Evolution of massive AGB stars^*

III. the thermally pulsing super-AGB phase

¹ Institut d'Astronomie et d'Astrophysique, Université Libre de Bruxelles, ULB, CP 226, 1050 Brussels, Belgium e-mail: siess@astro.ulb.ac.be
² Centre for Stellar and Planetary Astrophysics, School of Mathematical Sciences, Monash University, Victoria 3800, Australia

Received: 27 October 2009
Accepted: 15 December 2009

Abstract

Aims. We present the first simulations of the full evolution of super-AGB stars through the entire thermally pulsing AGB phase. We analyse their structural and evolutionary properties and determine the first SAGB yields.

Methods. Stellar models of various initial masses and metallicities were computed using standard physical assumptions which prevents the third dredge-up from occurring. A postprocessing nucleosynthesis code was used to compute the SAGB yields, to quantify the effect of the third dredge-up (3DUP), and to assess the uncertainties associated with the treatment of convection.

Results. Owing to their massive oxygen-neon core, SAGB stars suffer weak thermal pulses, have very short interpulse periods and develop very high temperatures at the base of their convective envelope (up to 14010⁸ K), leading to very efficient hot bottom burning. SAGB stars are consequently heavy manufacturers of ⁴He, ¹³C, and ¹⁴N. They are also able to inject significant amounts of ⁷Li, ¹⁷O, ²⁵Mg, and in the interstellar medium. The 3DUP mainly affects the CNO yields, especially in the lower metallicity models. Our post-processing simulations also indicate that changes in the temperature at the base of the convective envelope, which would result from a change in the efficiency of convective energy transport, have a dramatic impact on the yields and represent another major source of uncertainty.