Where have all the interstellar silicon carbides gone?
  The detection of the 11.3-micron emission feature characteristic of the Si--C
stretch in carbon-rich evolved stars reveals that silicon carbide (SiC) dust
grains are condensed in the outflows of carbon stars. SiC dust could be a
significant constituent of interstellar dust since it is generally believed
that carbon stars inject a considerable amount of dust into the interstellar
medium (ISM). The presence of SiC dust in the ISM is also supported by the
identification of presolar SiC grains of stellar origin in primitive
meteorites. However, the 11.3-micron absorption feature of SiC has never been
seen in the ISM and oxidative destruction of SiC is often invoked. In this work
we quantitatively explore the destruction of interstellar SiC dust through
oxidation based on molecular dynamics simulations and density functional theory
calculations. We find that the reaction of an oxygen atom with SiC molecules
and clusters is exothermic and could cause CO-loss. Nevertheless, even if this
is extrapolable to bulk SiC dust, the destruction rate of SiC dust through
oxidation could still be considerably smaller than the (currently believed)
injection rate from carbon stars. Therefore, the lack of the 11.3-micron
absorption feature of SiC dust in the ISM remains a mystery. A possible
solution may lie in the currently believed stellar injection rate of SiC (which
may have been overestimated) and/or the size of SiC dust (which may actually be
considerably smaller than submicron in size).
