Post Common Envelope Binaries from SDSS. I: 101 white dwarf main sequence binaries with multiple SDSS spectroscopy

TL;DR

This paper analyzes 101 white dwarf-main sequence binaries from SDSS, identifying post common envelope binaries and estimating their properties, revealing a minimum 15% PCEB fraction and discussing spectral and distance measurement discrepancies.

Contribution

It provides the first detailed analysis of a large SDSS WDMS sample, identifying PCEBs and examining spectral and distance estimation issues with implications for stellar activity.

Findings

At least 15% of WDMS are PCEBs based on radial velocity variations.

Approximately one third show significant distance estimate discrepancies.

Secondary star spectral types may be systematically too early by 1-2 classes.

Abstract

We present a detailed analysis of 101 white dwarf-main sequence binaries (WDMS) from the Sloan Digital Sky Survey (SDSS) for which multiple SDSS spectra are available. We detect significant radial velocity variations in 18 WDMS, identifying them as post common envelope binaries (PCEBs) or strong PCEB candidates. Strict upper limits to the orbital periods are calculated, ranging from 0.43 to 7880 d. Given the sparse temporal sampling and relatively low spectral resolution of the SDSS spectra, our results imply a PCEB fraction of >=15% among the WDMS in the SDSS data base. Using a spectral decomposition/fitting technique we determined the white dwarf effective temperatures and surface gravities, masses, and secondary star spectral types for all WDMS in our sample. Two independent distance estimates are obtained from the flux scaling factors between the WDMS spectra, and the white dwarf models and main sequence star templates, respectively. Approximately one third of the systems in our sample show a significant discrepancy between the two distance estimates. In the majority of discrepant cases, the distance estimate based on the secondary star is too large. A possible explanation for this behaviour is that the secondary star spectral types that we determined from the SDSS spectra are systematically too early by 1-2 spectral classes. This behaviour could be explained by stellar activity, if covering a significant fraction of the star by cool dark spots will raise the temperature of the inter-spot regions. Finally, we discuss the selection effects of the WDMS sample provided by the SDSS project.