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Ultra-massive white dwarfs ($ 1.05 \rm M_\odot \lesssim M_{WD}$) areparticularly interesting objects that allow us to study extreme astrophysicalphenomena 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 suggestthat some ultra-massive white dwarfs could harbour carbon-oxygen (CO) cores.Although several studies have attempted to elucidate the core composition ofultra-massive white dwarfs, to date, it has not been possible to distinguishthem through their observed properties. Here, we present a new method forrevealing the core-chemical composition in ultra-massive white dwarfs that isbased on the study of magnetic fields generated by convective mixing induced bythe crystallization process. ONe white dwarfs crystallize at higherluminosities than their CO counterparts. Therefore, the study of magneticultra-massive white dwarfs in the particular domain where ONe cores havereached the crystallization conditions but CO cores have not, may providevaluable support to their ONe core-chemical composition, since ONe white dwarfswould display signs of magnetic fields and CO would not. We apply our method toeight white dwarfs with magnetic field measurements and we suggest that thesestars are candidate ONe white dwarfs.

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