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 M ⊙ . 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, 1.7 × 10 −4 yr −1 , 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.

The ELM Survey. VIII. Ninety-eight Double White Dwarf Binaries

“…These facts motivated our follow-up radio and Xray search for millisecond pulsar companions in the ELM Survey. Previous searches for millisecond pulsar companions to known low mass WDs, mostly in the ELM Survey, have yielded no neutron star counterparts (van Leeuwen et al 2007;Agüeros et al 2009b,a;Kilic et al 2013Kilic et al , 2014bKilic et al , 2016Andrews et al 2018). We present our final set of radio and X-ray observations of low mass WDs in the ELM Survey sample.…”

“…We obtained Chandra X-ray Observatory (Weisskopf et al 2002) observations for ten low mass ELM candidates. Eight are previously published (Kilic et al 2013(Kilic et al , 2014b(Kilic et al , 2016. We present the results for the final two can-1 https://www.cv.nrao.edu/∼sransom/presto/ didates with Chandra observations, J0147+0113 and J2245+0750 here.…”

Discovery and characterization of five new eclipsing AM CVn systems

“…Shen 2015). Kilic et al (2016) show that the number of known white dwarfs with 𝑀 0.8 M ⊙ and low mass white dwarf companions (WDchannel) is sufficient to explain the AM CVn birth rate. However, the low-mass white dwarfs have a low entropy which would result in cool and small donors (Deloye et al 2007), inconsistent with our results (Figure 12).…”

A Very Fast and Angular Momentum Conserving Tree Code

“…One such system is a high mass ratio double white dwarf (DWD) with the larger, less massive star filling its Roche lobe. A system like this, if stable to mass transfer, is a potential progenitor of an AM Canum Venaticorum (AM CVn) type cataclysmic variable binary star (Marsh et al 2004;Kilic et al 2016). Our test problem is an approximation of such a system.…”