Bismuth abundance anomaly in a Hg—Mn star

Abstract

Strong absorption lines due to Bi II have been found in the Hg—Mn star HR7775 (HD193452) in high-resolution spectra obtained with the IUE. HR7775 was first recognized as a chemically peculiar star by Preston in 1972. Detailed examination of the optical spectrum at high resolution¹ showed that it is one of the most extreme stars of the ‘cool’ Hg—Mn group, with strong enhancements of Hg, Pt, Sr, Y, and Ga; the last of these is confirmed² by the very strong Ga II resonance line at 1,414 Å. Four-colour Strömgren photometry of HR7775 (ref. 3), interpreted with the aid of the model atmosphere calibrations by Relyea and Kurucz⁴, gives T_eff = 10,800 K, log g = 4.2, while the Hβ index gives log g = 4.0 according to the calibration of Schmidt⁵. We have adopted a fully blanketed model atmosphere⁶ with solar composition, T_eff = 11,000 K, log g = 4.0. Uncertainties in the exact composition and parameters have a very small effect on our results when compared with uncertainties due to oscillator strengths, identification of blending lines, and correct placement of the unblanketed continuum. The overabundance of Bi obtained from spectrum synthesis calculations is ∼10⁶. Close examination of the spectra of several other Hg—Mn stars shows that Bi II is not detected in any other star in this group.