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

摘要

Some very large (> 0.1 mu m) 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 greater than or similar to 10(8) yr; in particular, very large SiC grains can survive for similar to 1 Gyr. 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 1 Gyr, which may indicate that there is a more efficient mechanism than shattering in destroying VLGs, or that VLGs have larger velocity dispersions than 10 km s(-1). We also argue that the enhanced lifetime of SiC relative to graphite can be the reason why we find SiC among pm-sized presolar grains, while the abundance of SiC in the normal interstellar grains is much lower than graphite. (C) 2016 Elsevier Ltd. All rights reserved.