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 per cent, of magnetic white dwarfs among this population. In addition, we identify four outliers in transv erse v elocity and detect rapid rotation in five objects. Our results show that 56 (-10) (+ 9) per cent of the M asymptotic to 1 . 3 M-? 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.