Surface Detonations in Double Degenerate Binary Systems Triggered by Accretion Stream Instabilities

Abstract

We present three-dimensional simulations on a new mechanism for the detonation of a sub-Chandrasekhar CO white dwarf in a dynamically unstable system where the secondary is either a pure He white dwarf or a He/CO hybrid. For dynamically unstable systems where the accretion stream directly impacts the surface of the primary, the final tens of orbits can have mass accretion rates that range from 10 −5 to 10 −3 M ⊙ s −1 , leading to the rapid accumulation of helium on the surface of the primary. After ∼ 10 −2 M ⊙ of helium has been accreted, the ram pressure of the hot helium torus can deflect the accretion stream such that the stream no longer directly impacts the surface. The velocity difference between the stream and the torus produces shearing which seeds large-scale Kelvin-Helmholtz instabilities along the interface between the two regions. These instabilities eventually grow into dense knots of material that periodically strike the surface of the primary, adiabatically compressing the underlying helium torus. If the temperature of the compressed material is raised above a critical temperature, the timescale for triple-α reactions becomes comparable to the dynamical timescale, leading to the detonation of the primary's helium envelope. This detonation drives shockwaves into the primary which tend to concentrate at one or more focal points within the primary's CO core. If a relatively small amount of mass is raised above a critical temperature and density at these focal points, the CO core may itself be detonated.

Observational Clues to the Progenitors of Type Ia Supernovae

“…Instead, complex structures with regions of high temperature and density are found, in which explosive carbon ignition may occur. Such "violent mergers" may lead to a SN Ia, even if the total mass of the merger is below M Ch (Dan et al 2013, Guillochon et al 2010, van Kerkwijk, Chang & Justham 2010, Zhu et al 2013. Details depend on the WD masses (a primary WD mass ∼ > 0.9 M is likely required) and on the presence of He, which can facilitate ignition.…”

Three-dimensional simulations of gravitationally confined detonations compared to observations of SN 1991T

“…Since the spatial scales relevant to the initiation of a detonation cannot be resolved in full-star multi-dimensional explosion simulations (see the discussion in Seitenzahl et al 2009a), it is common practice (e.g. Jordan et al 2008Jordan et al , 2012aGuillochon et al 2010;Pakmor et al 2011Pakmor et al , 2012b to pick a certain critical density ρ crit and temperature T crit that a cell composed of nuclear fuel must exceed for a detonation to be initiated. However, these critical values are no more than informed guesses based on separate high-resolution onedimensional detonation initiation calculations (e.g.…”

NO X-RAYS FROM THE VERY NEARBY TYPE Ia SN 2014J: CONSTRAINTS ON ITS ENVIRONMENT

“…WD-WD mergers are, however, expected to enrich the ISM in a number of ways including (1) tidal tail ejections (Raskin & Kasen 2013), (2) mass outflows during the rapid accretion phase before the merger (Guillochon et al 2010;Dan et al 2011), (3) winds emanating from the disk during the viscous evolution (Ji et al 2013), and (4) shell ejections (Shen et al 2013; see also Soker et al 2013). Each of these mechanisms predicts the presence of relevant mass at different distances from the explosion center.…”