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