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

TL;DR

This study models the full evolution of a 9Msun progenitor star leading to an ultramassive white dwarf, providing detailed composition, cooling times, and the effects of different mass-loss and phase separation assumptions.

Contribution

It presents the first complete evolutionary sequence for an ultramassive white dwarf from progenitor to cooling, including detailed composition and phase separation effects.

Findings

Final white dwarf mass is approximately 1.313 Msun.

Cooling delay due to phase separation is about 16 Myr.

Minor differences observed when suppressing the TP-SAGB stage.

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