Abstract
Type Ia supernovae (SNe Ia) are among the brightest explosions in the universe. The intrinsic brightness of SNe Ia, and the relative uniformity of their light curves, enables them to be used as standardizable candles, which in turn has led to the discovery of the accelerated expansion of the universe. SNe Ia have long been believed to be caused by exploding white dwarfs, though the observational evidence in direct support of this view has emerged only recently. The exact mechanism by which SNe Ia explode is still not completely understood. However, what is clear is that white dwarfs may explode as SNe Ia when conditions for carbon burning are reached. Two pathways to achieve carbon ignition are either by accretion from a main sequence or red giant companion through the single-degenerate (SD) channel, or from a white dwarf companion through the double-degenerate (DD) channel. At present, there is no concordance between existing models and observations. The SD scenario may account for a range of outcomes, from superluminous events such as 1991T to subluminous SNe Ia such as 1991g, and very faint, low-velocity Iax events including 2002cx. However, a variety of observational constraints, including the delay-time distribution, favors the DD channel. Until very recently it was not clear how detonation conditions could be achieved in a self-consistent fashion during the merger of carbon-oxygen white dwarf binaries. With my collaborators, I have recently found, for the first time, that gravitational instability in the disk resulting from a merging white dwarf binary leads to a self consistent detonation of a primary white dwarf on a dynamical time scale. The effects of the spiral instability in the post-merger evolution of the tidally-disrupted disk surrounding the primary white dwarf are quite generic for a range of mass ratios, and may drive the merged systems towards carbon detonation in some cases. For our model reaching successful detonation, I have investigated the consequent light curves and spectra, and compared them against observation. This particular model corresponds to a slowly declining supenova (SN) Ia 2001ay. Furthermore, our work attempts to answer some of the key questions left unanswered in DD models - what is the fate of super-Mch mass white dwarf mergers over timescales longer than the dynamical time? If they do not detonate promptly, in a violent merger, will they never detonate? If they do detonate, what are the mechanisms responsible for their detonation? Would they look like normal SNe Ia?..