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

摘要

Some magnetic early B-type stars display Hα 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. Hα-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 ts,max. For stars with M* < 10 M_⊙ this comparison favours the Krticka recipe. For the most massive stars, both prescriptions yield t ts,max, a discrepancy which is difficult to explain via incorrect mass-loss rates alone.