The Merger Fraction of Ultramassive White Dwarfs

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 $M\approx 1.3M_{\odot }$ 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.