Stellar laboratories VI. New Mo IV-VII oscillator strengths and the molybdenum abundance in the hot white dwarfs G191-B2B and RE 0503-289

Context. 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. Aims. To identify molybdenum lines in the ultraviolet (UV) spectra of the DA-type white dwarf G191-B2B and the DO-type white dwarf RE 0503-289 and, to determine their photospheric Mo abundances, reliable Mo IV-VII oscillator strengths are used. Methods. We newly calculated Mo IV-VII oscillator strengths to consider their radiative and collisional bound-bound transitions in detail in our NLTE stellar-atmosphere models for the analysis of Mo lines exhibited in high-resolution and high S/N UV observations of RE 0503-289. Results. We identified 12 Mo V and 9 Mo VI lines in the UV spectrum of RE 0503-289 and measured a photospheric Mo abundance of 1.2-3.0 x 10(-4) (mass fraction, 22 500-56 400 times the solar abundance). In addition, from the As v and Sn IV resonance lines, we measured mass fractions of arsenic (0.5-1.3 x 10(-5), about 300-1200 times solar) and tin (1.3-3.2 x 10(-4), about 14 300-35 200 times solar). For G191-B2B, upper limits were determined for the abundances of Mo (5.3 x 10(-7), 100 times solar) and, in addition, for Kr (1.1 x 10(-6), 10 times solar) and Xe (1.7 x 10(-7), 10 times solar). The arsenic abundance was determined (2.3-5.9 x 10(-7), about 21-53 times solar). A new, registered German Astrophysical Virtual Observatory (GAVO) service, TOSS, has been constructed to provide weighted oscillator strengths and transition probabilities. Conclusions. Reliable measurements and calculations of atomic data are a prerequisite for stellar-atmosphere modeling. Observed Mo V-VI line profiles in the UV spectrum of the white dwarf RE 0503-289 were well reproduced with our newly calculated oscillator strengths. For the first time, this allowed the photospheric Mo abundance in a white dwarf to be determined.