The structure and fate of white dwarf merger remnants

Abstract

We present a large parameter study where we investigate the structure of white dwarf (WD) merger remnants after the dynamical phase. A wide range of WD masses and compositions are explored and we also probe the effect of different initial conditions. We investigated the degree of mixing between the WDs, the conditions for detonations as well as the amount of gas ejected. We find that systems with lower mass ratios have more total angular momentum and as a result more mass is flung out in a tidal tail. Nuclear burning can affect the amount of mass ejected. Many WD binaries that contain a helium-rich WD achieve the conditions to trigger a detonation. In contrast, for carbon-oxygen transferring systems only the most massive mergers with a total mass M 2.1M detonate. Even systems with lower mass may detonate long after the merger if the remnant remains above the Chandrasekhar mass and carbon is ignited at the centre. Finally, our findings are discussed in the context of several possible observed astrophysical events and stellar systems, such as hot subdwarfs, R Coronae Borealis stars, single massive white dwarfs, supernovae of type Ia and other transient events. A large database containing 225 white dwarf merger remnants is made available via a dedicated web page.

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

“…combinations. This tendency of weak dependence on the resolution has already been reported in previous studies (Raskin et al 2012;Dan et al 2014). …”

“…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.…”

“…We calculate both timescales for each particle in the merger phase and examine whether the particles satisfy Equation (3). In this work, C P is derived from the Helmholtz EOS of Timmes & Swesty (2000) and CC  is the same as that in Dan et al (2014), originally proposed by Blinnikov & KhoKhlov (1987). Its formulation is where n C is the number density of carbon and N a is Avogadro's number.…”

Theoretical uncertainties of the Type Ia supernova rate

“…In particular, Nomoto & Kondo (1991) show that the merger product evolves towards an accretion-induced collapse (AIC) rather than towards a normal SN Ia. Only recently, some studies indicate that the merger can lead to a SN Ia explosion under certain circumstances (Pakmor et al 2010(Pakmor et al , 2011(Pakmor et al , 2013Shen et al 2013;Dan et al 2014). Finally, binary population synthesis (BPS) studies show that neither channel reproduces the observed SN Ia rate (e.g.…”

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

“…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.…”