SUB-CHANDRASEKHAR WHITE DWARF MERGERS AS THE PROGENITORS OF TYPE Ia SUPERNOVAE

Abstract

Type Ia supernovae (SNe Ia) are generally thought to be due to the thermonuclear explosions of carbon–oxygen
\nwhite dwarfs (COWDs) with masses near the Chandrasekhar mass. This scenario, however, has two long-standing
\nproblems. First, the explosions do not naturally produce the correct mix of elements, but have to be finely tuned
\nto proceed from subsonic deflagration to supersonic detonation. Second, population models and observations
\ngive formation rates of near-Chandrasekhar WDs that are far too small. Here, we suggest that SNe Ia instead
\nresult from mergers of roughly equal-mass CO WDs, including those that produce sub-Chandrasekhar mass
\nremnants. Numerical studies of such mergers have shown that the remnants consist of rapidly rotating cores that
\ncontain most of the mass and are hottest in the center, surrounded by dense, small disks. We argue that the disks
\naccrete quickly, and that the resulting compressional heating likely leads to central carbon ignition. This ignition
\noccurs at densities for which pure detonations lead to events similar to SNe Ia. With this merger scenario, we
\ncan understand the type Ia rates and have plausible reasons for the observed range in luminosity and for the
\nbias of more luminous supernovae toward younger populations. We speculate that explosions of WDs slowly
\nbrought to the Chandrasekhar limit—which should also occur—are responsible for some of the “atypical” SNe Ia.