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), which is 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 ∼0.1 pc with respect to the center of the clump and is characterized by broad (>10 km s^-1) lines and abundances of ∼1.4 × 10^-8 with respect to H₂. Interestingly, SiO appears to be relatively depleted (χ_SiO ∼ 4 × 10^-9) within this radius, despite it being affected by molecular outflow activity. We attribute this to the fast condensation of SiO back onto dust grains and/or rapid gas-phase destruction of SiO, which is favored by the high density present in this area (>10⁶ cm^-3). Finally, we identify a peripheral, narrow-line (∼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 to understand the role of SiO as a chemical tracer of past star formation episodes in massive protoclusters.