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We investigate the magnetic dichotomy between Ap/Bp and other A-type stars bycarrying out a deep spectropolarimetric study of Am and HgMn stars. Using theNARVAL spectropolarimeter at the Telescope Bernard Lyot (Observatoire du Pic duMidi, France), we obtained high-resolution circular polarisation spectroscopyof 12 Am stars and 3 HgMn stars. Using Least Squares Deconvolution (LSD), nomagnetic field is detected in any of the 15 observed stars. Uncertaintiies aslow as 0.3 G (respectively 1 G) have been reached for surface-averagedlongitudinal magnetic field measurements for Am (respectively HgMn) stars.Associated with the results obtained previously for Ap/Bp stars, our studyconfirms the existence of a magnetic dichotomy among A-type stars. Our datademonstrate that there is at least one order of magnitude difference in fieldstrength between Zeeman detected stars (Ap/Bp stars) and non Zeeman detectedstars (Am and HgMn stars). This result confirms that thespectroscopically-defined Ap/Bp stars are the only A-type stars harbouringdetectable large-scale surface magnetic fields.

No detection of large-scale magnetic fields at the surfaces of Am and HgMn stars