Discovery of magnetic fields in hot subdwarfs

We present initial results of a project to measure mean longitudinal magnetic fields in a group of sdB/OB/O stars. The project was inspired by the discovery of three super-metal-rich sdOB stars, each having metals (e.g. Ti, V) enhanced by factors of 10 3 to 10 5. Similar behaviour is observed in chemically peculiar A stars, where strong magnetic fields are responsible for the enrichment. With this in mind, we obtained circularly polarised spectra of two of the super-metal-rich sdOBs, two " normal " sdBs and two sdOs using FORS1 on the ESO/VLT. By examining circular polarisation in the hydrogen Balmer lines and in helium lines, we have detected magnetic fields with strengths of 1-2 kG in most of our targets. This suggests that such fields are relatively common in hot subdwarfs. Recently, we have found several sdBs that show a very large enrichment of iron group elements (Edelmann et al. 2001). Three out of thirteen targets showed abundances of these elements of up to 10 5 times the solar value. Around 23% of the lines in the peculiar sdBs remain unidentified. These metal-enriched sdBs are reminiscent of the chemically peculiar A stars, where strong magnetic fields play a major role in the surface abundance patterns. Although no direct evidence for magnetic fields was found, we felt a proper examination of these peculiar objects for such a field was clearly required. Strittmatter & Norris (1971) studied the role of magnetic fields in chemically peculiar A stars, and several of their conclusions may be relevant to sdBs. The abundance pecularities in Ap stars are, in general, confined to the stellar surface. If the magnetic field is strong enough, Strittmatter & Norris suggest that magnetic braking will occur, causing angular momentum loss and resulting in a slowly rotating star. The slowing of the star's rotation can lead to peculiar surface abundances, however there is no well established or detailed theory. In the case of sdB stars, the vast majority are slowly rotating, with observational limits of v sin i < 5 km s −1. The only sdB star for which rotation has been detected is PG1605+072 (Heber et al. 1999). The metal lines of all the stars studied by Edelmann et al. (2001) are very sharp, consistent with projected rotational velocities of v sin i < 5 km s −1. Could the slow rotation of sdB stars,