2004ApJS..155..179A

Very heavy elements (Pt, Au, Hg, Tl, and Bi) are found to be enhanced in the atmospheres of the chemically peculiar stars of the upper main sequence by up to a million times the solar system levels. Such enhancements are believed to result from atmospheric dynamics (i.e., diffusion) rather than scenarios that dredge up nuclear-processed material to the surface or transfer processed material between binary companions. However, the theoretical framework needs to be further constrained by observations beyond the realm of the spectral types for which such abundance enhancements are observed at optical wavelengths.

The International Ultraviolet Explorer (IUE) satellite collected spectra of bright stars for which chemical peculiarities have been derived from ground-based data. For several elements the abundance enhancements have only been recently measured using Hubble Space Telescope data and have therefore not yet been exploited in the IUE data. We have initiated a program to analyze IUE high-dispersion spectra to more fully characterize the pattern of very heavy element enhancement for many mercury-manganese (HgMn) stars and to potentially extend the spectral class (effective temperature) boundaries over which these abundance anomalies are known to exist. The abundances of very heavy elements in chemically normal B and A-type stars provide a base level that may be compared with the solar system abundances. These early spectral type stars may therefore reveal clues for galactic chemical evolution studies since they were formed at a later epoch than the Sun in the history of the Galaxy. This first paper presents the motivation for the analyses to follow, outlines our spectral co-addition technique for IUE spectra, and discusses the choice of model atmospheres and the synthetic spectrum procedures, while initiating the study by highlighting the abundance of gold in several HgMn stars.