The first pre-supersoft X-ray binary

TL;DR

This paper reports the discovery of the first pre-supersoft X-ray binary, a close white dwarf and F dwarf star system, providing insights into its evolutionary history and potential as a progenitor of super-soft sources.

Contribution

It presents the identification and detailed characterization of a unique binary system that is a precursor to super-soft X-ray sources, expanding understanding of binary evolution pathways.

Findings

System consists of a hot white dwarf and an F8 star in a 12-hour orbit.

The system likely evolved from a common envelope phase involving an A-type star.

It is a progenitor of a super-soft source, not a supernova Ia.

Abstract

We report the discovery of an extremely close white dwarf plus F dwarf main-sequence star in a 12 hour binary identified by combining data from the RAdial Velocity Experiment (RAVE) survey and the Galaxy Evolution Explorer (GALEX) survey. A combination of spectral energy distribution fitting and optical and Hubble Space Telescope ultraviolet spectroscopy allowed us to place fairly precise constraints on the physical parameters of the binary. The system, TYC 6760-497-1, consists of a hot Teff~20,000K, M~0.6Ms white dwarf and an F8 star (M~1.23Ms, R~1.3Rs) seen at a low inclination (i~37 deg). The system is likely the descendent of a binary that contained the F star and a ~2Ms A-type star that filled its Roche-lobe on the thermally pulsating asymptotic giant branch, initiating a common envelope phase. The F star is extremely close to Roche-lobe filling and there is likely to be a short phase of thermal timescale mass-transfer onto the white dwarf during which stable hydrogen burning occurs. During this phase it will grow in mass by up to 20 per cent, until the mass ratio reaches close to unity, at which point it will appear as a standard cataclysmic variable star. Therefore, TYC 6760-497-1 is the first known progenitor of a super-soft source system, but will not undergo a supernova Ia explosion. Once an accurate distance to the system is determined by Gaia, we will be able to place very tight constraints on the stellar and binary parameters.