Discovery of six new strongly magnetic white dwarfs in the 20 pc local population

TL;DR

This study reports the discovery of six new magnetic white dwarfs within 20 parsecs, confirming that magnetism is common among white dwarfs and providing insights into their magnetic properties and polarisation characteristics.

Contribution

The paper presents the first systematic search for magnetic white dwarfs in the local 20 pc volume, discovering six new magnetic white dwarfs with fields up to 200 MG, and discusses their polarisation features.

Findings

At least 20% of local white dwarfs are magnetic.

Magnetic fields cause spectral line splitting and polarisation.

Continuum polarisation correlates with magnetic field strength.

Abstract

The sample of white dwarfs included in the local 20 pc volume documents, fairly accurately, the total production of white dwarfs over roughly 10 Gyr of stellar evolution in this part of the Milky Way Galaxy. In this sample, we have been systematically searching for magnetic white dwarfs. Here we report the discovery of six new magnetic white dwarfs, with a field strength from a few MG to about 200MG. Two of these stars show H lines that are split and polarised by the magnetic field. One star shows extremely weak spectral lines in intensity, to which highly polarised narrow features correspond. The three other stars have featureless flux spectra, but show continuum polarisation. These new discoveries support the view that at least 20% of all white dwarfs in the local 20 pc volume have magnetic fields, and they fully confirm the suspicion that magnetism is a common rather than a rare characteristic of white dwarfs. We discuss the level and the handedness of the continuum polarisation in the presence of a magnetic field in cool white dwarfs. We suggest that a magnetic field with a 15 MG longitudinal component produces 1% of continuum circular polarisation. We have also shown that the problem of cross-talk from linear to circular polarisation of the FORS2 instrument, used in our survey, represents an obstacle to accurate measurements of the circular polarisation of faint white dwarfs when the background is illuminated, and polarised, by the moon.