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 56−10+9\% of the M≈1.3M⊙ ultramassive white dwarfs form through mergers. This fraction is significantly higher than expected from the default binary population synthesis calculations using the α-prescription (with αλ=2), and provides further support for efficient orbital shrinkage, such as with low values of the common envelope efficiency.
@article{arxiv.2211.05938,
title = {The Merger Fraction of Ultramassive White Dwarfs},
author = {Mukremin Kilic and Adam G. Moss and Alekzander Kosakowski and P. Bergeron and Annamarie A. Conly and Warren R. Brown and Silvia Toonen and Kurtis A. Williams and P. Dufour},
journal= {arXiv preprint arXiv:2211.05938},
year = {2022}
}