No detection of large-scale magnetic fields at the surfaces of Am and HgMn stars⋆
Laboratoire d’Astrophysique de Toulouse-Tarbes, CNRS, Université de
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2 Department of Physics, Royal Military College of Canada, PO Box 17000, Station Forces, Kingston, Ontario K7K 4B4, Canada
3 Department of Physics & Astronomy, The University of Western Ontario, London, Ontario, N6A 3K7, Canada
4 Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland, UK
5 Institute of Astronomy, Bulgarian Academy of Sciences, 72 Tsarigradsko shose, 1784 Sofia, Bulgaria
Accepted: 12 August 2010
Aims. We investigate the magnetic dichotomy between Ap/Bp and other A-type stars by carrying out a deep spectropolarimetric study of Am and HgMn stars.
Methods. Using the NARVAL spectropolarimeter at the Télescope Bernard Lyot (Observatoire du Pic du Midi, France), we obtained high-resolution circular polarisation spectroscopy of 12 Am stars and 3 HgMn stars.
Results. Using least squares deconvolution (LSD), no magnetic field is detected in any of the 15 observed stars. Uncertaintiies as low as 0.3 G (respectively 1 G) have been reached for surface-averaged longitudinal magnetic field measurements for Am (respectively HgMn) stars.
Conclusions. Associated with the results obtained previously for Ap/Bp stars, our study confirms the existence of a magnetic dichotomy among A-type stars. Our data demonstrate that there is at least one order of magnitude difference in field strength between Zeeman detected stars (Ap/Bp stars) and non Zeeman detected stars (Am and HgMn stars). This result confirms that the spectroscopically-defined Ap/Bp stars are the only A-type stars harbouring detectable large-scale surface magnetic fields.
Key words: stars: chemically peculiar / stars: magnetic field / stars: peculiar / stars: early-type
© ESO, 2010