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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