How the merger of two white dwarfs depends on their mass ratio: orbital stability and detonations at contact

Abstract

Despite their unique astrophysical relevance, the outcome of white dwarf binary mergers has so far only been studied for a very restricted number of systems. Here we present the results of a survey with more than 200 simulations systematically scanning the white dwarf binary parameter space. We consider white dwarf masses ranging from 0.2 to 1.2 M⊙ and account for their different chemical compositions. We find excellent agreement with the orbital evolution predicted by mass transfer stability analysis. Much of our effort in this paper is dedicated to determining which binary systems are prone to a thermonuclear explosion just prior to merger or at surface contact. We find that a large fraction of He‐accreting binary systems explode: all dynamically unstable systems with accretor masses below 1.1 M⊙ and donor masses above ∼0.4 M⊙ are found to trigger a helium detonation at surface contact. A substantial fraction of these systems could explode at earlier times via detonations induced by instabilities in the accretion stream, as we have demonstrated in our previous work. We do not find definitive evidence for an explosion prior to merger or at surface contact in any of the studied double carbon–oxygen systems. Although we cannot exclude their occurrence if some helium is present, the available parameter space for a successful detonation in a white dwarf binary of pure carbon–oxygen composition is small. We demonstrate that a wide variety of dynamically unstable systems are viable Type Ia candidates. The next decade thus holds enormous promise for the study of these events, in particular with the advent of wide‐field synoptic surveys allowing a detailed characterization of their explosive properties.

A SYSTEMATIC STUDY OF CARBON–OXYGEN WHITE DWARF MERGERS: MASS COMBINATIONS FOR TYPE Ia SUPERNOVAE

“…Zhu et al (2013) mainly focused on the (early) remnant phase and the resolutions of their simulations are lower than those in our study. Although Dan et al (2012Dan et al ( , 2014 covered the merger phase and remnant phase, their numerical resolution is much lower than ours. We expect that SN Ia explosions occur not only in the early remnant phase but also in the dynamic merger phase.…”

“…Similar parameter surveys have already been done by Dan et al (2012Dan et al ( , 2014 and Zhu et al (2013). Zhu et al (2013) mainly focused on the (early) remnant phase and the resolutions of their simulations are lower than those in our study.…”

White Dwarf Mergers on Adaptive Meshes. I. Methodology and Code Verification

“…We do not intend here to directly compare results between the two simulation methods. We limit ourselves to the observation that at least for grid-based codes, a parameter study such as the ones performed by Dan et al (2012Dan et al ( , 2014 would likely not yield qualitatively convergent results if it were to use the same effective mass resolution. Instead the number of zones inside each star should at least be doubled.…”

“…For this paper we adopt the scheme of Dan et al (2012). Low-mass WDs are pure helium; low-to-intermediate-mass WDs are an even carbon-oxygen core with a relatively large helium envelope; intermediate-mass WDs are a carbon-oxygen core with slightly more oxygen than carbon; and, high-mass WDs are composed of oxygen, neon, and magnesium.…”

“…Notably, the detonation occurred self-consistently and did not need to be intentionally triggered using an external source term. Dan et al (2012Dan et al ( , 2014 performed a large sweep of the parameter space for merger pairs and found that pure carbon-oxygen systems would generally not lead to detonations (and thus be violent mergers) except for the most massive systems. They did find that for systems with WDs containing helium, many would detonate and potentially lead to SNe Ia, either through the aforementioned instabilities in the accretion stream, or during the contact phase, similar to the violent carbon-oxygen WD mergers.…”