Astron. Astrophys. 359, 1042-1058 (2000)

The chemical evolution of hot white dwarfs in the presence of diffusion and mass loss

K. Unglaub and I. Bues

Dr. Remeis-Sternwarte Bamberg, Astronomisches Institut der Universität Erlangen-Nürnberg, Sternwartstrasse 7, 96049 Bamberg, Germany

Received 14 March 2000 / Accepted 9 May 2000

Abstract

We present the results of new diffusion calculations with mass loss for hot white dwarfs with . In contrast to previous calculations, the stellar parameters , and the mass loss are allowed to vary. With stellar parameters from evolutionary tracks and two independent estimates of the mass loss rates the time-dependance of the composition during the cooling is investigated in an outer region of about . With an improved numerical method the elements H, He, C, N and O can be taken into account simultaneously.

In the - -diagram we expect a DAO/DA transition region near an approximately horizontal line characterized by . In hydrogen-rich objects with lower surface gravities helium lines should be detectable, because mass loss with prevents or retards the gravitational settling of helium. PG 1159 stars are predicted to evolve into DO's when the surface gravity reaches a value between and 8.0 and the mass loss rate decreases below about . From the results we suggest the existence of an evolutionary link between PG 1159 stars and most of the DO's. Calculations with various admixtures of hydrogen in PG 1159 stars show that most DAO's cannot be descendants of hydrogen-poor PG 1159 stars with a number ratio , because mass loss is expected to retard the floating up of hydrogen. The majority of hot DAO's and DA's must be descendants of hydrogen-rich precursors.

Key words: stars: abundances stars: evolution diffusion stars: white dwarfs

Send offprint requests to: K. Unglaub (unglaub@sternwarte.uni-erlangen.de)

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Contents

• 1. Introduction
• 2. Basic assumptions
• 3. Numerical method
  • 3.1. The primary grid
  • 3.2. The secondary grid
  • 3.3. The modified upwind scheme
  • 3.4. The monotonic transport scheme
  • 3.5. The evaluation of
  • 3.6. Boundary conditions
  • 3.7. The time step
  • 3.8. The stability of the method
• 4. Estimate of the mass loss rates
• 5. The DAO/DA transition
• 6. The PG 1159 / DO transition
• 7. The floating up of hydrogen
• 8. Summary and conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 2000

Online publication: July 13, 2000
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