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 (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 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).
- Publication:
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Astronomy and Astrophysics
- Pub Date:
- February 2007
- DOI:
- 10.1051/0004-6361:20066163
- arXiv:
- arXiv:astro-ph/0610875
- Bibcode:
- 2007A&A...462.1097J
- Keywords:
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- stars: white dwarfs;
- stars: magnetic fields;
- Astrophysics
- E-Print:
- 11 pages, 4 figures, accepted for publication in A&