Discovery of magnetic fields in five DC white dwarfs

¹ Department of Physics and Astronomy, University of Turku, Vesilinnantie 5, 20014 Turku, Finland
e-mail: andber@utu.fi
² Armagh Observatory & Planetarium, College Hill, Armagh BT61 9DG, UK
³ Department of Physics & Astronomy, University of Western Ontario, 1151 Richmond Street North, London, Ontario N6A 3K7, Canada
⁴ Leibniz-Institut für Sonnenphysik (KIS), Schöneckstr 6, Freibirg, Germany
⁵ IRSOL Istituto Ricerche Solari “Aldo e Cele Daccò”, Faculty of Informatics, Università della Svizzera italiana, Via Patocci 57, Locarno, Switzerland
⁶ Euler Institute, Faculty of Informatics, Università della Svizzera italiana, Via la Santa 1, 6962 Lugano, Switzerland

Received: 5 October 2022
Accepted: 6 December 2022

Abstract

About half of white dwarfs (WDs) evolve to the DC state as they cool; the others become DQ or (temporarily?) DZ WDs. The recent magnetic survey of the local 20 pc volume has established a high frequency of magnetic fields among WDs older than 2–3 Gyr, demonstrating that in low- and average-mass WDs, the effects of magnetism become more common as they age, and the fields on average become stronger. However, the available statistics of WDs older than about 5 Gyr do not clearly establish how fields evolve beyond this age. We are carrying out a survey to clarify the occurrence of magnetism in DC-type WDs in order to better understand this late evolution. We use broadband filter polarimetry, arguably the most efficient way to detect magnetic fields in featureless WDs via continuum circular polarization. Here we report the discovery of a magnetic field in five DC WDs (of 23 observed), almost doubling the total sample of known magnetic WDs belonging to the DC spectral class.