The ELM Survey. IV. 24 White Dwarf Merger Systems

TL;DR

This paper reports the discovery of 24 white dwarf merger systems, including new binaries and potential gravitational wave sources, significantly expanding the known population and providing insights into their evolution and future observability.

Contribution

It presents new observations identifying seven binary systems, increasing the known merger white dwarf population, and highlights potential gravitational wave sources for upcoming space missions.

Findings

Identified seven new binary white dwarf systems with 1-18 h periods.

Discovered five systems that will merge within 10 Gyr due to gravitational waves.

Found three systems that are excellent gravitational wave sources for LISA-like missions.

Abstract

We present new radial velocity and X-ray observations of extremely low-mass (ELM, 0.2 Msol) white dwarf candidates in the Sloan Digital Sky Survey (SDSS) Data Release 7 area. We identify seven new binary systems with 1-18 h orbital periods. Five of the systems will merge due to gravitational wave radiation within 10 Gyr, bringing the total number of merger systems found in the ELM Survey to 24. The ELM Survey has now quintupled the known merger white dwarf population. It has also discovered the eight shortest period detached binary white dwarf systems currently known. We discuss the characteristics of the merger and non-merger systems observed in the ELM Survey, including their future evolution. About half of the systems have extreme mass ratios. These are the progenitors of the AM Canum Venaticorum systems and supernovae .Ia. The remaining targets will lead to the formation of extreme helium stars, subdwarfs, or massive white dwarfs. We identify three targets that are excellent gravitational wave sources. These should be detected by the Laser Interferometer Space Antenna (LISA)-like missions within the first year of operation. The remaining targets are important indicators of what the Galactic foreground may look like for gravitational wave observatories.