Astrocrash

For this project we performed three-dimensional simulations of double-degenerate white dwarf binaries in FLASH to determine if anything interesting happens during the high-accretion rate phase that precedes merger 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 $10^{-3} M_{\odot}$ ${\rm s}^{-1}$, leading to the rapid accumulation of helium on the surface of the primary. After $\sim 10^{-2} M_{\odot}$ 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-$\alpha$ 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.

For a detailed discussion, please read our paper:
Surface Detonations in Double Degenerate Binary Systems Triggered by Accretion Stream Instabilities. 2009. J. Guillochon, M. Dan, E. Ramirez-Ruiz, S. Rosswog. 2010 ApJL, 709, 64L.

Movies from our simulations

Run A	$\log_{10} \rho$ at early times, orbital plane. (MOV, 17.1 MB) $T$ at late times, orbital plane. (MOV, 173.9 MB) SPH particle density, used as boundary condition. (AVI, 22.1 MB)
Run B	$\log_{10} T$, volume rendering. (MOV, 22.9 MB) $T$, orbital plane. (MOV, 94.6 MB) $\log_{10} (\rho X_{\rm He})$, volume rendering. (MOV, 70.4 MB) $\log_{10} (\rho|\vec{v}|^2)$ of CO core during surface detonation, volume rendering. (MOV, 6.6 MB) $\log_{10} X_{\rm He}$ during surface detonation, orbital plane. (MOV, 1.4 MB) $\log_{10} X_{\rm C}$ during surface detonation, orbital plane. (MOV, 4.8 MB) $\log_{10} X_{\rm O}$ during surface detonation, orbital plane. (MOV, 3.0 MB) $\log_{10} X_{\rm Ne}$ during surface detonation, orbital plane. (MOV, 7.4 MB) $\log_{10} X_{\rm Mg}$ during surface detonation, orbital plane. (MOV, 3.7 MB) $\log_{10} X_{\rm Si}$ during surface detonation, orbital plane. (MOV, 6.0 MB) $\log_{10} X_{\rm S}$ during surface detonation, orbital plane. (MOV, 1.9 MB) $\log_{10} X_{\rm Ar}$ during surface detonation, orbital plane. (MOV, 1.7 MB) $\log_{10} X_{\rm Ca}$ during surface detonation, orbital plane. (MOV, 1.5 MB) $\log_{10} X_{\rm Ti}$ during surface detonation, orbital plane. (MOV, 3.6 MB) $\log_{10} X_{\rm Cr}$ during surface detonation, orbital plane. (MOV, 868 KB) $\log_{10} X_{\rm Ni}$ during surface detonation, orbital plane. (MOV, 276 KB) SPH particle density, used as boundary condition. (AVI, 15.6 MB)
Run Ba	$\log_{10} \rho$, orbital plane. (MOV, 6.9 MB) $T$, orbital plane. (MOV, 10.2 MB)
Run C	$\log_{10} \rho$, orbital plane. (MOV, 16.5 MB) $\log_{10} T$ during surface detonation, volume rendering. (MOV, 852 KB) $T$, orbital plane. (MOV, 24.0 MB) $\log_{10} (\rho|\vec{v}|^2)$ during surface detonation, orbital plane. (MOV, 3.1 MB) $\sum_{A = 20}^{56} X_A$ during surface detonation, orbital plane. (MOV, 3.6 MB)