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Astron. Astrophys. 318, 171-178 (1997)

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1. Introduction

In a preceding paper (Kähler 1995, hereafter K95) we investigated the conditions for thermal equilibrium in contact binaries. In unevolved (or slighly evolved) late-type systems the conditions are generally severe since rapid internal mass motions in the primary's envelope are required. We failed to notice that highly supersonic mass motions in the primary's outer layers are incompatible with the observed broadening of spectral lines. Combining this observational fact with the results of K95 we already have a strong argument that unevolved or slightly evolved systems are usually in thermal disequilibrium, in accordance with the results of Lucy (1976), Flannery (1976) and Robertson & Eggleton (1977).

For evolved systems, however, the situation is less clear. Lucy (1976) suggested that A-type systems are evolved and in thermal equilibrium. Robertson & Eggleton (1977) and Refsdal & Stabell (1981) showed indeed the existence of evolved theoretical configurations in stable thermal equilibrium, but these configurations are possibly not realistic.

The purpose of the present paper is to show that thermal equilibrium in late-type contact binaries, evolved or not, usually cannot be established. Sect. 2 concerns theoretical contact configurations in thermal equilibrium. In Sect. 3 we discuss observational properties of contact binaries and derive a test which is (within the framework of some assumptions) sufficient to disprove thermal equilibrium in an observed system. This test is applied to a number of W-type and A-type systems. Sect. 4 concerns uncertainties in the present analysis. In the last section the results are summarized.

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998