Si isotopic compositions of presolar silicate grains from red giant stars and supernovae

We have measured the silicon isotopic compositions of 38 presolar silicate grains from the carbonaceous chondrite Acfer 094, which have been previously studied for oxygen isotopes. The goals of this study are to further disentangle stellar sources of the O-18-enriched (Group IV) and the most O-17-enriched (Group I) silicate grains and to put further constraints on the Galactic chemical evolution (GCE) of the Si isotopes. Our results show that the majority (6 out of 8) of the O-18 enriched silicates have enhanced Si-28, in qualitative agreement with the signature of presolar silicon carbide (SiC) X grains from supernovae. Three highly 17O-enriched grains (O-17/O-16 > 3 x 10(-3)) have close to solar Si-29/Si-28 but enhanced Si-30/Si-28, possibly indicating an origin in binary systems. Alternative stellar sources are 3.5-4 M-circle dot asymptotic giant branch (AGB) stars. Si isotopic compositions of the majority of presolar silicates fall along the SiC mainstream line, although most grains plot to the Si-30-poor side of this line. Most presolar silicate grains therefore formed in red giant branch (RGB)/AGB stars of roughly solar metallicity, and incorporated less (or no) Si processed by slow neutron capture reactions ("s-process") than presolar SiC grains, because silicates form before large amounts of C-12 and s-processed material has been dredged up to the surface. The inferred shift of the Si isotopes due to the different dredge-up of matter from the He intershell in C-rich AGB stars is Delta Si-29 = 3 parts per thousand-2 parts per thousand and Delta Si-30 = 17 parts per thousand-30 parts per thousand, which is compatible with predictions for 1.5-3 M-circle dot AGB stars of solar metallicity when C/O > 1.