The Merger Fraction of Ultramassive White Dwarfs

TL;DR

This study investigates the origins of ultramassive white dwarfs, revealing that over half are likely merger products, supported by high magnetic incidence, rapid rotation, and velocity outliers, challenging existing binary evolution models.

Contribution

It provides the first empirical estimate that approximately 56% of ultramassive white dwarfs form through mergers, indicating more efficient orbital shrinkage than previously modeled.

Findings

56% of ultramassive white dwarfs are merger products

High magnetic white dwarf fraction (40%) among the sample

Detection of rapid rotation in five objects

Abstract

We search for merger products among the 25 most massive white dwarfs in the Montreal White Dwarf Database 100 pc sample through follow-up spectroscopy and high-cadence photometry. We find an unusually high fraction, 40%, of magnetic white dwarfs among this population. In addition, we identify four outliers in transverse velocity and detect rapid rotation in five objects. Our results show that $56^{+9}_{-10}$\% of the $M\approx1.3~M_{\odot}$ ultramassive white dwarfs form through mergers. This fraction is significantly higher than expected from the default binary population synthesis calculations using the $\alpha$-prescription (with $\alpha \lambda = 2$), and provides further support for efficient orbital shrinkage, such as with low values of the common envelope efficiency.