White dwarf-red dwarf binaries in the Sloan Digital Sky Survey^*

I. Sample definition

¹ Nordic Optical Telescope, Apartado 474, 38700 Santa Cruz de La Palma, Spain e-mail: tau@not.iac.es
² Isaac Newton Group of Telescopes, Apartado de correos 321, 38700 Santa Cruz de la Palma, Spain
³ Institute of Physics, University of Graz, Universitätsplatz 5, 8010 Graz, Austria e-mail: rgreimel@gmail.com
⁴ Visiting scientist, Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK
⁵ Department of Astrophysics, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands e-mail: [besselaar;pgroot;lmr]@astro.ru.nl

Received: 31 August 2007
Accepted: 16 October 2007

Abstract

Context. A significant fraction of binary stars consisting of a white dwarf and a low-mass main-sequence star (red dwarf) are expected to be close binaries that are the end products of common-envelope (CE) evolution.

Aims. To gain a better understanding of CE evolution, we want to study white dwarf-red dwarf binaries. For this it is fundamental to establish a well-defined sample.

Methods. To reduce contamination by more distant sources, such as quasars, we have selected candidate white dwarf-red dwarf binaries from the catalogue of proper motion stars drawn from the intersection of the Sloan Digital Sky Survey (SDSS) and the USNO-B1.0 catalogue. To separate single from binary sources and to cut spurious sources out, we define selection criteria based on a combination of the , , and  colours.

Results. We evaluate and discuss the selection criteria on the basis of both the publicly available SDSS spectra and the predicted colours of white dwarf-red dwarf binaries. We define a sample of 651 binary candidates. However, we find that for r magnitudes brighter than ~16.5 the proper motion catalogue is heavily contaminated with sources that have incorrect colours due to the r magnitudes being relatively too faint, most likely due to saturation effects.

Conclusions. We show that the level of contamination can be reduced by either excluding sources brighter than  mag or by excluding sources bluer than or and brighter than mag. We expect ~85% of the remaining sources to be genuine white dwarf-red dwarf binaries. We estimate that we exclude ~5-10% of the white dwarf-red dwarf binaries in the proper motion catalogue.