Massive Double White Dwarfs and the AM CVn Birthrate

TL;DR

This study investigates four low-mass white dwarf binaries, finds they are likely to evolve into AM CVn systems through stable mass transfer, and estimates the galactic rate of such events.

Contribution

It provides new observational constraints on the nature of low-mass white dwarf binaries and demonstrates their role as progenitors of AM CVn systems, supporting their contribution to the galactic population.

Findings

X-ray observations rule out neutron star companions.

Total binary masses exceed 1.02 to 1.39 solar masses.

The inspiral rate of these systems aligns with the observed AM CVn density.

Abstract

We present Chandra and Swift X-ray observations of four extremely low-mass (ELM) white dwarfs with massive companions. We place stringent limits on X-ray emission from all four systems, indicating that neutron star companions are extremely unlikely and that the companions are almost certainly white dwarfs. Given the observed orbital periods and radial velocity amplitudes, the total masses of these binaries are greater than 1.02 to 1.39 Msun. The extreme mass ratios between the two components make it unlikely that these binary white dwarfs will merge and explode as Type Ia or underluminous supernovae. Instead, they will likely go through stable mass transfer through an accretion disk and turn into interacting AM CVn. Along with three previously known systems, we identify two of our targets, J0811 and J2132, as systems that will definitely undergo stable mass transfer. In addition, we use the binary white dwarf sample from the ELM Survey to constrain the inspiral rate of systems with extreme mass ratios. This rate, 0.00017/year, is consistent with the AM CVn space density estimated from the Sloan Digital Sky Survey. Hence, stable mass transfer double white dwarf progenitors can account for the entire AM CVn population in the Galaxy.