Astron. Astrophys. 359, 1085-1106 (2000)

Non-LTE line formation for neutral oxygen

Model atom and first results on A-type stars ^*

N. Przybilla ^1,2, K. Butler ¹, S.R. Becker ¹, R.P. Kudritzki ^1,2,3 and K.A. Venn <sup>4

1</sup> Universitäts-Sternwarte München, Scheinerstrasse 1, 81679 München, Germany
² Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Strasse 1, 85740 Garching bei München, Germany
³ University of Arizona, Steward Observatory, 933 N. Cherry Av., Tucson AZ 85721, USA
⁴ Macalester College, Department of Physics and Astronomy, 1600 Grand Ave., St. Paul, MN 55105, USA

Received 25 June 1999 / Accepted 22 May 2000

Abstract

An extensive model atom for non-LTE line formation calculations for OI is presented, taking into account recent improvements in the atomic data. Based on line-blanketed LTE model atmospheres equivalent widths are computed in LTE and non-LTE for the diagnostic OI lines of A- and late B-type stars in the range to and luminosity classes V to Ia. Non-LTE abundance corrections are provided: they span a wide range in magnitude, from less than 0.1 dex for the weak lines in main sequence stars to more than 1.5 dex for the near-infrared lines in some supergiants.

The dependence of the non-LTE effects on the atmospheric parameters is discussed with special emphasis on supergiants. In particular, the near-infrared transitions are found to react sensitively to the collisional excitation cross sections used in the calculations. Further investigations concentrate on the influence of microturbulence and on the rôle of wind outflow velocity fields on the line formation.

As a test and first application of the model, oxygen abundances for Vega (A0 V), Leo (A0 Ib) and HD 92207 (A0 Iae) are derived. The analysis of Vega confirms a slight oxygen underabundance (0.3 dex) in this star while Leo and HD 92207 show a nearly solar value. For Vega the observed spectrum can be reproduced accurately by the calculated line profiles. In supergiants consistent abundances can be derived from the weak lines in the visible. At high luminosities the prominent near-infrared features - among other strong lines from different elemental species - are subject to additional broadening by an unidentified process which prevents an equally accurate theoretical interpretation.

Key words: atomic data line: formation stars: abundances stars: supergiants

* Based on observations collected at the European Southern Observatory, Chile (ESO N^o 62.H-0176)

Send offprint requests to: N. Przybilla (nob@usm.uni-muenchen.de)

SIMBAD Objects

Contents

• 1. Introduction
• 2. Model calculations
  • 2.1. Model atmospheres and programs
  • 2.2. The model atom
    • 2.2.1. Energy levels
    • 2.2.2. Radiative transitions
    • 2.2.3. Collisional transitions
    • 2.2.4. Charge exchange reactions
  • 2.3. Error estimates for the oxygen non-LTE calculations
• 3. Discussion
  • 3.1. Results of our model calculations
  • 3.2. The non-LTE effects
  • 3.3. The strong near-infrared lines
  • 3.4. Comparison with other studies
• 4. Application to observations
  • 4.1. The spectra
  • 4.2. Abundance analysis
  • 4.3. Comparison with other analyses
• 5. The impact of velocity fields on the line formation
  • 5.1. Microturbulence
  • 5.2. Wind outflow velocity fields
• 6. Summary and conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 2000

Online publication: July 13, 2000
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