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

Abstract

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 × 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 × 10^-5, about 300-1200 times solar) and tin (1.3-3.2 × 10^-4, about 14 300-35 200 times solar). For G191-B2B, upper limits were determined for the abundances of Mo (5.3 × 10^-7, 100 times solar) and, in addition, for Kr (1.1 × 10^-6, 10 times solar) and Xe (1.7 × 10^-7, 10 times solar). The arsenic abundance was determined (2.3-5.9 × 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.

Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26666.Based on observations made with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer.Tables A.10-A.13 are only available via the German Astrophysical Virtual Observatory (GAVO) service TOSS (http://dc.g-vo.org/TOSS).