Gravitational Settling of ²²Ne in Liquid White Dwarf Interiors

The nuclear reactions that occur in the stellar progenitors of white dwarfs (WDs) lead to an internal composition of ¹²C, ¹⁶O, and a "contaminant" nucleus, ²²Ne. The ²²Ne is produced by helium captures on ¹⁴N left from hydrogen burning via the CNO cycle. By virtue of its two excess neutrons (relative to the predominant A = 2Z nuclei), a downward force of ≈2m_pg is exerted on ²²Ne in the WD interior. This biases its diffusive equilibrium, forcing ²²Ne to settle toward the center of the WD. We discuss the physics of the gravitational settling when the WD is in the liquid state and the luminosity generated by it. This modifies the cooling of WDs with masses in excess of M_☉. The current uncertainties in the microphysics even allow for solutions where a 1.2 M_☉ WD remains mostly liquid for a few gigayears because of the internal heating from ²²Ne sedimentation. This highlights the need for an accurate calculation of the interdiffusion coefficient, especially in the quantum liquid regime relevant for high-mass WDs. There is also time in old, liquid WDs (such as those found in cataclysmic variables and possibly in accreting Type Ia progenitors) for partial settling.