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

Figures (12)

• Figure 1: Hertzsprung-Russel (HR) Diagram following the evolution of a sequence with $\rm M_\mathrm{ZAMS} = 9\, \rm M_\odot$ and Z=0.02, from the zero age main sequence (ZAMS) to the WD cooling track
• Figure 2: Evolution of central (top panel) and total (bottom panel) abundances of $^1$H (grey), $^4$He (dotted orange), $^{12}$C (dotted blue), $^{16}$O(red), and $^{20}$Ne (pink) from MS to the white dwarf stage, before phase separation sets in. Once phase separation occurs, the central fractions of oxygen and neon change.
• Figure 3: Top Panel: Helium luminosity $\rm L_\mathrm{He}$ (black), carbon luminosity $\rm L_\mathrm{C}$ (red), and neutrino luminosity $\rm L_\nu;$ (blue) during C-burning. Middle Panel: Radius (pink) and Surface Luminosity (orange). Bottom panel: central temperature (pink) and density (blue dotted). The time axis is reset so that zero marks the start of the EAGB.
• Figure 4: Kippenhahn Diagram following carbon burning in the EAGB. The color bar represents the same quantity used in the Kippenhahn plots of timmes+2015, where purple regions indicate cooling, primarily due to thermal neutrino losses. Darker purple indicates a logarithmic increase in thermal neutrino production rates. Yellow/orange regions indicate significant nuclear burning, with redder colors indicating a logarithmic increase to the nuclear reaction rates; light blue regions indicate convection. For clarity, in this plot we show only the inner 1.75 $\rm M_\odot$, and the time axis is reset so that zero marks the start of the EAGB.
• Figure 5: Abundance profile in the inner 1.4$\,\rm M_\odot$ for our main species right after the dredge-out has occurred.