Seismology and diffusion of ultramassive white dwarf magnetic fields

Abstract Ultramassive white dwarfs (UMWDs; defined by masses ≳ 1. 1 M⊙) are prime targets for seismology, because they pass through the ZZ Ceti instability strip at the same time that their cores crystallize. Recent studies suggest that crystallization may magnetize white dwarf interiors with a strong magnetic field B0 up to a radius r ₎ₔₓ⁰, either through a magnetic dynamo or by transporting a pre-existing fossil field. We demonstrate that seismology can probe these buried fields before they break out at the surface, because even the weak exponential tail of the outwardly diffusing field can disrupt the propagation of gravity waves near the surface. Based on the observed oscillation modes of WD J0135+5722 – the richest pulsating UMWD to date – we constrain its surface field Bsurf ≲ 2 kG. We solve the induction equation and translate this to an upper limit on the internal field B0. For a carbon–oxygen (CO) core we find Bsurf ≪ B0 ≲ 0. 6 MG, consistent with the crystallization dynamo theory. For an oxygen–neon (ONe) core, on the the other hand, r ₎ₔₓ⁰ is larger, such that the magnetic field breaks out and Bsurf ≲ B0 ≲ 7 kG. This low magnetic field rules out an ONe composition or, alternatively, an intense dynamo during crystallization or merger. Either way, the imprint of magnetic fields on UMWD seismology may reveal the uncertain composition and formation paths of these stars.