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

TL;DR

This paper reevaluates the lifetime of large presolar grains in the interstellar medium, highlighting interstellar shattering as a potential dominant destruction mechanism and discussing implications for grain survival and composition.

Contribution

It introduces the idea that interstellar shattering could be the main process destroying large presolar grains, revising previous lifetime estimates and explaining observed grain compositions.

Findings

Large presolar grains may survive over 10^8 years if shattering dominates.

Most presolar SiC grains have shorter residence times than 1 Gyr, suggesting other destruction mechanisms.

SiC's longer lifetime compared to graphite explains its higher abundance among presolar grains.

Abstract

Some very large (>0.1 um) 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 $\gtrsim 10^8$ yr; in particular, very large SiC grains can survive 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 $\mu$m-sized presolar grains, while the abundance of SiC in the normal interstellar grains is much lower than graphite.