The fraction of da white dwarfs with kilo-Gauss magnetic fields

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 (LTT7987), we detected a statistically significant (97% confidence level) longitudinal magnetic field varying between (-1±0.5) kG and (+1±0.5) 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). © ESO 2007.