Doppler imaging of abundance features on AP stars - The SI II distribution on Gamma2 ARIETIS

It is shown that an improved version of the Doppler imaging technique which incorporates the maximum entropy formalism can be used to obtain accurate maps of the surface distribution of elements on rapidly rotating Ap stars strictly from the spectral line variations. Not only can the method accurately recover complex spot distributions such as rings, but it also can reconstruct spot distributions with both depleted and enhanced abundance regions in spite of the assumption that the intrinsic line profile for both regions is the same. The technique is applied to the B9pSiCr star Gamma2 Arieti using the Si II 6347 A line. The resulting reconstruction shows that silicon is depleted near the magnetic poles and that it is enhanced in an incomplete, nonuniform annulus near the negative magnetic pole. This image provides direct observational support for the theory that diffusion is the process responsible for the abundance variations in Ap stars. 22 refs.