A precision study of two eclipsing white dwarf plus M dwarf binaries

S G Parsons,A D Schwope,D Koester,A Rebassa-Mansergas,R D G Hickman,S Pyrzas,T R Marsh,C Tappert,A Nebot Gómez-Morán,S P Littlefair,C D J Savoury,P R Steele,P Kerry,L Schmidtobreick,M R Burleigh,C M Copperwheat,V S Dhillon,M R Schreiber,B T Gänsicke

doi:10.1111/j.1365-2966.2011.20251.x

Abstract

We use a combination of X-shooter spectroscopy, ULTRACAM high-speed photometry and SOFI near-infrared photometry to measure the masses and radii of both components of the eclipsing post common envelope binaries SDSS J1212-0123 and GK Vir. For both systems we measure the gravitational redshift of the white dwarf and combine it with light curve model fits to determine the inclinations, masses and radii. For SDSS J1212-0123 we find a white dwarf mass and radius of 0.439 +/- 0.002 Msun and 0.0168 +/- 0.0003 Rsun, and a secondary star mass and radius of 0.273 +/- 0.002 Msun and 0.306 +/- 0.007 Rsun. For GK Vir we find a white dwarf mass and radius of 0.564 +/- 0.014 Msun and 0.0170 +/- 0.0004 Rsun, and a secondary star mass and radius of 0.116 +/- 0.003 Msun and 0.155 +/- 0.003 Rsun. The mass and radius of the white dwarf in GK Vir are consistent with evolutionary models for a 50,000K carbon-oxygen core white dwarf. Although the mass and radius of the white dwarf in SDSS J1212-0123 are consistent with carbon-oxygen core models, evolutionary models imply that a white dwarf with such a low mass and in a short period binary must have a helium core. The mass and radius measurements are consistent with helium core models but only if the white dwarf has a very thin hydrogen envelope, which has not been predicted by evolutionary models. The mass and radius of the secondary star in GK Vir are consistent with evolutionary models after correcting for the effects of irradiation by the white dwarf. The secondary star in SDSS J1212-0123 has a radius ~9 per cent larger than predicted.

Highlights

Detached eclipsing binaries are a primary source of accurate physical properties of stars and stellar remnants
We have used a combination of ULTRACAM and SOFI photometry and X-shooter spectroscopy to measure precise masses and radii for both components of the eclipsing post common envelope binaries (PCEBs) Sloan Digital Sky Survey (SDSS) J1212−0123 and GK Vir
We use our results from SDSS J1212−0123 and information from the emission lines to determine the correction factor to apply to the emission-line radial velocities in GK Vir to measure the centre-ofmass radial velocity of the secondary star

Summary

Introduction

Detached eclipsing binaries are a primary source of accurate physical properties of stars and stellar remnants. There is disagreement between models and observations, consistently resulting in radii up to 15 per cent larger and effective temperatures 400 K or more below theoretical predictions (Ribas 2006; Lopez-Morales 2007). These inconsistencies are seen in M dwarf eclipsing binaries (Bayless & Orosz 2006; Kraus et al 2011) and in field stars (Berger et al 2006; Morales, Ribas & Jordi 2008) and the host stars of transiting extrasolar planets (Torres 2007)

Results

Discussion

Conclusion