22Ne distillation and the cooling sequence of the old metal-rich open cluster NGC 6791

Abstract

Recent Monte Carlo plasma simulations carried out to study the phase separation of $^ Ne $ in crystallizing carbon-oxygen (CO) white dwarfs (WDs; the most abundant metal after carbon and oxygen) have shown that, under the right conditions, a distillation process that transports $^ Ne $ towards the WD centre is efficient and releases a considerable amount of gravitational energy. This can lead to cooling delays of up to several Gyr. Here we present the first CO WD stellar evolution models that self-consistently include the effect of neon distillation and cover the full range of CO WD masses for a twice-solar progenitor metallicity, which is appropriate for the old open cluster NGC 6791. The old age (about 8.5 Gyr) and high metallicity of this cluster -- and hence the high neon content (about 3<!PCT!> by mass) in the cores of its WDs -- maximize the effect of neon distillation in the models. We discuss the effect of distillation on the internal chemical stratification and cooling time of the models, confirming that distillation causes cooling delays of up to several Gyr that depend in a non-monotonic way on the mass. We also show how our models produce luminosity functions (LFs) that can match the faint end of the observed WD LF in NGC 6791, for ages consistent with the range determined from a sample of cluster eclipsing binary stars and the main sequence turn-off. Without the inclusion of distillation, the theoretical WD cooling sequences reach magnitudes that are too faint compared to observations. We also propose James Webb Space Telescope observations that would independently demonstrate the efficiency of neon distillation in the interiors of NGC 6791 WDs and help resolve the current uncertainty on the treatment of the electron conduction opacities for the hydrogen-helium envelope of WD models.