INTERSTELLAR RESIDENCE TIMES OF PRESOLAR SiC DUST GRAINS FROM THE MURCHISON CARBONACEOUS METEORITE

The time span between the formation of presolar grains in stellar outflows and their incorporation into early solar-system solids is poorly constrained. Knowledge of this time span is essential for a better understanding of the processing of grains in the interstellar medium (ISM) and formation processes of the solar system. Here, we report interstellar residence times of [?]3-1100 Myr for large ([?]5-50 mm) presolar SiC grains, based on their content of He and Ne produced by Galactic cosmic rays. A majority of these grains have interstellar residence times on the order of a few tens up to less than 200 Myr, considerably shorter than theoretical estimates of interstellar dust lifetimes ([?]500 Myr), but long enough to require formation of the majority of the grains before that of the presolar molecular cloud. The age distribution may be explained by starburst activity 1-2 billion years prior to the birth of the Sun. Our findings provide essential "ground truth" to constrain models of interstellar dust lifetimes and destructive processes in the ISM.