Stellar laboratories

IV. New Ga iv, Ga v, and Ga vi oscillator strengths and the gallium abundance in the hot white dwarfs G191−B2B and 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 November 2014
Accepted: 23 January 2015

Abstract

Context. For the spectral analysis of high-resolution and high-signal-to-noise (S/N) spectra of hot stars, advanced non-local thermodynamic equilibrium (NLTE) model atmospheres are mandatory. These atmospheres are strongly dependent on the reliability of the atomic data that are used to calculate them.

Aims. Reliable Ga iv–vi oscillator strengths are used to identify Ga lines in the spectra of the DA-type white dwarf G191−B2B and the DO-type white dwarf RE 0503−289 and to determine their photospheric Ga abundances.

Methods. We newly calculated Ga iv–vi oscillator strengths to consider their radiative and collisional bound-bound transitions in detail in our NLTE stellar-atmosphere models for analyzing of Ga lines exhibited in high-resolution and high-S/N UV observations of G191−B2B and RE 0503−289.

Results. We unambiguously detected 20 isolated and 6 blended (with lines of other species) Ga v lines in the Far Ultraviolet Spectroscopic Explorer (FUSE) spectrum of RE 0503−289. The identification of Ga iv and Ga vi lines is uncertain because they are weak and partly blended by other lines. The determined Ga abundance is 3.5 ± 0.5 × 10^-5 (mass fraction, about 625 times the solar value). The Ga iv/Ga v ionization equilibrium, which is a very sensitive indicator for the effective temperature, is well reproduced in RE 0503−289. We identified the strongest Ga iv lines (at 1258.801, 1338.129 Å) in the HST/STIS spectrum of G191−B2B and measured a Ga abundance of 2.0 ± 0.5 × 10^-6 (about 22 times solar).

Conclusions. Reliable measurements and calculations of atomic data are a prerequisite for stellar-atmosphere modeling. The observed Ga iv–v line profiles in two white dwarf (G191−B2B and RE 0503−289) ultraviolet spectra were well reproduced with our newly calculated oscillator strengths. For the first time, this allowed us to determine the photospheric Ga abundance in white dwarfs.