SIC PARTICLES FROM ASYMPTOTIC GIANT BRANCH STARS - MG BURNING AND THE S-PROCESS

We address whether isotopically anomalous SiC particles found in meteorites originate in AGB stars. We show that if the peak helium shell flash temperatures of massive (6-9 M.) AGB stars are about 10% larger than they are normally assumed to be, alpha particle reactions with the magnesium will become significant. Then the Mg-26(alpha, n)Si-29 reaction produces a large excess of Si-29. With a light element nuclear reaction network, we calculate the evolution of the silicon isotopic composition during AGB evolution and find that the experimentally determined (slope = 1.4) correlation between excess Si-29 and excess Si-30 in SiC particles from carbonaceous chondrites can indeed be naturally produced in this way. This demonstration contrasts with previous negative conclusions for temperatures too low for magnesium burning to contribute. We are therefore led to suggest that if the large isotopically anomalous SiC particles carrying nearly pure s-process krypton and xenon do indeed originate in AGB stars, those stars were massive and have peak shell flash temperatures near 450 MK. We also demonstrate that this suggestion may answer some interesting questions related to the sizes of the meteoritic SiC particles.