Constraining the Merger Fraction and the Upper Mass Limit of Ultramassive White Dwarfs
We performed a spectroscopic survey of all known white dwarfs with $M \geq 1.3~M_\odot$ and $T_{\rm eff}\geq11,000$ K identified through Gaia DR3 astrometry from the Gentile-Fusillo et al. (2021) catalog. Of the 293 white dwarf candidates that met our criteria, 259 had no optical spectroscopy or spectral classification available in the literature. We obtained follow-up spectroscopy on 97% of this sample and performed a detailed model atmosphere analysis to constrain the atmospheric composition and mass for each object. For a selection of magnetic white dwarfs, we use offset dipole models to constrain the magnetic field strength and geometry and calculate the magnetic fraction as a function of mass. We use magnetism, unusual atmospheric composition, kinematics, and rapid rotation rates to identify merger remnants, and constrain the merger fraction to be $53_{-6}^{+7}$% for white dwarfs more massive than $1.3~M_\odot$. Hence, roughly half of the most massive white dwarfs likely form through binary mergers. We also use our sample to derive an empirical upper mass limit for white dwarfs in the solar neighborhood. Including the General Relativistic effects, the most massive white dwarf in our sample, J0318+7923, has $M=1.33-1.35~M_\odot$ depending on the unknown core composition. Furthermore, because our temperature criteria overlap with the ZZ Ceti instability strip, we find a new selection of ultramassive pulsator candidates that may provide an excellent opportunity to probe the interiors of crystallized CO or ONe-core white dwarfs.