Evolution and formation of ultramassive white dwarf stars: The case for a 9Msun progenitor

Abstract

We study the full evolution of a 1.313 Msun white dwarf star that descended from a 9 Msun 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% O16, 39.7% Ne20, 4.2% Mg24, 3.3% Na23 and 0.386% C12 -- corresponding to a total mass of 5 x 10^-3 Msun of C --, surrounded by a 1.5 x 10^-5 Msun 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