Post-common envelope binaries from SDSS - XV. Accurate stellar parameters for a cool 0.4 M⊙ white dwarf and a 0.16 M⊙ M dwarf in a 3h eclipsing binary

Abstract

We identify SDSS J121010.1+334722.9 as an eclipsing post-common-envelope binary, with an orbital period of Porb = 2.988 h, containing a very cool, low-mass, DAZ white dwarf and a low-mass main-sequence star of spectral type M5. A model atmosphere analysis of the metal absorption lines detected in the blue part of the optical spectrum, along with the Galaxy Evolution Explorer near-ultraviolet flux, yields a white dwarf temperature of Teff,WD = 6000±200K and a metallicity value of log [Z/H]=-2.0 ± 0.3. The Na I λλ8183.27, 8194.81 absorption doublet is used to measure the radial velocity of the secondary star, Ksec = 251.7 ± 2.0kms-1, and Fe I absorption lines in the blue part of the spectrum provide the radial velocity of the white dwarf, KWD = 95.3 ± 2.1kms-1, yielding a mass ratio of q = 0.379 ± 0.009. Light-curve model fitting, using the Markov chain Monte Carlo method, gives the inclination angle as i = (79.°05-79.°36) ± 0.°15, and the stellar masses as MWD = 0.415 ± 0.010M⊙ and Msec = 0.158 ±0.006M⊙. Systematic uncertainties in the absolute calibration of the photometric data influence the determination of the stellar radii. The radius of the white dwarf is found to be RWD = (0.0157-0.0161) ± 0.0003R⊙ and the volume-averaged radius of the tidally distorted secondary is Rsec,vol.aver. = (0.197-0.203) ± 0.003R⊙. The white dwarf in SDSS J121010.1+334722.9 is a very strong He-core candidate. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.