Magnetic dynamos in white dwarfs – II. Relating magnetism and pollution

Abstract

We investigate whether the recently suggested rotation and crystallization driven dynamo can explain the apparent increase of magnetism in old metal polluted white dwarfs. We find that the effective temperature distribution of polluted magnetic white dwarfs is in agreement with most/all of them having a crystallizing core and increased rotational velocities are expected due to accretion of planetary material which is evidenced by the metal absorption lines. We conclude that a rotation and crystallization driven dynamo offers not only an explanation for the different occurrence rates of strongly magnetic white dwarfs in close binaries, but also for the high incidence of weaker magnetic fields in old metal polluted white dwarfs.

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

“…About a third of the sample is consistent with a crystallization-driven dynamo, with the other white dwarfs being either too hot to be crystallizing or having magnetic fields well above our maximum, requiring a different explanation. Interestingly, the recently discovered class of single white dwarfs with Zeemansplit Balmer emission lines (Gänsicke et al 2020;Walters et al 2021) exhibits fields that are fairly close to our maximum, potentially explaining the clustering of these white dwarfs at 𝑇 eff ≈ 7500 K, where 𝐵(𝑇 eff ) peaks for 0.6−0.8 M white dwarfs (see also Schreiber et al 2021b).…”

“…In addition to the white dwarfs presented in Fig. 4, we can also reproduce the 𝐵 ∼ 1 − 10 MG fields measured for metal-polluted white dwarfs, which were plausibly spun up to periods of minutes to hours by planetary material accretion (Schreiber et al 2021b). Almost all of these metal-polluted magnetic white dwarfs have 𝑇 eff < 8000 K, strongly supporting a crystallization-driven dynamo scenario.…”

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

“…Our finding that the dynamo scenario can consistently explain the magnetic nature of NLTT 12758 (and the absence of clear signs of magnetism among close double white dwarfs otherwise) adds another piece of evidence to the already available support for the dynamo scenario for the generation of strong magnetic fields in white dwarfs: the dynamo is the only scenario that can explain the absence of strongly magnetic white dwarfs among young post common envelope binaries, it offers an explanation for the existence of the radio pulsing white dwarf binary AR Sco, it is consistent with the high occurrence rate of magnetic white dwarfs in cataclysmic variables (Schreiber et al 2021a), it naturally explains the absence of high accretion rate intermediate polars in globular clusters (Belloni et al 2021), and seems to be the reason behind the observed relation between magnetism and metal pollution (Schreiber et al 2021b). Considering that also single white dwarfs that are currently not metal polluted, might have accreted planetary material in the past and therefore gained angular momentum (Schreiber et al 2021b), the model might also explain the magnetism of old white dwarfs that currently do not show signs of accretion (Bagnulo & Landstreet 2021).…”

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

“…Finally, we cannot disregard that the magnetic fields in these WDs could be fossil fields (Braithwaite & Spruit 2004), generated during the merger of two WDs (Garcia Berro et al 2012) or in a common envelope phase, if the binary companion has been destroyed or is in a wide orbit (Tout et al 2008). However, none of these scenarios can account for the number of observed magnetic cataclysmic variables, intermediate polar WDs in globular clusters and cold metal polluted WDs (Schreiber et al 2021a;Belloni et al 2021;Schreiber et al 2021b), implying that the crystallization-induced dynamos must take place. Furthermore, the analysis of the complete volume-limited WD sample within 20 pc from the Sun (Bagnulo & Landstreet 2021) has shown that the occurrence of magnetic fields is significantly higher in WDs that have fully or partially crystallized cores than in WDs with fully liquid cores.…”