NanoSIMS STUDIES OF SMALL PRESOLAR SiC GRAINS: NEW INSIGHTS INTO SUPERNOVA NUCLEOSYNTHESIS, CHEMISTRY, AND DUST FORMATION

We have studied more than 2000 presolar silicon carbide (SiC) grains from the Murchison CM2 chondrite in the size range 0.2-0.5 mu m for C- and Si-isotopic compositions. In a subset of these grains, we also measured N-, Mg-Al-, S-, and Ca-Ti-isotopic compositions as well as trace element concentrations. The overall picture emerging from the isotope data is quite comparable with that of larger grains, except for the abundances of grains from Type II supernovae (SNeII) and low-metallicity asymptotic giant branch (AGB) stars. Especially, the latter are much more abundant among submicrometer-sized grains than among micrometer-sized grains. This implies that SiC grains from lower-than-solar-metallicity AGB stars are on average smaller than those from solar metallicity AGB stars which provided the majority of presolar SiC grains. We identified five grains with large enrichments in (29)Si (up to 3.5x solar) and (30)Si (up to 3.9x solar in three of these grains). These grains are most likely from SNeII. The isotopically light S ((32)S/(34)S of 2x solar) together with the heavy Si in one of these grains suggests that molecule formation precedes macroscopic mixing and dust formation in SNII ejecta. This adds to the complexity of SN mixing calculations and should be considered in future studies. In total, about 2% of the presolar SiC grains in the size range 0.2-0.5 mu m appear to come from SNeII. This is about a factor of 2 higher than for micrometer-sized grains and suggests that SNeII, on average, produce smaller SiC grains than solar metallicity AGB stars. The high (29)Si/(30)Si ratio in one of the SN grains suggests that current SN models underestimate the (29)Si production in the C- and Ne-burning regions by about a factor of 2. It is shown that with this adjustment the solar (29)Si/(28)Si ratio can be well reproduced in Galactic chemical evolution models and that a merger of our Galaxy with a low-metallicity satellite some 1.5 Gyr before solar system formation could account for the slope 1.3 of the Si mainstream line.