The Q Branch Cooling Anomaly Can Be Explained by Mergers of White Dwarfs and Subgiant Stars

TL;DR

This paper proposes that mergers between white dwarfs and subgiant stars can explain the Q branch cooling anomaly in white dwarfs, offering a new mechanism involving nuclear distillation that accounts for observed stellar populations.

Contribution

It introduces a novel merger scenario involving white dwarfs and subgiants that explains the Q branch anomaly without requiring unusual initial metallicities.

Findings

Merger of WDs with subgiants creates 26Mg and 22Ne, releasing energy during distillation.

This energy release can account for the prolonged cooling delays observed in the Q branch.

The high fraction of old stars on the Q branch supports the merger hypothesis.

Abstract

Gaia's exquisite parallax measurements allowed for the discovery and characterization of the Q branch in the Hertzsprung-Russell diagram, where massive C/O white dwarfs (WDs) pause their dimming due to energy released during crystallization. Interestingly, the fraction of old stars on the Q branch is significantly higher than in the population of WDs that will become Q branch stars or that were Q branch stars in the past. From this, Cheng et al. inferred that ~6% of WDs passing through the Q branch experience a much longer cooling delay than that of standard crystallizing WDs. Previous attempts to explain this cooling anomaly have invoked mechanisms involving super-solar initial metallicities. In this paper, we describe a novel scenario in which a standard composition WD merges with a subgiant star. The evolution of the resulting merger remnant leads to the creation of a large amount of 26Mg, which, along with the existing 22Ne, undergoes a distillation process that can release enough energy to explain the Q branch cooling problem without the need for atypical initial abundances. The anomalously high number of old stars on the Q branch may thus be evidence that mass transfer from subgiants to WDs leads to unstable mergers.