Slow convection and fast rotation in crystallization-driven white dwarf dynamos

Abstract

It has been recently suggested that white dwarfs generate magnetic fields in a process analogous to the Earth. The crystallization of the core creates a compositional inversion that drives convection, and combined with rotation, this can sustain a magnetic dynamo. We reanalyse the dynamo mechanism, arising from the slow crystallization of the core, and find convective turnover times 𝑡 conv of weeks to months -longer by orders of magnitude than previously thought. With white dwarf spin periods 𝑃 𝑡 conv , crystallization-driven dynamos are almost always in the fast rotating regime, where the magnetic field 𝐵 is at least in equipartition with the convective motion and is possibly further enhanced by a factor of 𝐵 ∝ (𝑡 conv /𝑃) 1/2 , depending on the assumed dynamo scaling law. We track the growth of the crystallized core using and compute the magnetic field 𝐵(𝑇 eff ) as a function of the white dwarf's effective temperature 𝑇 eff . We compare this prediction with observations and show that crystallization-driven dynamos can explain some -but not all -of the ∼MG magnetic fields measured for single white dwarfs, as well as the stronger fields measured for white dwarfs in cataclysmic variables, which were spun up by mass accretion to short 𝑃. Our 𝐵(𝑇 eff ) curves might also explain the clustering of white dwarfs with Balmer emission lines around 𝑇 eff ≈ 7500 K.

Multiple channels for the onset of magnetism in isolated white dwarfs

“…First, that the field generated by this dynamo is orders of magnitude stronger than that originally estimated, in such a way to justify the observed range of field strengths (Schreiber et al 2021). Secondly, that the convection is much slower than originally estimated (Ginzburg et al 2022). These modifications lead to a dynamo that requires much less rapid rotation and one that readily produces fields of the order of 100 MG, suggesting more strongly that crystallisation might play an important part in producing the fields in about 20% of normal-mass WDs older than about ∼ 2 − 3 Gyr.…”

“…The origin of magnetic fields in WDs is not well understood, but several ideas have been put forward to explain their presence, for example that the currently visible field is a descendant of a field that was present when the star was in a previous evolutionary stage, or that the observed field is generated by a contemporary dynamo excited in the core of a rotating WD by the convection produced by sinking solids when crystallisation begins (Isern et al 2017;Ginzburg et al 2022). Another family of field generation mechanisms suggest that a field is produced by close binary evolution, for example by a dynamo acting during a post common envelope phase (Tout et al 2008;Briggs et al 2018), or by a dynamo that is active during the merger of a binary pair of WDs which become a single star (García-Berro et al 2012).…”

Catalog of magnetic white dwarfs with hydrogen dominated atmospheres

“…We could also mention other possibilities that could account for the magnetic field in white dwarfs, like the crystallization of its core (Isern et al 2017;Ginzburg et al 2022) and the interaction with orbiting planets (Schreiber et al 2021).The later effect will not be further studied in this work, as we have yet no evidence that it is statistically significant to the complete sample.…”

Magnetic dynamos in white dwarfs -III: explaining the occurrence of strong magnetic fields in close double white dwarfs

“…However, existing scaling laws are most likely not appropriate for white dwarfs as the much higher magnetic Prandtl number in white dwarfs could imply a much larger field strength (Brandenburg 2014;Bovino et al 2013). Very recently, Ginzburg et al (2022) argued that the convective turn-over time is much larger than assumed by Isern et al (2017) which would imply that white dwarfs are almost always in the fast rotation regime. Combining this finding with a scaling law based on the balance between the Lorentz and Coriolis forces, even permits the generation of strong magnetic fields (reaching 100 MG) without postulating a magnetic field enhancement due to the white dwarf's Prandtl number.…”