Why are magnetic Ap stars slowly rotating?

Observational data on rotation of Ap stars suggest that the bulk of their rotation rates form a separate Maxwellian distribution with an average value 3-4 times lower than the normal star distribution. No evidences for a significant angular momentum (AM) loss on the main sequence (MS) have been found. It is thus concluded that Ap stars must lose a large fraction of their initial angular momentum (AM) in the pre-MS phase of evolution, most probably as a result of the interaction of their primordial magnetic fields with accretion disks and stellar winds. The observationally most acceptable values of accretion rate from the disk, 10^(-8) M_sun/year, of mass loss rate via a magnetized wind, 10^(-8) M_sun/year, and of the surface magnetic field, 1 kG on the ZAMS, result in the AM loss in full agreement with observations. There exists a separate group of extremely slowly rotating Ap stars, with periods of the order of 10-100 years. They are too numerous to come from the distribution describing the bulk of Ap stars. It is conjectured that their extremely low rotation rates are the result of additional AM loss on the MS.

K. Stepien · 1998