New insight into the magnetism of degenerate stars from the analysis of a volume limited sample of white dwarfs

Abstract

Many stars evolve into magnetic white dwarfs, and observations may help to understand when the magnetic field appears at the stellar surface, if and how it evolves during the cooling phase, and, above all, what are the mechanisms that generate it. After obtaining new spectropolarimetric observations and combining them with previous literature data, we have checked almost the entire population of about 152 white dwarfs within 20 pc from the Sun for the presence of magnetic fields, with a sensitivity that ranges from better than 1 kG for most of the stars of spectral class DA, to 1 MG for some of the featureless white dwarfs. We find that 33 white dwarfs of the local 20 pc volume are magnetic. Statistically, the data are consistent with the possibility that the frequency of the magnetic field occurrence is similar in stars of all spectral classes, except that in the local 20 pc volume, either DQ stars are more frequently magnetic, or host much stronger fields than average. The distribution of the observed field strength ranges from 40 kG to 300 MG and is uniform per decade, in striking contrast to the field frequency distribution resulting from spectroscopic surveys. Remarkably, no fields weaker than 40 kG are found. We confirm that magnetic fields are more frequent in white dwarfs with higher than average mass, especially in younger stars. We find a marked deficiency of magnetic white dwarfs younger than 0.5 Gyr, and we find that the frequency of the occurrence of the magnetic field is significantly higher in white dwarfs that have undergone the process of core crystallisation than in white dwarfs with fully liquid core. There is no obvious evidence of field strength decay with time. We discuss the implications of our findings in relation to some of the proposals that have been put forward to explain the origin and evolution of magnetic fields in degenerate stars, in particular those that predict the presence of a dynamo acting during the crystallisation phase.

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).…”

“…From an observational point of view, we need to expand the work of Bagnulo & Landstreet (2021) who presented the first volume limited (but still rather small) sample of white dwarfs and investigated how the incidence of magnetism depends on key parameters. As in SDSS-V white dwarfs are for the first time prime targets of an SDSS survey, we may indeed be able to analyze a large volume limited sample of white dwarfs in the near future.…”