The ELM Survey. II. Twelve Binary White Dwarf Merger Systems

TL;DR

This paper reports the discovery of twelve binary white dwarf systems, including four new short-period binaries, and discusses their implications for stellar evolution, mergers, and related transient phenomena.

Contribution

It presents new observations of ELM white dwarf binaries, increasing the known sample, and explores their characteristics, merger outcomes, and potential links to various astrophysical phenomena.

Findings

12 systems will merge within a Hubble time due to gravitational waves

Tripled the number of known merging white dwarf systems

Evidence for a new formation channel for single low-mass WDs

Abstract

We describe new radial velocity and X-ray observations of extremely low-mass white dwarfs (ELM WDs, ~0.2 Msol) in the Sloan Digital Sky Survey Data Release 4 and the MMT Hypervelocity Star survey. We identify four new short period binaries, including two merger systems. These observations bring the total number of short period binary systems identified in our survey to 20. No main-sequence or neutron star companions are visible in the available optical photometry, radio, and X-ray data. Thus, the companions are most likely WDs. Twelve of these systems will merge within a Hubble time due to gravitational wave radiation. We have now tripled the number of known merging WD systems. We discuss the characteristics of this merger sample and potential links to underluminous supernovae, extreme helium stars, AM CVn systems, and other merger products. We provide new observational tests of the WD mass-period distribution and cooling models for ELM WDs. We also find evidence for a new formation channel for single low-mass WDs through binary mergers of two lower mass objects.