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 C- and 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 C- and 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 C- and 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.