Astron. Astrophys. 360, 603-616 (2000)

The effect of diffusion on pulsations of stars on the upper main sequence

 Scuti and metallic A stars

S. Turcotte ^1,2, J. Richer ³, G. Michaud ³ and J. Christensen-Dalsgaard <sup>2,4

1</sup> Service d'Astrophysique/DAPNIA, CEA-Saclay, 91191 Gif sur Yvette, France (turcotte@discovery.saclay.cea.fr)
² Teoretisk Astrofysik Center, Danmarks Grundforskningsfond
³ Université de Montréal and Centre de Recherche en Calcul Appliqué, Montréal, Canada (richer,michaudg@cerca.umontreal.ca)
⁴ Institut for Fysik og Astronomi, Aarhus Universitet, 8000 Århus C, Denmark (jcd@ifa.au.dk)

Received 30 March 2000 / Accepted 19 May 2000

Abstract

Recent dramatic improvements in the modeling of abundance evolution due to diffusion in A stars have been achieved with the help of monochromatic opacity tables from the OPAL group. An important result in the context of stellar pulsations is the substantial helium abundance shown to be left over in the driving region of  Scuti-type pulsations in chemically peculiar Am stars. An accurate opacity profile in the entire stellar envelope including the full effect of heavy elements is also now available for the first time.

Pulsations are shown to be excluded for young Am stars but occur naturally when these stars evolve off the ZAMS. The predicted variable metallic A stars all lie towards the red edge of the instability strip, in qualitative agreement with the observed variable  Delphini and mild Am stars.

Results show little direct excitation from iron-peak elements in A-type stars. The main abundance effect is due to the settling of helium, along with a marginal effect due to the enhancement of hydrogen.

Key words: diffusion stars: abundances stars: evolution stars: variables: ffi Sct

Send offprint requests to: S. Turcotte

SIMBAD Objects

Contents

• 1. Introduction
• 2. Metallicism and pulsations: observations
• 3. Metallicism and pulsations: theory
• 4. The new Montreal models
  • 4.1. New results with the NMM
  • 4.2. The basic properties of the NMM
• 5. Diffusion and the mechanism
• 6. Opacity and its derivatives
• 7. The models of A stars
• 8.  Scuti-type pulsations in Am stars
  • 8.1. The variability of an Am star across the instability strip
  • 8.2. The effect of diffusion on the instability strip
  • 8.3. Comparison with variable evolved CP stars
  • 8.4. The excitation of g modes in A stars with diffusion
• 9. Is there a seismic signature of diffusion in  Scuti stars?
• 10. Pulsation and turbulence
• 11. Conclusion
• Acknowledgements
• References

© European Southern Observatory (ESO) 2000

Online publication: August 17, 2000
helpdesk.link@springer.de