Astronomy and Astrophysics, volume 489, 395-402 (2008/10-1)
Production of 26Al by super-AGB stars.
SIESS L. and ARNOULD M.
Abstract (from CDS):
Super AGB (SAGB) stars have initial masses ranging between ∼7-11M☉ and develop efficient hydrogen burning at the base of their convective envelope during their AGB evolution, leading to a substantial production of 26Alg. We present the first discussion of the contribution of the SAGB stars to the galactic 26Alg production, and we estimate the main uncertainties that affect the determination of the 26Alg yields. The results of full stellar evolution computations are presented, with special emphasis on the 26Alg 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. Hot bottom burning leads to individual SAGB 26Alg yields that are larger than those from intermediate mass stars, amounting to typical values as high as 5x10–5M☉. The overall SAGB contribution remains modest, however, not exceeding ∼0.3M☉ of the estimated galactic content of 2.8M☉. 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 26Alg 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 Tenv by ±10% leads to variations in the 26Alg 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.