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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. [ABSTRACT FROM AUTHOR]

After an extensive assessment of the effective collision strengths available to model the He-like triplet of C V, N VI, O VII, Ne IX, Mg XI and Si XIII in collisionally dominated plasmas, new accurate effective collision strengths are reported for Ne IX. The uncertainty intervals of the density and temperature diagnostics due to the atomic data errors are also determined for both Maxwell-Boltzmann and κ electron-energy distributions. It is shown that these uncertainty bands limit the temperature range where the temperature line-ratio diagnostic can be applied and its effectiveness to discern the electron-energy distribution type. These findings are benchmarked with Chandra and XMM-Newton spectra of stellar coronae and with tokamak measurements. [ABSTRACT FROM AUTHOR]