Termination of AGB-evolution by hydrogen recombination.

Astronomy and Astrophysics – Astrophysics

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Stars: Interiors, Evolution, Asymptotic Giant Branch, Planetary Nebulae: General

Scientific paper

The evolution of stars with mass between 0.8 and 5Msun_ along the Asymptotic Giant Branch has been calculated. We find that for the mass range of =~1-4Msun_ there exists a critical luminosity above which the stars experience a runaway expansion during the luminosity peak of a thermal pulse. The physical reason for this lies in the recombination of hydrogen within a thermally and dynamically unstable envelope. The calculations are supplemented by various stability tests. For the first time this long-predicted instability has been found in complete stellar evolution models. While our models indicate the complete and fast ejection of the envelope, the assumptions of hydrostatic equilibrium and instantaneous convective mixing limit the significance of the results, although they confirm and complete earlier dynamical investigations of static stellar envelopes. Our approach does not allow to decide conclusively whether the instability has any connection with the ejection of Planetary Nebulae from real stars; instead, we demonstrate a definitive physical limit of standard hydrostatic AGB calculations.

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