Abstract
We have identified a star in the WASP archive photometry with an unusual lightcurve due to the total eclipse of a small, hot star by an apparently normal A-type star and with an orbital period of only 0.668d. From an analysis of the WASP lightcurve together with V-band and I_C-band photometry of the eclipse and a spectroscopic orbit for the A-type star we estimate that the companion star has a mass of (0.23+-0.03)Msun and a radius of (0.33+-0.01)Rsun, assuming that the A-type star is a main-sequence star with the metalicity appropriate for a thick-disk star. The effective temperature of the companion is (13400+-1200)K from which we infer a luminosity of (3+-1)Lsun. From a comparison of these parameters to various models we conclude that the companion is most likely to be the remnant of a red giant star that has been very recently stripped of its outer layers by mass transfer onto the A-type star. In this scenario, the companion is currently in a shell hydrogen-burning phase of its evolution, evolving at nearly constant luminosity to hotter effective temperatures prior to ceasing hydrogen burning and fading to become a low-mass white dwarf composed of helium (He-WD). The system will then resemble the pre-He-WD/He-WD companions to A-type and B-type stars recently identified from their Kepler satellite lightcurves (KOI-74, KOI-81 and KIC10657664). This newly discovered binary offers the opportunity to study the evolution of a stripped red giant star through the pre-He-WD stage in great detail.
Highlights
Wide-area surveys for transiting extra-solar planets such as WASP (Wide Angle Search for Planets, Pollacco et al 2006), HATnet (Bakos et al 2004), XO (McCullough et al.2005) and TrES (O’Donovan et al 2006) provide high cadence photometry for millions of bright stars across a large fraction of the sky
In the case of the white-dwarf companions to early-type stars KOI-74 and KOI-81, these were identified from the eclipses and transits in the Kepler lightcurves, even though these features are much less than 1% deep (Rowe et al 2010; van Kerkwijk et al 2010)
In this paper we present the discovery from WASP photometry of an eclipsing binary star that is related to KOI-74, KOI-81 but that has much deeper eclipses, i.e. the companion to the early-type star is much brighter and larger than the white-dwarf companions to these Kepler discoveries
Summary
Wide-area surveys for transiting extra-solar planets such as WASP (Wide Angle Search for Planets, Pollacco et al 2006), HATnet (Bakos et al 2004), XO The photometric precision achieved by these surveys with modest equipment ( 0.01 magnitudes at V≈12) is impressive, but cannot compete with the micro-magnitude photometry acheived from space by surveys such as CoRoT (Baglin et al 2006) and Kepler (Borucki et al 2009) Photometry with this precision has made it possible to identify new types of variable star that are difficult or impossible to study from the ground, e.g., triply eclipsing binary stars (Carter et al 2011), stars with tidally excited pulsations (Welsh et al 2011), subdwarfs with white dwarf companions (Bloemen et al 2011), and white dwarf companions to early-type main sequence stars (Rowe et al 2010; van Kerkwijk et al 2010). We present the data used to identify this new eclipsing binary star and the follow-up photometry and spectroscopy we have obtained; we analyse these data to determine the masses, radii and luminosities of the stars; and we outline how our discovery of this bright pre-white dwarf companion to an early-type star makes it possible to study the formation of an LMWD in detail
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