On the empirical evidence for the existence of ultra-massive white dwarfs

TL;DR

This paper critically reviews observational evidence for ultra-massive white dwarfs, emphasizing the importance of spectral analysis and parallax measurements in confirming their existence and understanding their origins.

Contribution

It provides a comprehensive re-evaluation of methods used to identify ultra-massive white dwarfs and highlights the significance of parallax data in establishing their population.

Findings

Balmer line profiles are key evidence for high-mass white dwarfs.

The companion to HR 8210 is a confirmed ultra-massive white dwarf.

Parallax measurements are crucial for accurately determining white dwarf masses.

Abstract

We re-examine the evidence for the existence of ultra-massive (M > 1.1 M_sun) 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-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 ultra-massive white dwarfs supports the concept of a steeper initial-mass to final-mass relations linking 6 M_sun progenitors with approximately greater than 1.1 M_sun white dwarfs.