The fraction of DA white dwarfs with kilo-Gauss magnetic fields ^*,**

¹ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstr. 12-14, 69120 Heidelberg, Germany e-mail: jordan@ari.uni-heidelberg.de
² Max-Planck-Institut für Sonnensystemforschung, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany e-mail: [aznar;solanki]@linmpi.mpg.de
³ Centre for Astrophysics Research, University of Hertfordshire, Hatfield AL10 9AB, UK e-mail: rn@star.herts.ac.uk
⁴ Institut für Astronomie, ETH Zürich, 8092 Zürich, Switzerland e-mail: schmid@astro.phys.ethz.ch

Received: 2 August 2006
Accepted: 12 October 2006

Abstract

Context.Weak magnetic fields have been searched for on only a small number of white dwarfs. Current estimates find that about 10% of all white dwarfs have fields in excess of 1 MG; according to previous studies this number increases up to about 25% in the kG regime.

Aims.Our aim is to improve on these statistics by a new sample of ten white dwarfs in order to determine the ratio of magnetic to field-free white dwarfs.

Methods.Mean longitudinal magnetic fields strengths were determined by means of high-precision circular polarimetry of Hβ and Hγ with the FORS1 spectrograph of the VLT “Kueyen” 8 m telescope.

Results.In one of our objects (LTT 7987), we detected a statistically significant (97% confidence level) longitudinal magnetic field varying between () kG and () kG. This would be the weakest magnetic field ever found in a white dwarf, but systematic errors cannot completely be ruled out at this level of accuracy. We also observed the sdO star EC 11481-2303 but could not detect a magnetic field.

Conclusions. VLT observations with uncertainties typically of 1000 G or less suggest that 15-20% of WDs have kG fields. Together with previous investigations, the fraction of kG magnetic fields in white dwarfs amounts to about 11-15%, which is close to the current estimations for highly magnetic white dwarfs (>1 MG).