Gravitational Settling of22Ne in Liquid White Dwarf Interiors: Cooling and Seismological Effects

Abstract

We assess the impact of the trace element 22Ne on the cooling and seismology of a liquid C/O white dwarf (WD). Because of this element's neutron excess, it sinks toward the interior as the liquid WD cools. The subsequent gravitational energy released slows the cooling of the WD by 0.25-1.6 Gyr by the time it has completely crystallized, depending on the WD mass and the adopted sedimentation rate. The effects make massive WDs or those in metal-rich clusters (such as NGC 6791) appear younger than their true age. Our diffusion calculations show that the 22Ne mass fraction in the crystallized core actually increases outward. The stability of this configuration has not yet been determined. In the liquid state, the settled 22Ne enhances the internal buoyancy of the interior and changes the periods of the high radial order g-modes by ≈1%. Although a small adjustment, this level of change far exceeds the accuracy of the period measurements. A full assessment and comparison of mode frequencies for specific WDs should help constrain the still uncertain 22Ne diffusion coefficient for the liquid interior.