The ELM Survey. IX. A Complete Sample of Low Mass White Dwarf Binaries in the SDSS Footprint

TL;DR

This paper reports the discovery of 17 low mass white dwarf binaries using Gaia and SDSS data, expanding the sample and identifying potential gravitational wave sources for LISA.

Contribution

It introduces a new method combining Gaia and SDSS data to identify and complete a sample of low mass white dwarf binaries, including the discovery of ultra-compact systems.

Findings

Discovered 17 new low mass white dwarf binaries.

Achieved 90% completeness in the ELM survey within the specified color range.

Identified two ultra-compact binaries as future LISA gravitational wave sources.

Abstract

We present the discovery of 17 double white dwarf (WD) binaries from our on-going search for extremely low mass (ELM) <0.3 Msun WDs, objects that form from binary evolution. Gaia parallax provides a new means of target selection that we use to evaluate our original ELM Survey selection criteria. Cross-matching the Gaia and Sloan Digital Sky Survey (SDSS) catalogs, we identify an additional 36 ELM WD candidates with 17<g<19 mag and within the 3-sigma uncertainties of our original color selection. The resulting discoveries imply the ELM Survey sample was 90% complete in the color range -0.4 < (g-r)_0 < -0.1 mag (approximately 9,000 K < Teff < 22,000 K). Our observations complete the sample in the SDSS footprint. Two newly discovered binaries, J123950.370-204142.28 and J232208.733+210352.81, have orbital periods of 22.5 min and 32 min, respectively, and are future LISA gravitational wave sources.