Astron. Astrophys. 357, 587-596 (2000)

Element abundances in cool white dwarfs

I. The DZA white dwarfs L 745-46A and Ross 640 ^*

D. Koester ^** and B. Wolff

Institut für Theoretische Physik und Astrophysik, Universität Kiel, 24098 Kiel, Germany (koester,wolff@astrophysik.uni-kiel.de)

Received 28 January 2000 / Accepted 14 March 2000

Abstract

The standard explanation for the presence of photospheric metals in white dwarfs of spectral type DZ and related types is a combination of episodic accretion from the interstellar medium and diffusion of heavy elements downwards in the atmosphere on time scales of typically 10⁶ years. Much of the empirical data on atmospheric parameters and element abundances available in the literature is based on International Ultraviolet Explorer spectra with low signal-to-noise and on atmospheric modeling, which does not include recent advances in input physics. We have therefore decided to address this problem again, using modern UV observations of the Hubble Space Telescope (HST), if available, and a new generation of theoretical models. In this paper we start with an analysis of the DZA white dwarfs L 745-46A and Ross 640. These are key objects since they contain both hydrogen and metals in their atmospheres. Excellent UV spectra obtained with the Faint Object Spectrograph (FOS) onboard HST make them good test cases for the methods to be applied in future work on other objects and for the accuracy, which can be achieved. We have determined the atmospheric parameters and as well as hydrogen abundances and abundances of several heavy elements. The results are in good agreement with the theoretical predictions of Dupuis and collaborators. Particular emphasis is placed on a discussion of the remaining problems in the input physics and the corresponding uncertainties of the final results.

Key words: stars: abundances stars: atmospheres stars: white dwarfs ultraviolet: stars

* Based on observations made 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 NAS 5-26555. These observations are associated with proposal ID 6450. Based on observations collected at the European Southern Observatory, Chile (ESO No. 54.D-0402)
** Visiting Astronomer, German-Spanish Astronomical Centre, Calar Alto, operated by the Max-Planck-Institute for Astronomy, Heidelberg, jointly with the Spanish National Commission for Astronomy

Send offprint requests to: B. Wolff

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Contents

• 1. Introduction
• 2. Model atmospheres and input physics
  • 2.1. Equation of state and ionization fraction
  • 2.2. Lyman wing
• 3. Observations
• 4. L 745-46A
  • 4.1. Effective temperature and hydrogen abundance
  • 4.2. Metal abundances
• 5. Ross 640
  • 5.1. Effective temperature and hydrogen abundance
  • 5.2. Metal abundances
• 6. Discussion
• Acknowledgements
• References

© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000
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