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

Abstract

Weak magnetic field has been searched for on only a small number of white dwarfs. Current estimates for white dwarfs with fields in excess of 1MG are about 10%; according to previous studies this number increases up to about 25% in the kG regime. Our aim is to improve the statistics by a new sample of ten white dwarfs in order to determine the ratio of magnetic to field-free white dwarfs. Mean longitudinal magnetic fields strengths were determined by means of high-precision circular polarimetry of Hbeta and Hgamma with the FORS1 spectrograph of the VLT ``Kueyen'' 8 m telescope. In one of our objects (LTT 7987) 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 at this level of accuracy, systematic errors cannot completely be ruled out. We also observed the sdO star EC11481-2303 but could not detect a magnetic field. VLT observations with uncertainties of typically 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 (>1MG).