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

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.