Stellar laboratories: new Ge V and Ge VI oscillator strengths and their validation in the hot white dwarf RE 0503–289^{⋆,⋆⋆,⋆⋆⋆}

¹ Institute for Astronomy and Astrophysics, Kepler Center for Astro and Particle Physics, Eberhard Karls University, Sand 1, 72076 Tübingen, Germany
e-mail: rauch@astro.uni-tuebingen.de
² Astrophysique et Spectroscopie, Université de Mons – UMONS, 7000 Mons, Belgium
³ IPNAS, Université de Liège, Sart Tilman, 4000 Liège, Belgium
⁴ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA

Received: 13 July 2012
Accepted: 14 August 2012

Abstract

Context. State-of-the-art spectral analysis of hot stars by means of non-LTE model-atmosphere techniques has arrived at a high level of sophistication. The analysis of high-resolution and high-S/N spectra, however, is strongly restricted by the lack of reliable atomic data for highly ionized species from intermediate-mass metals to trans-iron elements. Especially data for the latter has only been sparsely calculated. Many of their lines are identified in spectra of extremely hot, hydrogen-deficient post-AGB stars. A reliable determination of their abundances establishes crucial constraints for AGB nucleosynthesis simulations and, thus, for stellar evolutionary theory.

Aims. In a previous analysis of the UV spectrum of RE 0503−289, spectral lines of highly ionized Ga, Ge, As, Se, Kr, Mo, Sn, Te, I, and Xe were identified. Individual abundance determinations are hampered by the lack of reliable oscillator strengths. Most of these identified lines stem from Ge V. In addition, we identified Ge VI lines for the first time. We calculated Ge V and Ge VI oscillator strengths in order to reproduce the observed spectrum.

Methods. We newly calculated Ge V and Ge VI oscillator strengths to consider their radiative and collisional bound-bound transitions in detail in our non-LTE stellar-atmosphere models for the analysis of the Ge iv–vi spectrum exhibited in high-resolution and high-S/N FUV (FUSE) and UV (ORFEUS/BEFS, IUE) observations of RE 0503−289.

Results. In the UV spectrum of RE 0503−289, we identify four Ge IV, 37 Ge V, and seven Ge VI lines. Most of these lines are identified for the first time in any star. We can reproduce almost all Ge IV, Ge V, and Ge VI lines in the observed spectrum of RE 0503−289 (T_eff = 70   kK, log g = 7.5) at log Ge = −3.8 ± 0.3 (mass fraction, about 650 times solar). The Ge iv/v/vi ionization equilibrium, that is a very sensitive T_eff indicator, is reproduced well.

Conclusions. Reliable measurements and calculations of atomic data are a prerequisite for stellar-atmosphere modeling. Our oscillator-strength calculations have allowed, for the first time, Ge V and Ge VI lines to be successfully reproduced in a white dwarf’s (RE 0503−289) spectrum and to determine its photospheric Ge abundance.