Abundance and Isotopic Anomalies of Thallium in the Atmosphere of the HgMn Star χ LUPI

Abstract

We announce the detection of two fully resolved hyperfine components of the intercombination line of Tl II (Z = 81) at 1908.6 Å in the ultraviolet spectrum of the sharp-lined, nonmagnetic, chemically peculiar star χ Lupi (B9.5pHgMn + A2 Vm). The wavelengths and profiles of these absorption lines strongly suggest that they arise from the heaviest isotope, ²⁰⁵Tl. The echelle observations were obtained with the Hubble Space Telescope Goddard High Resolution Spectrograph (HST/GHRS). Spectrum synthesis fits to the line profiles yield a logarithmic overabundance of thallium, relative to the solar system abundance, of +3.8 dex. We believe this to be the first detection of thallium in a star other than the Sun. It is also the first time hyperfine structure has been resolved in the satellite ultraviolet spectrum of an astrophysical source. The large overabundance and the presence of only the heaviest isotope of the element continue the pattern observed previously in χ Lup for platinum (Z = 78), gold (Z = 79), and mercury (Z = 80).