Constraining the very heavy elemental abundance peak in the chemically peculiar star χ Lupi, with new atomic data for Os II and Ir II

The determination of elemental abundances plays an important role in understanding the atmospheric processes that lead to anomalies in spectra of chemically peculiar stars. Ongoing study of the HgMn star χ Lupi has defined much of the elemental abundance distribution but progress is dependent upon the availability and accuracy of the atomic data. Here we present experimentally determined wavelengths and oscillator strengths for 27 Os II and 23 Ir II transitions. The oscillator strengths have been determined by combining radiative lifetimes measured using the laser-induced fluorescence technique with branching fractions determined from Fourier transform spectrometer emission line intensities. The new data were used in a synthetic spectrum analysis of high-resolution ultraviolet spectra of χ Lupi obtained with the Hubble Space Telescope. Weak abundance enhancements were determined for both elements, (Os/H) =+ 1.3 dex and (Ir/H) =+ 0.7 dex, which now clearly defines the heavy element abundance peak in this star to be comprised of only the four elements Pt, Au, Hg, and Tl. This result offers a new constraint on theoretical efforts to explain the observed spectrum anomalies.