Its bottom is separated from the underlying secondary's convection zone by a radiative transition layer acting as an insulator. For a typically observed degree of contact the heat capacity of the stream matter is much larger than radiative losses during its flow around the secondary. As a result, its effective temperature and entropy decrease very little before it returns to the primary. The existence of the stream changes insignificantly specific entropies of both convective envelopes and sizes of the components. Substantial oversize of the secondaries, required by the Roche geometry, cannot be explained in this way. The situation can, however, be explained by assuming that the primary is a main-sequence star whereas the secondary is in an advanced evolutionary stage with hydrogen depleted in its core. Such a configuration is reached past mass transfer with mass ratio reversal. Good agreement with observations is demonstrated by model calculations applied to actual W UMa-type binaries. In particular, a presence of the equatorial bulge moving with a relative velocity of 10–30 km s−! around both components of AW UMa is accounted for.