White Dwarf Merger Remnants with Cooling Delays on the Q Branch Lack Strong Magnetism

TL;DR

This study investigates the properties of anomalous ultra-massive white dwarfs with cooling delays, revealing they lack strong magnetism and pulsate, challenging existing merger models and suggesting alternative formation scenarios.

Contribution

The paper provides the first spectroscopic, volume-limited analysis of the Q branch white dwarfs, showing they do not exhibit expected strong magnetism from mergers, and identifies pulsations in some of these stars.

Findings

White dwarfs with long cooling delays lack strong magnetism.

Some Q branch white dwarfs show pulsations, extending the DAV instability strip.

Differences in atmospheric composition and rotation rates correlate with tangential velocity.

Abstract

A population of anomalous ultra-massive white dwarfs discovered with Gaia, often referred to as the Q branch, show high (multi-Gyr) cooling delays produced by exotic physical mechanisms. They are believed to be the products of stellar mergers, but the exact origin and formation channel remain unclear. We obtained a spectroscopically complete, volume-limited sample of the Q branch region within 100 pc, and found significant differences in atmospheric composition and rotation rates as a function of tangential velocity. In particular, we discover that stellar remnants with the longest cooling delays do not show strong magnetism nor detectable short-period rotational variability, as opposed to what is generally believed for double-degenerate mergers. This indicates that either these white dwarfs arise from a formation channel with no strong magnetism induced, or that the magnetism produced from the merger dissipates over the cooling delay timescales. Our follow-up photometry has also discovered pulsations in the second and third hydrogen-dominated DAQ white dwarfs, one hotter than 15,500 K, possibly extending the boundaries of the DAV instability strip for white dwarfs with thin hydrogen layers.