2014A&A...564A..41R

For the spectral analysis of high-resolution and high-signal-to-noise (S/N) spectra of hot stars, state-of-the-art non-local thermodynamic equilibrium (NLTE) model atmospheres are mandatory. These are strongly dependent on the reliability of the atomic data that is used for their calculation. In a recent analysis of the ultraviolet (UV) spectrum of the DA-type white dwarf G191-B2B, 21 ZnIV lines were newly identified. Because of the lack of ZnIV data, transition probabilities of the isoelectronic GeVI were adapted for a first, coarse determination of the photospheric Zn abundance. Reliable ZnIV and ZnV oscillator strengths are used to improve the Zn abundance determination and to identify more Zn lines in the spectra of G191-B2B and the DO-type white dwarf RE0503-289. We performed new calculations of ZnIV and ZnV oscillator strengths to consider their radiative and collisional bound-bound transitions in detail in our NLTE stellar-atmosphere models for the analysis of the ZnIV - V spectrum exhibited in high-resolution and high-S/N UV observations of G191-B2B and RE0503-289. In the UV spectrum of G191-B2B, we identify 31 ZnIV and 16 ZnV lines. Most of these are identified for the first time in any star. We can reproduce well almost all of them at logZn=-5.52±0.2 (mass fraction, about 1.7 times solar). In particular, the ZnIV/ ZnV ionization equilibrium, which is a very sensitive Teff indicator, is well reproduced with the previously determined T_eff= 60000±2000K and logg=7.60±0.05. In the spectrum of RE0503-289, we identified 128 ZnV lines for the first time and determined logZn=-3.57±0.2 (155 times solar). Reliable measurements and calculations of atomic data are a pre-requisite for stellar-atmosphere modeling. Observed ZnIV and ZnV line profiles in two white dwarf (G191-B2B and RE0503-289 ultraviolet spectra were well reproduced with our newly calculated oscillator strengths. This allowed us to determine the photospheric Zn abundance of these two stars precisely.