An Unusual Presolar Silicon Carbide Grain from a Supernova: Implications for the Production of Silicon-29 in Type Ii Supernovae

We report the discovery of a presolar SiC grain (KJB2-11-17-1) with unusual Si-isotopic composition. The grain has 29Si/28Si = 1.63 × solar, 30Si/28Si = 0.82 × solar, 12C/13C = 265 ( = 3 × solar), and evidence for the presence of radiogenic 44Ca from the decay of 44Ti. A comparison of these isotopic signatures with stellar models suggests an origin in a 15 M Type II supernova. It is possible to achieve a very good match between the 30Si/28Si, 12C/13C, and inferred 44Ti/48Ti ratios in KJB2-11-17-1 and the model predictions if matter from different supernova zones is mixed in appropriate proportions. The 29Si/28Si ratio, however, cannot be reproduced and is clearly higher than predicted. It was suggested previously by Travaglio et al. that supernova models underestimate the 29Si yield in the Cand Ne-burning regions by about a factor of 2. Because of its very high 29Si/30Si of two times the solar ratio, grain KJB2-11-17-1 provides the opportunity to make a stringent test of this hypothesis. With a twofold enhanced 29Si yield in the Cand Ne-burning zones, we find a perfect match for 29Si/28Si between the model predictions and the grain. Nuclear network calculations show that a twofold increase in the 29Si yield in the Cand Ne-burning regions requires roughly a threefold higher 26Mg(α, n)29Si reaction rate, the most important reaction for the production of 29Si, in the temperature range 1–3 × 109 K than currently used in supernova models. This increase is qualitatively within current uncertainties of this reaction rate.