Gravitational Settling of 22Ne in Liquid White Dwarf Interiors

Abstract

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 towards 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 WD's with masses in excess of a solar mass. The current uncertainties in the microphysics even allow for solutions where a 1.2M_\odot WD remains mostly liquid for a few Gyrs due to the internal heating from 22Ne sedimentation. This highlights the need for an accurate calculation of the inter-diffusion coefficient, especially in the quantum liquid regime relevant for high mass WD's. There is also time in old, liquid WD's (such as those found in cataclysmic variables and possibly in accreting Type Ia progenitors) for partial settling.