Large scale circulations and energy transport in contact binaries

TL;DR

This paper presents a hydrodynamic model explaining energy transport in contact binaries through large-scale circulation driven by baroclinic structures, aligning well with observed features of W UMa-type binaries.

Contribution

It introduces a detailed hydrodynamic model of energy transfer in contact binaries, emphasizing the role of large-scale circulation and its effects on stellar structure and observed phenomena.

Findings

The circulation transports high entropy matter effectively between components.

The model explains the observed equatorial bulge and velocity features.

Secondary star oversize is better explained by advanced evolutionary stage, not circulation alone.

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 nonuniform 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.