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

Abstract

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 M ⊙ , 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  ∼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  ≳1.05 M ⊙ . 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 ultramassive CO-core WDs. As the remnant contracts toward a compact configuration, it experiences a “bottleneck” that sets the characteristic total angular momentum that can be retained. This limit predicts that single WDs formed from WD–WD mergers have rotational periods of ≈10–20 minutes on the WD cooling track. Similarly, it predicts remnants that collapse can form NSs with rotational periods  ∼10 ms.