Evolution and formation of ultramassive white dwarf stars: the case for a 9M⊙ progenitor

ABSTRACT We study the full evolution of a 1.313 $\rm M_\odot$ white dwarf star that descended from a 9 $\rm M_\odot$ main-sequence progenitor with an initial metallicity of Z = 0.02. Using MESA r24.08.01, we calculate its entire evolution from pre-ZAMS to the WD cooling curve, including both the evolution through 139 thermal pulses and the post-AGB phase. The resulting remnant is an ultramassive H-deficient WD, for which the composition, in mass fraction, is 47.7 per cent $^{16}$O, 39.7 per cent $^{20}$Ne, 4.2 per cent $^{24}$Mg, 3.3 per cent $^{23}$Na, and 0.386 per cent $^{12}\rm C$ – corresponding to a total mass of $5 \times 10^{-3} \rm \, M_\odot$ of C –, surrounded by a $1.5 \times 10^{-5}\rm M_\odot$ He layer. We also investigate the effects of fully suppressing the TP-SAGB stage by adopting a high mass-loss rate only after the second dredge-up, and find only minor differences in the final mass and composition. In addition, we calculate models with and without phase separation during the WD stage, estimating a cooling delay of only 16 Myr. This is the first ultramassive white dwarf sequence for which both the TP-SAGB and post-AGB stages are calculated and, to our knowledge, the most massive WD model from complete evolution for which cooling times and detailed abundance profiles are published.