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 etal. 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.