Violent mergers revisited: The origin of the fastest stars in the Galaxy

Abstract

Binary systems of two carbon-oxygen white dwarfs are one of the most promising candidates for the progenitor systems of Type Ia supernovae. Violent mergers, where the primary white dwarf ignites when the secondary white dwarf smashes onto it while being disrupted on its last orbit, were the first proposed double degenerate merger scenario that ignites dynamically. However, violent mergers likely contribute only a few per cent to the total Type Ia supernova rate and do not yield normal Type Ia supernova light curves. Here we revisit the scenario, simulating a violent merger with better methods, and in particular a more accurate treatment of the detonation. We find good agreement with previous simulations, with one critical difference. The secondary white dwarf, being disrupted and accelerated towards the primary white dwarf, and impacted by its explosion, does not fully burn. Its core survives as a bound object. The explosion leaves behind a $0.16\,\mathrm{M_\odot}$ carbon-oxygen white dwarf travelling $2800\,\mathrm{km/s}$, making it an excellent (and so far the only) candidate to explain the origin of the fastest observed hyper-velocity white dwarfs. We also show that before the explosion, $5\times10^{-3}\,\mathrm{M_\odot}$ of material consisting predominantly of helium, carbon, and oxygen has already been ejected at velocities above $1000\,\mathrm{km/s}$. Finally, we argue that if a violent merger made D6-1 and D6-3, and violent mergers require the most massive primary white dwarfs in binaries of two carbon-oxygen white dwarfs, there has to be a much larger population of white dwarf mergers with slightly lower-mass primary white dwarfs. Because of its size, this population can essentially only give rise to normal Type Ia supernovae, likely exploding via the quadruple detonation channel and leaving no bound object behind.

Figures (8)

• Figure 1: Time evolution of the separation between the two white dwarfs. We first shrink the binary system at a constant rate of $100\,\mathrm{km/s}$ for $108\,\mathrm{s}$. We then evolve it for another $395\,\mathrm{s}$ self-consistently, that is, the total angular momentum in the simulation is conserved. In this phase mass transfer only shrinks it at a rate of $\approx 0.5\,\mathrm{km/s}$. We then actively shrink it again at a rate of $10\,\mathrm{km/s}$ until it merges $166\,\mathrm{s}$ later. The cross denotes the time of explosion.
• Figure 2: Density (left panel) and temperature (right panel) slices of the binary system during the merger at the time when it reaches conditions for carbon ignition. The purple circle shows the point where $32$ cells reach a temperature of $10^9\,\mathrm{K}$ at a density of $2\times10^6\,\mathrm{g\,cm^{-3}}$. We ignite a detonation there (see text). The black contours in the left panel include $95\%$ of the material that will become the bound remnant after the explosion.
• Figure 3: Slices through the ejecta $100\,\mathrm{s}$ after the explosion in the plane of rotation of the binary system (top rows) and perpendicular to it (bottom rows). At this time the ejecta are fully in homologous expansion. The panels show the density in the top left panel and mass fractions of different isotopes in the other panels. The open circle indicates the centre of the ejecta. The filled white circle appears where the material of the bound remnant is located, which has been removed here. The structures look very similar to older violent merger simulations, showing a clear global asymmetry and material originating from the secondary white dwarf close to the centre.
• Figure 4: Slices of density (first column), pressure (second column), radial velocity (third column), and azimuthal velocity (fourth panel) of the bound remnant $1000\,\mathrm{s}$ after the explosion. The top row shows slices in the plane of rotation of the initial binary system, the bottom row slices perpendicular to it. The velocities are relative to the rest frame velocity of the bound remnant. The inner part of the $0.16\,\mathrm{M_\odot}$ bound white dwarf remnant is essentially spherical, the outer parts are clearly not.
• Figure 5: Radial profiles of the bound remnant $1000\,\mathrm{s}$ after the explosion. The panels show from left to right and top to bottom profiles of density, specific energies, temperature, cumulative mass, cumulative angular momentum, and mean atomic weight. The essentially spherical part of the remnant seen in Figure \ref{['fig:remnant']} extends out to a radius of about $0.04\,\mathrm{R_\odot}$. It is essentially non-rotating and contains about half of the bound mass of the remnant. The outer parts are rotating, cold, and contain a little bit of the heavier elements from the ashes of the explosion.