What can magnetic early B-type stars tell us about early B-type stars in general?

International audienceSome magnetic early B-type stars display Halpha emission originating in their Centrifugal Magnetospheres (CMs). To determine the rotational and magnetic properties necessary for the onset of emission, we analyzed a large spectropolarimetric dataset for a sample of 51 B5-B0 magnetic stars. New rotational periods were found for 15 stars. We determined physical parameters, dipolar magnetic field strengths, magnetospheric parameters, and magnetic braking timescales. Halpha-bright stars are more rapidly rotating, more strongly magnetized, and younger than the overall population. We use the high sensitivity of magnetic braking to the mass-loss rate to test the predictions of Vink et al. (2001) and Krticka (2014) by comparing ages t to maximum spindown ages t <SUB>S, max</SUB>. For stars with M <SUB>*</SUB> < 10 M <SUB>&sun;</SUB> this comparison favours the Krticka recipe. For the most massive stars, both prescriptions yield t << t <SUB>S, max</SUB>, a discrepancy which is difficult to explain via incorrect mass-loss rates alone