White dwarf cooling anomaly

[6] At a certain point, within the cooling core of a carbon-oxygen white dwarf, electrostatic interactions will start to dominate over thermal motion, and the ions will crystallize into a lattice structure.

[7] This will produce a pile-up on the HR diagram as the aging white dwarfs spend up to a billion years in the region where crystallization occurs.

[9][8] On the HR diagram for these stars, three branch-like groupings are visible, dubbed the A, B, and Q branches, after the DA, DB, and DQ classifications for white dwarf atmospheres, respectively.

This pile-up of higher mass white dwarfs is narrower and more luminous than is predicted by standard crystallization models, suggesting an extra, anomalous cooling delay is at work.

[2] This element is produced in stellar cores that generate energy from the CNO cycle,[2] which is the dominant fusion process in ordinary main sequence stars with at least 1.3 times the mass of the Sun.

This neutron-heavy isotope of neon experiences a downward pressure in the degenerate interior of a carbon-oxygen white dwarf, causing it to settle toward the core.

[11] The initial gravitational energy potential of the neon isotope within a solar mass white dwarf is 6.8×1047 erg, which is sufficient to delay the cooling for about 8.9 billion years.

In a liquid state that is approaching crystalization, groups of neon ions could become more strongly coupled to each other compared to the surrounding mix of carbon and oxygen.

[12] However, simulations of this hypothesis demonstrated that the formation of 22Ne clusters can not take place at the low abundance levels found in typical carbon-oxygen white dwarfs.

If this distillation process begins when crystallization is first initiated in a white dwarf, the result is a neon-rich core and a large release of gravitational energy.