Trapped discs and the rotation rates of Ap stars

The rotation rates of chemically peculiar A stars span more than four orders of magnitude, from a few days to hundreds of years. While it has long been suggested that the strong magnetic fields of these stars act to regulate their spin, the exact mechanism for the spin down remains unclear. Additionally, the large variation in spin period and lack of clear association with magnetic field strength remains unexplained. I present recent results from studying "dead discs", which are circumstellar discs that remain coupled to a star's magnetic field long after active accretion onto the star has ceased. For a moderately large disc-field coupling, spin-down to rotational periods of 10 years is possible. This model could also explain the recently discovered correlation between slow rotation and binarity in A-stars, whereby the angular momentum from the star is transported through the disc and is finally transferred to the orbit of the binary.