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