We explore collisions between two white dwarfs as a pathway for making Type Ia supernovae (SNIa). White dwarf number densities in globular clusters allow 10-100, redshift 1 collisions per year, and observations by Chomiuk et al. of globular clusters in the nearby S0 galaxy NGC 7457 have detected what is likely to be a SNIa remnant. We carry out simulations of the collision between two 0.6 M white dwarfs at various impact parameters and mass resolutions. For impact parameters less than half the radius of the white dwarf, we find such collisions produce ≈0.4 M of 56 Ni, making such events potential candidates for underluminous SNIa or a new class of transients between Novae and SNIa.

Merging white dwarfs are a possible progenitor of Type Ia supernovae (SNe Ia). While it is not entirely clear if and when an explosion is triggered in such systems, numerical models suggest that a detonation might be initiated before the stars have coalesced to form a single compact object. Here we study such "peri-merger" detonations by means of numerical simulations, modeling the disruption and nucleosynthesis of the stars until the ejecta reach the coasting phase. Synthetic light curves and spectra are generated for comparison with observations. Three models are considered with primary masses 0.96 M , 1.06 M , and 1.20 M . Of these, the 0.96 M dwarf merging with an 0.81 M companion, with a 56 Ni yield of 0.58 M , is the most promising candidate for reproducing common SNe Ia. The more massive mergers produce unusually luminous SNe Ia with peak luminosities approaching those attributed to "super-Chandrasekhar" mass SNe Ia. While the synthetic light curves and spectra of some of the models resemble observed SNe Ia, the significant asymmetry of the ejecta leads to large orientation effects. The peak bolometric luminosity varies by more than a factor of 2 with the viewing angle, and the velocities of the spectral absorption features are lower when observed from angles where the light curve is brightest. The largest orientation effects are seen in the ultraviolet, where the flux varies by more than an order of magnitude. Despite the large variation with viewing angle, the set of three models roughly obeys a width-luminosity relation, with the brighter light curves declining more slowly in the B-band. Spectral features due to unburned carbon from the secondary star are also seen in some cases.

We carry out a comprehensive smooth particle hydrodynamics simulation survey of doubledegenerate white dwarf binary mergers of varying mass combinations in order to establish correspondence between initial conditions and remnant configurations. We find that all but one of our simulation remnants share general properties such as a cold, degenerate core surrounded by a hot disk, while our least massive pair of stars forms only a hot disk. We characterize our remnant configurations by the core mass, the rotational velocity of the core, and the half-mass radius of the disk. We also find that some of our simulations with very massive constituent stars exhibit helium detonations on the surface of the primary star before complete disruption of the secondary. However, these helium detonations are insufficiently energetic to ignite carbon, and so do not lead to prompt carbon detonations.

We present a comprehensive study of white dwarf collisions as an avenue for creating type Ia supernovae. Using a smooth particle hydrodynamics code with a 13-isotope, α-chain nuclear network, we examine the resulting 56 Ni yield as a function of total mass, mass ratio, and impact parameter. We show that several combinations of white dwarf masses and impact parameters are able to produce sufficient quantities of 56 Ni to be observable at cosmological distances. We find the 56 Ni production in double-degenerate white dwarf collisions ranges from sub-luminous to the super-luminous, depending on the parameters of the collision. For all mass pairs, collisions with small impact parameters have the highest likelihood of detonating, but 56 Ni production is insensitive to this parameter in high-mass combinations, which significantly increases their likelihood of detection. We also find that the 56 Ni dependence on total mass and mass ratio is not linear, with larger mass primaries producing disproportionately more 56 Ni than their lower mass secondary counterparts, and symmetric pairs of masses producing more 56 Ni than asymmetric pairs.

The merger of two white dwarfs may be preceded by the ejection of some mass in "tidal tails", creating a circumstellar medium around the system. We consider the variety of observational signatures from this material, which depend on the lag time between the start of the merger and the ultimate explosion (assuming one occurs) of the system in a Type Ia supernova. If the time lag is fairly short, the interaction of the supernova ejecta with the tails could lead to detectable shock emission at radio, optical, and/or x-ray wavelengths. At somewhat later times, the tails produce relatively broad NaID absorption lines with velocity widths of order the white dwarf escape speed (∼ 1000 km s −1 ). That none of these signatures have been detected in normal SNe Ia constrains the lag time to be either very short ( 100 s) or fairly long ( 100 yr). If the tails have expanded and cooled over timescales ∼ 10 4 yr, they could be observable through narrow NaID and CaII H&K absorption lines in the spectra, which are seen in some fraction of SNe Ia. Using a combination of 3D and 1D hydrodynamical codes, we model the mass-loss from tidal interactions in binary systems, and the subsequent interactions with the interstellar medium, which produce a slow-moving, dense shell of gas. We synthesize NaID line profiles by ray-casting through this shell, and show that in some circumstances tidal tails could be responsible for narrow absorptions similar to those observed.