Discovery of kilogauss magnetic fields in three DA white dwarfs^*

¹ Max-Planck-Institut für Aeronomie, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany e-mail: aznar@linmpi.mpg.de
² Institut für Astronomie und Astrophysik, Eberhard-Karls-Universität Tübingen, Sand 1, 72076 Tübingen, Germany
³ Astronomisches Rechen-Institut, Mönchhofstr. 12–14, 69120 Heidelberg, Germany
⁴ Department of Physics & Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK
⁵ Institut für Astronomie, ETH Zentrum, 8092 Zürich, Switzerland
⁶ European Southern Observatory, Casilla 19001, Santiago 19, Chile

Received: 27 February 2004
Accepted: 12 May 2004

Abstract

We have detected longitudinal magnetic fields between 2 and 4 kG in three (WD 0446-790, WD 1105-048, WD 2359-434) out of a sample of 12 normal DA white dwarfs by using optical spectropolarimetry done with the VLT Antu 8 m telescope equipped with FORS1. With the exception of 40 Eri B (4 kG) these are the first positive detections of magnetic fields in white dwarfs below 30 kG. Although suspected, it was not clear whether a significant fraction of white dwarfs contain magnetic fields at this level. These fields may be explained as relics from magnetic fields in the main-sequence progenitors considerably enhanced by magnetic flux conservation during the shrinkage of the core. A detection rate of 25% (3/12) may indicate now for the first time that a substantial fraction of white dwarfs have a weak magnetic field. This result, if confirmed by future observations, would form a cornerstone for our understanding of the evolution of stellar magnetic fields.