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

TL;DR

This study targets the Gaia-identified Q branch of ultra-massive white dwarfs, where long cooling delays may reveal merger histories or alternative evolution. By constructing a spectroscopically complete 100 pc sample and separating objects into a kinematically delayed (merger-like) subset and a young subset, the authors use atmospheric composition, magnetism, and rotation as diagnostic probes, complemented by 26 new spectra and a comparison to canonical-mass WDs. They find that the most delayed WDs show no strong magnetism and a high fraction of carbon-dominated atmospheres, while the younger Q branch remnants can be magnetized but with an overall distinct distribution from field, single-star WDs; pulsations are detected in two DAQ WDs, potentially extending the DAV instability strip for thin-H layers. These results imply that Q branch cooling delays may stem from multiple channels, including merger remnants with little or dissipated magnetism, or alternative formation pathways, with significant implications for cosmological age dating and our understanding of WD evolution.

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.

Figures (2)

• Figure 1: Color-magnitude diagram of the Gaia white dwarfs (gray points, 2019MNRAS.482.4570G) within 150 pc (left) and 100 pc (right). The left panel highlights the over-density referred to as the Q branch. The right panel zooms in on the region to show our full sample of 75 white dwarfs in the Q branch region within 100pc as defined in 2019ApJ...886..100C, where the targets filled in orange make the delayed subset with high cooling delay as indicated by their high kinematics ($v_{\mathrm{t}} >$ 50 km s$^{-1}$). White dwarfs of a given mass are expected to cool from the top left to the bottom right along theoretical cooling tracks shown here (solid pink lines) for 0.6 M$_{\odot}$, 0.8 M$_{\odot}$, and 1.0 M$_{\odot}$2020AA...640L..11B.
• Figure 2: Sunburst plot of the spectral type distribution for each of the two kinematically selected subsets in the Q branch region and a comparison sample: the delayed Q branch subset ($v_{\mathrm{t}} >$ 50 km s$^{-1}$), the young Q branch subset ($v_{\mathrm{t}} <$ 50 km s$^{-1}$), and the single star evolution average mass sample from SDSS ($M <$ 0.8 M$_\odot$ and $v_{\mathrm{t}} <$ 50 km s$^{-1}$). The inner ring shows the dominant composition from spectroscopy, and the outer ring specifies the spectral type based on all elements detected in a low-resolution spectra. The physical meaning of each spectral type is described in Section \ref{['sec:spec']}. The delayed subset has the highest fraction of white dwarfs with carbon-dominated atmospheres. There are striking differences between the populations in the fraction of magnetism (0%, 27%, 14% respectively), helium-dominated atmospheres (0%, 0%, 28% respectively), and metal pollution (0%, 0%, 6% respectively).