Large-scale circulations and energy transport in contact binaries

Abstract

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.

Evolutionary history of four binary blue stragglers from the globular clustersωCen, M 55, 47 Tuc, and NGC 6752

“…AW UMa looks apparently as a typical contact system of W UMa-type with the secondary radius of about 0.5 R ⊙ , yet the detailed observations of this binary obtained by Rucinski (2015) indicate a much more complex geometry of the system, where both components do not actually fill their Roche lobes but are surrounded by the equatorial flow extending up to the inner critical surface. The existence of such a flow in contact and near contact binaries was predicted by Stȩpień (2009) and Stȩpień & Kiraga (2013). The flow redistributes energy between the components resulting in an apparent equality of their effective temperatures what we also see in V8 (see Table 2).…”

“…An obvious improvement would be to replace the present code with a binary evolution code solving the equations of the internal structure of each component at every time step and allowing for the mass transfer between the components. The next step would be to incorporate the dynamical processes which take place in close binaries but are still poorly understood, like the rapid mass transfer following RLOF with an unknown amount of mass and AM lost from the system (Sarna & De Greve 1996), or the mass flow carrying thermal energy from the primary to secondary component in a contact binary (Stȩpień 2009;Rucinski 2015).…”

“…The following search for progenitors of the investigated binaries is based on a model developed by one of us (Stȩpień 2006a;Gazeas & Stȩpień 2008;Stȩpień 2009;Stȩpień & Kiraga 2015). It describes the evolution of a cool close binary from the zero-age main sequence (ZAMS) until a stage preceding the merger of the components.…”

Evolution of the progenitor binary of V1309 Scorpii before merger

“…Following the idea put forward by Lucy (1968Lucy ( , 1976 and Flannery (1976), many authors assume that the expanded component does not relax thermally and stays oversized, transferring mass back to its more massive companion on the evolutionary time scale. It was, however, demonstrated by Stȩpień (2009) that the energy transfer between components of a contact binary takes place in the form of a large-scale circulation bound to the equatorial region of the less massive component. The core energy of that component can be freely radiated by the polar regions and the star remains in thermal equilibrium.…”

The First Photometric Study of NSVS 1461538: A New W-subtype Contact Binary with a Low Mass Ratio and Moderate Fill-out Factor

“…The other explanation suggested by Stepien (2007Stepien ( , 2009) comes from the knowledge that the primary star and the secondary star have very different status of evolution, as shown in Figs. 8 and 9.…”