Can we reveal the core-chemical composition of ultra-massive white dwarfs through their magnetic fields?

TL;DR

This paper proposes a novel method to determine the core composition of ultra-massive white dwarfs by analyzing their magnetic fields, which are influenced by the crystallization process and differ between ONe and CO cores.

Contribution

The study introduces a new approach using magnetic field observations to distinguish between ONe and CO core compositions in ultra-massive white dwarfs, addressing a longstanding challenge.

Findings

Magnetic fields are present in some ultra-massive white dwarfs.

Crystallization occurs at different luminosities for ONe and CO cores.

Eight white dwarfs are identified as candidate ONe white dwarfs.

Abstract

Ultra-massive white dwarfs ($ 1.05 \rm M_\odot \lesssim M_{WD}$) are particularly interesting objects that allow us to study extreme astrophysical phenomena such as type Ia supernovae explosions and merger events. Traditionally, ultra-massive white dwarfs are thought to harbour oxygen-neon (ONe) cores. However, recent theoretical studies and new observations suggest that some ultra-massive white dwarfs could harbour carbon-oxygen (CO) cores. Although several studies have attempted to elucidate the core composition of ultra-massive white dwarfs, to date, it has not been possible to distinguish them through their observed properties. Here, we present a new method for revealing the core-chemical composition in ultra-massive white dwarfs that is based on the study of magnetic fields generated by convective mixing induced by the crystallization process. ONe white dwarfs crystallize at higher luminosities than their CO counterparts. Therefore, the study of magnetic ultra-massive white dwarfs in the particular domain where ONe cores have reached the crystallization conditions but CO cores have not, may provide valuable support to their ONe core-chemical composition, since ONe white dwarfs would display signs of magnetic fields and CO would not. We apply our method to eight white dwarfs with magnetic field measurements and we suggest that these stars are candidate ONe white dwarfs.