The incidence of magnetic fields in cool DZ white dwarfs

TL;DR

This study identifies magnetic fields in cool DZ white dwarfs using metal lines as indicators, revealing a higher incidence of magnetism in these old stars compared to hotter white dwarfs.

Contribution

It demonstrates that metal lines can be used to detect magnetism in the coolest white dwarfs, showing a higher magnetic incidence than previously known.

Findings

Identified 7 magnetic cool DZ white dwarfs with fields 1.9-9.6 MG.

Estimated a magnetic incidence of 13+/-4% among cool DZ white dwarfs.

Magnetic incidence is higher in cool DZ white dwarfs than in hot DA types.

Abstract

Little is known about the incidence of magnetic fields among the coolest white dwarfs. Their spectra usually do not exhibit any absorption lines as the bound-bound opacities of hydrogen and helium are vanishingly small. Probing these stars for the presence of magnetic fields is therefore extremely challenging. However, external pollution of a cool white dwarf by, e.g., planetary debris, leads to the appearance of metal lines in its spectral energy distribution. These lines provide a unique tool to identify and measure magnetism in the coolest and oldest white dwarfs in the Galaxy. We report the identification of 7 strongly metal polluted, cool (T_eff < 8000 K) white dwarfs with magnetic field strengths ranging from 1.9 to 9.6 MG. An analysis of our larger magnitude-limited sample of cool DZ yields a lower limit on the magnetic incidence of 13+/-4 percent, noticeably much higher than among hot DA white dwarfs.