Massive Double White Dwarfs and the AM CVn Birthrate

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.

MNRAS, in press

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Cited by in corpus (8)

• The ELM Survey. VIII. 98 Double White Dwarf Binaries
• Discovery and characterization of five new eclipsing AM CVn systems
• The ELM Survey South. II. Two dozen new low mass white dwarf binaries
• Discovery of a Detached, Eclipsing 40 min Period Double White Dwarf Binary and a Friend: Implications for He+CO White Dwarf Mergers
• A Very Fast And Angular Momentum Conserving Tree Code
• Compact Objects in close orbits as Gravitational Wave Sources: Formation Scenarios and Properties
• Broad Observational Perspectives Achieved by the Accreting White Dwarf Sciences in the XMM-Newton and Chandra Eras
• Tidal heating in detached double white dwarf binaries