Revisiting the lifetime estimate of large presolar grains in the interstellar medium

Abstract

Some very large (>0.1μm) presolar grains are sampled in meteorites. We reconsider the lifetime of very large grains (VLGs) in the interstellar medium focusing on interstellar shattering caused by turbulence-induced large velocity dispersions. This path has never been noted as a dominant mechanism of destruction. We show that, if interstellar shattering is the main mechanism of destruction of VLGs, their lifetime is estimated to be ≳108yr; in particular, very large SiC grains can survive for ~1Gyr. The lifetimes obtained for VLGs are comparable to the longest residence time derived for some presolar grains based on the cosmic-ray exposure time. However, most presolar SiC grains show residence times significantly shorter than 1Gyr, which may indicate that there is a more efficient mechanism than shattering in destroying VLGs, or that VLGs have larger velocity dispersions than 10kms−1. We also argue that the enhanced lifetime of SiC relative to graphite can be the reason why we find SiC among μm-sized presolar grains, while the abundance of SiC in the normal interstellar grains is much lower than graphite.