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On the evolutionary model of progenitors of extreme horizontal branch objects after critical Roche lobe overfilling

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It has been shown quite recently that dwarf B stars, which are extreme horizontal branch (EHB) objects, in high probability all belong to binary systems. We study in detail the mass and angular momentum loss from the giant progenitors of subdwarf B stars in an attempt to clarify why binarity must be a crucial factor in producing EHB objects. It was found that considerable shrinkage of the orbit can be achieved after Roche lobe overfilling by the EHB progenitor, because of the combined effect of angular momentum loss from the red giant and appreciable accretion on its low-mass companion on the hydrodynamic timescale of the donor, which results in the formation of a helium white dwarf with mass roughly equal to half the solar mass, thus evading the common envelope stage. A simple approximating analytical formula for mass loss rate from the Roche-lobe-filling giant donor is proposed and the ranges of its applicability are studied.

Keywords: Binaries; Close stars; Evolutionary model

Document Type: Research Article


Affiliations: 1: Tallinn Technical University, Tallinn, Estonia,Tartu Observatory, Tõravere, Estonia 2: Tartu Observatory, Tõravere, Estonia

Publication date: February 1, 2007

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