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Evolutionary Models for the Remnant of the Merger of Two Carbon-Oxygen Core White Dwarfs

We construct evolutionary models of the remnant of the merger of two carbon-oxygen (CO) core white dwarfs (WDs). With total masses in the range $1-2 {\rm M_\odot}$, these remnants may either leave behind a single massive WD or undergo a merger-induced collapse to a neutron star (NS). On the way to their final fate, these objects generally experience a $\sim 10$ kyr luminous giant phase, which may be extended if sufficient helium remains to set up a stable shell-burning configuration. The uncertain, but likely significant, mass loss rate during this phase influences the final remnant mass and fate (WD or NS). We find that the initial CO core composition of the WD is converted to oxygen-neon (ONe) in remnants with final masses $\gtrsim 1.05 {\rm M_\odot}$. This implies that the CO core / ONe core transition in single WDs formed via mergers occurs at a similar mass as in WDs descended from single stars, and thus that WD-WD mergers do not naturally provide a route to producing ultra-massive CO-core WDs. As the remnant contracts towards a compact configuration, it experiences a "bottleneck" that sets the characteristic total angular momentum that can be retained. This limit predicts single WDs formed from WD-WD mergers have rotational periods of $\approx 10-20$ min on the WD cooling track. Similarly, it predicts remnants that collapse can form NSs with rotational periods $\sim 10$ ms.