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Preprint B/04

Discovery of kilogauss magnetic fields in three DA white dwarfs

R. Aznar Cuadrado (1), S. Jordan (2 and 3), R. Napiwotzki (4), H. M. Schmid (5), S. K. Solanki (1), G. Mathys (6)

(1) Max-Planck-Institut für Aeronomie, Katlenburg-Lindau, Germany
(2) Institut für Astronomie und Astrophysik, Tübingen, Germany
(3) Astronomisches Rechen-Institut, Heidelberg, Germany
(4) Department of Physics & Astronomy, Leicester, UK
(5) Institut für Astronomie, ETH Zürich
(6) European Southern Observatory, Santiago, Chile

To be published in: A&A

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 fossil 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 on the evolution of stellar magnetic fields.

Key words: stars: white dwarfs - stars: magnetic fields - stars: individual: WD0446-790, WD1105-048, WD2359-434


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