DOPPLER IMAGING OF THE SILICON DISTRIBUTION ON CU VIR - EVIDENCE FOR A DECENTERED MAGNETIC DIPOLE

A Doppler image of the silicon distribution on the magnetic Ap star CU Vir is presented. This distribution is characterized by a large circu lar spot of relative silicon depletion (with respect to the mean value across the star) that is coincident with the positive magnetic pole. An elongated spot of enhanced silicon is centred at the expected locat ion of the negative magnetic pole. If silicon is enhanced where the ma gnetic field is horizontal, as is predicted by diffusion theory, then this distribution is consistent with a magnetic dipole whose axis is d ecentred by about 0.2 in stellar radius along the dipole axis.