Elemental abundances in normal late-B and HgMn stars from co-added IUE spectra. I. Iron-peak elements.

The abundances of five iron-peak elements (chromium through nickel) in a sample of forty normal, superficially normal, and HgMn-type late-B stars are derived by spectrum-synthesis analysis of co-added high-resolution International Ultraviolet Explorer (IUE) spectra. Approximately solar abundances of these elements are obtained for the normal stars, with the notable exception of α Lyr which is shown to be mildly metal-deficient in accordance with published analyses of its visual-region spectrum. The superficially normal stars are generally indistinguishable from the normal stars in terms of their iron-peak elemental abundances. The HgMn stars exhibit a large diversity in chromium and iron abundances, a striking effective- temperature-dependent manganese enhancement, and substantially sub-solar abundances of cobalt and nickel. Even for Mn, Co, and Ni there are some HgMn stars which depart from the more general behaviour of the group as a whole, and thus these results tend to confirm the rather heterogeneous nature of this peculiarity class suspected in previous surveys of their visual-region spectra. By making schematic corrections for the effects on our analysis of atmospheric chemical stratification, we show that the predictions of radiative diffusion theory for manganese are in excellent accord with the observed upper envelope of abundances in the HgMn stars.