Production of ²⁶Al by super-AGB stars

Institut d'Astronomie et d'Astrophysique, Université Libre de Bruxelles, CP 226, 1050 Brussels, Belgium e-mail: siess@astro.ulb.ac.be

Received: 7 May 2008
Accepted: 17 July 2008

Abstract

Context. Super AGB (SAGB) stars have initial masses ranging between ~7–11  and develop efficient hydrogen burning at the base of their convective envelope during their AGB evolution, leading to a substantial production of Al_g.

Aims. We present the first discussion of the contribution of the SAGB stars to the galactic Al_g production, and we estimate the main uncertainties that affect the determination of the Al_g yields.

Methods. The results of full stellar evolution computations are presented, with special emphasis on the Al_g yields from SAGB stars. We also use a postprocessing nucleosynthesis code to quantify the uncertainties associated with the nuclear reaction rates and with the treatment of convection that modifies the thermodynamical conditions at the base of the convective envelope.

Results. Hot bottom burning leads to individual SAGB Al_g yields that are larger than those from intermediate mass stars, amounting to typical values as high as 5 10^-5 . The overall SAGB contribution remains modest, however, not exceeding ~0.3  of the estimated galactic content of 2.8 . On the other hand, the SAGB ²⁶Al/²⁷Al ratios always exceed 0.01, which is commensurable with the values measured in some SiC grains considered to originate in C-rich AGB stars. However, the isotopic composition of some other elements, particularly nitrogen, is clearly at variance with the observations. We find that the Al_g yields are not affected by the pollution induced by the third dredge-ups, but that they strongly depend on the evolution of the temperature at the base of the convective envelope, the determination of which remains highly dependent on the specific convection model used in the stellar computations. Modifications of T_env by ± 10% leads to variations in the Al_g yields by a factor of 0.2 to 6. In comparison, the nuclear reaction rate uncertainties have less of an impact, altering the yields by less than a factor of 2.