2002A&A...384..554S

Landstreet & Mathys (2000A&A...359..213L) have discovered that almost all the magnetic Ap stars having rotation periods longer than about one month have their magnetic and rotation axes fairly closely aligned, in contrast to the more common magnetic Ap stars of shorter period, in which the two axes are usually inclined to one another at a large angle. Furthermore, as shown earlier by Mathys et al. (1997A&AS..123..353M) and by Hubrig et al. (2000ApJ...539..352H), these most slowly rotating magnetic Ap stars have also magnetic fields several times larger than are typical for shorter period magnetic Ap stars, and generally have masses below 3M_☉. In this paper, we present calculations to show that the observed axial alignment in the slowest rotators may have arisen during the pre-main sequence (PMS) phase, when stars of a few solar masses are observed as Herbig Ae/Be stars. During the PMS phase, a star is expected to exchange angular momentum with a disk and with a stellar wind. As shown by Stepien (2000A&A...353..227S), a disk around a PMS magnetic Ap star tends to lock the rotation rate at a few days, while mass loss in the absence of a disk is able to slow the star to quite long periods. We argue that is it plausible to assume that the fraction of the PMS phase during which a disk persists depends on the orientation of the field axis to the plane of the disk, and that disk dissipation may have occurred more quickly in magnetic stars with aligned magnetic and rotation axes than in stars in which the angle between these axes is large. In this case, the magnetic Ap stars with aligned rotation and magnetic axes, especially those with large fields, could lose much more angular momentum than the stars of large obliquity. However, it is also shown that loss of nearly all the angular momentum is only possible for stars with masses below about 3M_☉ (and with relatively large fields); for higher masses the available time is too short. Our model thus provides a reasonable explanation for all of the principle observational facts about the very slowly rotating magnetic Ap stars.