Surface abundance distribution models of Si, Cr, Mn, Fe, Pr and Nd for the slowly rotating Ap star HD 187474

Abstract

The very slowly rotating magnetic Ap star HD 187474 has strong and quite variable spectral lines of several elements. An axisymmetric low-order multipole magnetic model has been obtained for this star by Landstreet & Mathys (2000). HD 187474 is one of the few very slowly rotating magnetic Ap stars that is suitable for abundance distribution modelling; because of the unusually large angle between the field and rotation axes, the line of sight goes well into both magnetic hemispheres. We have used CASPEC and CES spectra with good phase coverage together with the programme ZEEMAN to search for simple three-ring abundance distributions of Si, Cr, Mn, Fe, Pr and Nd, symmetric about the assumed magnetic field distribution axis, that match observed line profiles as a function of rotational phase. Reasonably satisfactory fits to the observations are found for all of these elements. In all cases, three-ring models work as well as more finely gridded models. The deduced abundances are somewhat overabundant and somewhat non-uniform for Fe; for the other elements quite large overabundances (up to 5 dex for Nd) are found, with substantially higher (but somewhat unequal) abundances around the two magnetic poles compared to the magnetic equator. All the elements studied are overabundant everywhere (except perhaps Si around the magnetic equator). This star poses a real challenge to theories of chemical peculiarity.