Cooling Models for the Most Massive White Dwarfs

TL;DR

This paper develops models for ultramassive white dwarfs above 1.3 solar masses, highlighting neutrino cooling effects and core composition stability, with implications for recent observations.

Contribution

It introduces parameterized models for ultramassive white dwarfs considering different formation channels and emphasizes neutrino cooling and core composition stability.

Findings

Neutrino cooling dominates the early cooling phase for ultramassive white dwarfs.

The ultramassive white dwarf ZTF J190132.9+145808.7 likely experienced Urca process cooling.

Core compositions are unlikely to be significantly altered before crystallization.

Abstract

We present a set of ultramassive white dwarf models, focused on masses above $1.3\,M_\odot$. Given the uncertainties about the formation and compositions of such objects, we construct parameterized model sequences, guided by evolutionary calculations including both single star and double white dwarf merger formation channels. We demonstrate that the cooling of objects with central densities in excess of $10^9\,\rm g\,cm^{-3}$ is dominated by neutrino cooling via the Urca process in the first $\approx 100$ Myr after formation. Our models indicate that the recently discovered ultramassive white dwarf ZTF J190132.9+145808.7 is likely to have experienced this Urca-dominated cooling regime. We also show that the high densities imply that diffusion is unlikely to significantly alter the core compositions of these objects before they crystallize.