Institut für Astronomie und Astrophysik Abteilung Astronomie Sand 1, D-72076 Tübingen, Germany

Preprint 01/15

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 RE0503-289

T. Rauch (1), K. Werner (1), P. Quinet (2,3), J.W. Kruk (4)

(1) Institute for Astronomy and Astrophysics, Eberhard Karls University, Tuebingen, Germany
(2) Astrophysique et Spectroscopie, Universite de Mons - UMONS, Belgium
(3) IPNAS, Universite de Liege, Liege, Belgium
(4) NASA Goddard Space Flight Center, Greenbelt, USA

To be published in: A&A

Abstract. 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.
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 RE0503-289 and to determine their photospheric Ga abundances.
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 RE0503-289.
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 RE0503-289. 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 x 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 RE0503-289. We identified the strongest Ga IV lines (at 1258.801, 1338.129 A) in the HST/STIS (Hubble Space Telescope / Space Telescope Imaging Spectrograph) spectrum of G191-B2B and measured a Ga abundance of 2.0 ± 0.5 x 10^-6 (about 22 times solar).
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 RE0503-289) 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.
 
Key words: atomic data, line: identification, stars: abundances, stars: individual: G191-B2B, stars: individual: RE0503-289, virtual observatory tools

Preprint (657 kb PDF file including figures)

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