Astron. Astrophys. 330, 641-650 (1998)

The stability of late-type stars close to the Eddington limit

Martin Asplund

Astronomiska observatoriet, Box 515, S-751 20  Uppsala, Sweden

Received 7 May 1997 / Accepted 10 September 1997

Abstract

The opacity-modified Eddington limit has been computed for hydrogen-deficient model atmospheres. The R Coronae Borealis (R CrB) stars are found to be located strikingly close to the limit, which suggests that the unknown trigger mechanism for their visual declines of the stars are instabilities in connection with the stars encountering the Eddington limit in their evolution. It also points to a similarity between the eruptive behaviours of the R CrB stars and the Luminous Blue Variables (LBVs).

Super-Eddington luminosities in hydrostatic model atmospheres manifest themselves by the presence of gas pressure inversions. Such inversions are not an artifact of the assumption of hydrostatic equilibrium but can also be present in hydrodynamical model atmospheres. Only for very large mass loss rates hardly realized in supergiants will the inversions be removed. Instabilities may, however, still be present in such inversions, which is investigated for both H-rich and H-deficient late-type supergiant model atmospheres. Dynamical instabilities may occur in surface ionization zones, which might lead to ejections of gas. A local, non-adiabatic, linear stability analysis reveals that sound waves can be amplified due to the strong radiative forces. However, despite the super-Eddington luminosities, the efficiency of the radiative instabilities is fairly low compared to for early-type stars with growth rates of s^-1.

Key words: stars: model atmospheres instabilities stars: AGB and post-AGB stars: mass-loss stars: evolution stars: variables: R Coronae Borealis stars: variables: Luminous Blue Variables

Present address: NORDITA, Blegdamsvej 17,DK-2100  Copenhagen O, Denmark

Send offprint requests to: martin@nordita.dk

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Contents

• 1. The Eddington limit and the development of gas pressure inversions
• 2. The Eddington limit and the R CrB stars
  • 2.1. The Eddington limit in the -log g diagram
  • 2.2. Possible consequences for H-deficient stars
• 3. The disappearance of gas pressure inversions
  • 3.1. Effects of convection
  • 3.2. Effects of mass loss
• 4. Dynamical instabilities
• 5. Radiative instabilities
  • 5.1. Radiatively driven sound waves in -inversions
  • 5.2. The simplifying case of a isothermal, non-ionizing atmosphere
• 6. Conclusions and speculations
• Acknowledgements
• References

© European Southern Observatory (ESO) 1998

Online publication: January 16, 1998
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