White Dwarf Merger Remnants: The DAQ Subclass

TL;DR

This paper reports the discovery of five DAQ white dwarfs, likely merger remnants, characterized by specific mass, temperature, and rotational properties, and explores their evolutionary implications within white dwarf populations.

Contribution

The study identifies and characterizes a new subclass of white dwarfs, DAQ, and discusses their potential origin from mergers and evolutionary pathways.

Findings

All five DAQ white dwarfs have similar mass and temperature ranges.

At least two DAQs show rapid rotation with ~10 min periods.

DAQ white dwarfs are old kinematically but appear young photometrically.

Abstract

Four years after the discovery of a unique DAQ white dwarf with a hydrogen-dominated and carbon-rich atmosphere, we report the discovery of four new DAQ white dwarfs, including two that were not recognized properly in the literature. We find all five DAQs in a relatively narrow mass and temperature range of $M=1.14-1.19~M_{\odot}$ and $T_{\rm eff}=13,000-17,000$ K. In addition, at least two show photometric variations due to rapid rotation with $\approx10$ min periods. All five are also kinematically old, but appear photometrically young with estimated cooling ages of about 1 Gyr based on standard cooling tracks, and their masses are roughly twice the mass of the most common white dwarfs in the solar neighborhood. These characteristics are smoking gun signatures of white dwarf merger remnants. Comparing the DAQ sample with warm DQ white dwarfs, we demonstrate that there is a range of hydrogen abundances among the warm DQ population, and the distinction between DAQ and warm DQ white dwarfs is superficial. We discuss the potential evolutionary channels for the emergence of the DAQ subclass, and suggest that DAQ white dwarfs are trapped on the crystallization sequence, and may remain there for a significant fraction of the Hubble time.