The magnetic fields of hot subdwarf stars^⋆

¹ Armagh Observatory, College Hill, Armagh, BT61 9DG, Northern Ireland, UK
e-mail: jls@arm.ac.uk; sba@arm.ac.uk
² Department of Physics & Astronomy, University of Western Ontario, London, ON N6A 3K7, Canada
e-mail: jlandstr@uwo.ca
³ The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
e-mail: l.fossati@open.ac.uk
⁴ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstr. 12–14, 69120 Heidelberg, Germany
e-mail: jordan@ari.uni-heidelberg.de
⁵ Australian Astronomical Observatory, PO Box 296, 1710 Epping NSW, Australia
e-mail: otoole@aao.gov.au

Received: 6 March 2012
Accepted: 29 March 2012

Abstract

Context. Detection of magnetic fields has been reported in several sdO and sdB stars. Recent literature has cast doubts on the reliability of most of these detections. The situation concerning the occurrence and frequency of magnetic fields in hot subdwarfs is at best confused.

Aims. We revisit data previously published in the literature, and we present new observations to clarify the question of how common magnetic fields are in subdwarf stars.

Methods. We consider a sample of about 40 hot subdwarf stars. About 30 of them have been observed with the FORS1 and FORS2 instruments of the ESO VLT. Results have been published for only about half of the hot subdwarfs observed with FORS. Here we present new FORS1 field measurements for 17 stars, 14 of which have never been observed for magnetic fields before. We also critically review the measurements already published in the literature, and in particular we try to explain why previous papers based on the same FORS1 data have reported contradictory results.

Results. All new and re-reduced measurements obtained with FORS1 are shown to be consistent with non-detection of magnetic fields. We explain previous spurious field detections from data obtained with FORS1 as due to a non-optimal method of wavelength calibration. Field detections in other surveys are found to be uncertain or doubtful, and certainly in need of confirmation.

Conclusions. There is presently no strong evidence for the occurrence of a magnetic field in any sdB or sdO star, with typical longitudinal field uncertainties of the order of 2–400 G. It appears that globally simple fields of more than about 1 or 2 kG in strength occur in at most a few percent of hot subdwarfs. Further high-precision surveys, both with high-resolution spectropolarimeters and with instruments similar to FORS1 on large telescopes, would be very valuable.