The end of super AGB and massive AGB stars

I. The instabilities that determine the final mass of AGB stars

¹ Argelander Institute for Astronomy, University of Bonn, Auf dem Huegel 71, 53121 Bonn, Germany
e-mail: h.lau@monash.edu
² Department of Applied Physics, Polytechnical University of Catalonia, 08860 Barcelona, Spain
³ Monash Centre for Astrophysics, School of Mathematical Sciences, Monash University, Victoria 3800, Australia

Received: 16 January 2012
Accepted: 17 April 2012

Abstract

Context. The literature is rich in analysis and results related to thermally pulsing-asymptotic giant branch (TP-AGB) stars, but the problem of the instabilities that arise and cause the divergence of models during the late stages of their evolution is rarely addressed.

Aims. We investigate the physical conditions, causes and consequences of the interruption in the calculations of massive AGB stars in the late thermally-pulsing AGB phase.

Methods. We have thoroughly analysed the physical structure of a solar metallicity 8.5 M_⊙ star and described the physical conditions at the base of the convective envelope (BCE) just prior to divergence.

Results. We find that the local opacity maximum caused by M-shell electrons of Fe-group elements lead to the accumulation of an energy excess, to the departure of thermal equilibrium conditions at the base of the convective envelope and, eventually, to the divergence of the computed models. For the 8.5 M_⊙ case we present in this work the divergence occurs when the envelope mass is about 2 M_⊙. The remaining envelope masses range between somewhat less than 1 and more than 2 M_⊙ for stars with initial masses between 7 and 10 M_⊙ and, therefore, our results are relevant for the evolution and yields of super-AGB stars. If the envelope is ejected as a consequence of the instability we are considering, the occurrence of electron-capture supernovae would be avoided at solar metallicity.