A white dwarf cooling age of 8 Gyr for NGC 6791 from physical separation processes

TL;DR

This study demonstrates that physical separation processes in white dwarf cores explain the age discrepancy of NGC 6791, aligning white dwarf cooling ages with main sequence turn-off ages.

Contribution

It provides observational evidence that phase separation and element sinking in white dwarf cores resolve the age discrepancy in NGC 6791.

Findings

Physical separation processes occur in white dwarf cores.

These processes explain the age discrepancy in NGC 6791.

White dwarf cooling ages are consistent with main sequence ages after considering separation.

Abstract

NGC 6791 is a well studied open cluster1 that it is so close to us that can be imaged down to very faint luminosities. The main sequence turn-off age (~8 Gyr) and the age derived from the termination of the white dwarf cooling sequence (~6 Gyr) are significantly different. One possible explanation is that as white dwarfs cool, one of the ashes of helium burning, 22Ne, sinks in the deep interior of these stars. At lower temperatures, white dwarfs are expected to crystallise and phase separation of the main constituents of the core of a typical white dwarf, 12C and 16O, is expected to occur. This sequence of events is expected to introduce significant delays in the cooling times, but has not hitherto been proven. Here we report that, as theoretically anticipated, physical separation processes occur in the cores of white dwarfs, solving the age discrepancy for NGC 6791.