A new and improved ephemeris for the hot subdwarf eclipsing binary SDSS J082053.53+000843.4
2016 March 22
SDSS J082053.53+000843.4, for brevity here referred to as J08205, is a 15th magnitude short period, 0.0962404(1)d (~2h 18.6m), detached eclipsing binary system thought to consist of a post-main sequence subdwarf B star with a brown dwarf companion. J08205 belongs to a subclass of the HW Vir family of very short period (<4 hours) eclipsing binary systems comprising a hot subdwarf O or B-type star and a dwarf M star or brown dwarf companion. The hot subdwarf is a core helium-burning star with a thin hydrogen shell, and is situated at the extreme end of the horizontal branch on the H-R diagram.
Formation of an HW Vir type system has been speculatively explained as the evolution of a binary system consisting of two main-sequence stars of different masses. As soon as the more massive star reaches the red-giant phase and fills its Roche lobe, mass transfer takes place to the companion star. If the mass transfer becomes unstable, the envelope of the more massive star will engulf the companion star and form a common envelope. Drag forces on the two stellar objects transfer angular momentum from the binary pair to the common envelope. When the envelope has gained sufficient energy it is ejected and the momentum loss from the binary system results in a much reduced binary separation and period.
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