The role of SiO as a tracer of past star-formation events: The case of
  the high-mass protocluster NGC 2264-C
  NGC 2264-C is a high-mass protocluster where several star-formation events
are known to have occurred. To investigate whether past protostellar activity
has left a chemical imprint in this region, we mapped it in SiO($J = 2-1$), a
shock tracer, and several other molecular lines with the Nobeyama 45 m
telescope. Our observations show the presence of a complex network of
protostellar outflows. The strongest SiO emission lies beyond a radius of $\sim
0.1$ pc with respect to the center of the clump, and is characterized by broad
($> 10$ km s$^{-1}$) lines and abundances of $\sim 1.4 \times 10^{-8}$ with
respect to H$_2$. Interestingly, SiO appears relatively depleted
($\chi_\mathrm{SiO} \sim 4 \times 10^{-9}$) within this radius, despite it
being affected by molecular outflow activity. We attribute this to fast
condensation of SiO back onto dust grains and/or rapid gas-phase destruction of
SiO, favored by the high density present in this area ($> 10^6$ cm$^{-3}$).
Finally, we identify a peripheral, narrow-line ($\sim 2$ km s$^{-1}$)
component, where SiO has an abundance of a few times 10$^{-11}$. After
considering different options, we conclude that this weak emission may be
tracing protostellar shocks from the star formation episode that preceded the
current one, which have decelerated over time and eventually resulted in SiO
being largely depleted/destroyed. Alternatively, a population of unresolved
low-mass protostars may be responsible for the narrow SiO emission.
High-angular resolution observations are necessary to distinguish between these
two possibilities and thus understand the role of SiO as a chemical tracer of
past star-formation episodes in massive protoclusters.
