On the empirical evidence for the existence of ultramassive white dwarfs

We re-examine the evidence for the existence of ultramassive (M > 1.1 M_⊙) white dwarfs based on gravitational redshift of white dwarfs in common proper motion binaries or in clusters, on parallax measurements, on orbital solutions and, finally, on the analysis of hydrogen-line profiles. We conclude that the best evidence is largely based on the analysis of Balmer-line profiles although the companion to the A8V star HR 8210 is a compelling case made initially using the large binary mass function and confirmed by an analysis of the Lyman-line spectrum. The confirmation and identification of high-mass white dwarfs, more particularly non-hydrogen-rich (non-DA) white dwarfs, using parallax measurements may prove critical in establishing the population fraction of these objects and in constraining the high end of empirical initial-mass to final-mass relations. The existence of a substantial population of ultramassive white dwarfs supports the concept of a steeper initial-mass to final-mass relations linking 6 M_⊙ progenitors with ≳1.1 M_⊙ white dwarfs.