Probing the formation of intermediate- to high-mass stars in protoclusters: A detailed millimeter study of the NGC 2264 clumps

We present the results of dust continuum and molecular line observations of two massive cluster-forming clumps, NGC 2264-C and NGC 2264-D, including extensive mapping performed with the MAMBO bolometer array and the HERA heterodyne array on the IRAM 30m telescope. Both NGC 2264 clumps are located in the Mon OB1 giant molecular cloud complex, adjacent to one another. Twelve and fifteen compact millimeter continuum sources (i.e. MMSs) are identified in clumps C and D, respectively. Evidence for widespread infall motions is found in, e.g., HCO+(3-2) or CS(3-2) in both NGC 2264-C and NGC 2264-D. A sharp velocity discontinuity ~ 2 km/s in amplitude is observed in N_2H+(1-0) and H^{13}CO+(1-0) in the central, innermost part of NGC 2264-C, which we interpret as the signature of a strong dynamical interaction between two MMSs and their possible merging with the central MMS C-MM3. Radiative transfer modelling supports the idea that NGC 2264-C is a highly unstable prolate clump in the process of collapsing along its long axis on a near free-fall dynamical timescale ~ 1.7x10^5 yr. Our model fit of this large-scale collapse suggests a maximum mass inflow rate ~ 3x10^{-3} Msun/yr toward the central protostellar object C-MM3. Such infall rates are sufficiently high to overcome radiation pressure and allow the formation of ~ 20 Msun stars by accretion in ~ 1.7x10^5 yr, i.e., a time similar to the global dynamical timescale of the central part of NGC 2264-C. We conclude that we are likely witnessing the formation of a high-mass (> 10 Msun) protostar in the central part of NGC 2264-C. Our results suggest a picture of massive star formation intermediate between the scenario of stellar mergers of Bonnell et al. (1998) and the massive turbulent core model of McKee & Tan (2003).