A PARAMETER-SPACE STUDY OF CARBON-OXYGEN WHITE DWARF MERGERS
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abstract
The merger of two carbon-oxygen white dwarfs can lead either to a spectacular
transient, stable nuclear burning or a massive, rapidly rotating white dwarf.
Simulations of mergers have shown that the outcome strongly depends on whether
the white dwarfs are similar or dissimilar in mass. In the similar-mass case,
both white dwarfs merge fully and the remnant is hot throughout, while in the
dissimilar case, the more massive, denser white dwarf 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 white dwarf mergers for a
large range of masses using smoothed-particle hydrodynamics. We find that the
structure of the merger remnant varies smoothly as a function of the ratio of
the central densities of the two white dwarfs. A density ratio of 0.6
approximately separates similar and dissimilar mass mergers. Confirming
previous work, we find that the temperatures of most merger remnants are not
high enough to immediately ignite carbon fusion. During subsequent viscous
evolution, however, the interior will likely be compressed and heated as the
disk accretes and the remnant spins down. We find from simple estimates that
this evolution 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.
