Detection of cesium in the atmosphere of the hot He-rich white dwarf HD 149499B

We report the first detection of cesium (Z = 55) in the atmosphere of a white dwarf. Around a dozen absorption lines of Cs IV, Cs V, and Cs VI have been identified in the Far Ultraviolet Spectroscopic Explorer spectrum of the He-rich white dwarf HD 149499B (T eff  = 49 500 K, log g = 7.97). The lines have equivalent widths ranging from 2.3 to 26.9 mÅ. We performed a spectral synthesis analysis to determine the cesium content in the atmosphere. Non-local thermodynamic equilibrium atmosphere models were computed by considering cesium explicitly in the calculations. For this purpose we calculated oscillator strengths for the bound–bound transitions of Cs IV–Cs VI with both AUTOSTRUCTURE (multiconfiguration Breit–Pauli) and GRASP2K (multiconfiguration Dirac–Fock) atomic structure codes as neither measured nor theoretical values are reported in the literature. We determined a cesium abundance of log N(Cs)/N(He) = −5.45 ± 0.35, which can also be expressed in terms of the mass fraction log X Cs  = −3.95 ± 0.35.