Chenchong Zhu, Philip Chang, Marten van Kerkwijk, James Wadsley
(Abbrev.) The merger of two carbon-oxygen white dwarfs (WDs) can lead to a spectacular transient--an SN Ia or AIC--or the formation of a carbon star or massive, rapidly rotating WD. Simulations of mergers have shown that the outcome strongly depends on whether the WDs are similar or dissimilar in mass. In the similar-mass case, both WDs merge fully and the remnant is hot throughout, while in the dissimilar case, the more massive, denser WD remains cold and essentially intact, with the disrupted lower mass one wrapped around it in a hot envelope and disk. In order to determine what constitutes "similar in mass" and more generally how the properties of the merger remnant depend on the input masses, we simulated unsynchronized carbon-oxygen WD mergers for a large range of masses using smoothed-particle hydrodynamics. Generally, we find that the properties of the remnants vary smoothly as a function of the two masses, with the remnant structure determined primarily by the ratio of the central densities of the two WDs. A density ratio of 0.6, equivalent to a mass difference of about 0.1 Msun, approximately separates similar and dissimilar mass mergers. Confirming previous work, we find that the temperatures of the merger remnants are not high enough to immediately ignite carbon fusion, except possibly for WD masses approaching 1 Msun. During subsequent viscous evolution, however, the interior will likely be compressed and heated as the disk accretes and the remnant spins down. From first-order estimates of the evolution, where we assume that the remnant spins down completely, that all rotational energy is used to expel matter to large distances, and that the remaining mass evolves adiabatically, we find that this can lead to ignition for many remnants. For similar-mass mergers, this would likely occur under sufficiently degenerate conditions that a thermonuclear runaway would ensue.
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http://arxiv.org/abs/1210.3616
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