2007A&A...464..983P

The earliest phases of massive star formation in clusters are still poorly understood. Here, we test the hypothesis for high-mass star formation proposed in our earlier paper (Peretto et al., 2006A&A...445..979P) stating that a massive, ultra-dense core may be currently forming at the center of the collapsing NGC 2264-C protocluster via the gravitational coalescence of several intermediate-mass Class 0 objects. In order to confirm the physical validity of this hypothesis, we carried out IRAM Plateau de Bure interferometer observations of NGC 2264-C and performed SPH numerical simulations of the collapse of a Jeans-unstable, prolate dense clump. A detailed comparison between these hydrodynamic simulations and both our earlier IRAM 30m observations and the new interferometer observations is presented. Our Plateau de Bure observations provide evidence for disk emission in three of the six Class 0-like objects identified earlier with the 30m in the NGC 2264-C clump. Furthermore, they reveal the presence of a new compact source (C-MM13) located only ∼10000AU away, but separated by ∼1.1km/s in (projected) velocity, from the most massive Class 0 object (C-MM3) lying at the very center of NGC 2264-C. Detailed comparison with our numerical SPH simulations supports the view that NGC 2264-C is an elongated cluster-forming clump in the process of collapsing and fragmenting along its long axis, leading to a strong dynamical interaction and possible protostar merger in the central region of the clump. The marked velocity difference observed between the two central objects C-MM3 and C-MM13, which can be reproduced in the simulations, is interpreted as an observational signature of this dynamical interaction. The present study also sets several quantitative constraints on the initial conditions of large-scale collapse in NGC 2264-C. Our hydrodynamic simulations indicate that the observed velocity pattern characterizes an early phase of protocluster collapse which survives for an only short period of time (i.e., ≤1x10⁵yr). To provide a good match to the observations the simulations require an initial ratio of turbulent to gravitational energy of only ∼5%, which strongly suggests that the NGC 2264-C clump is structured primarily by gravity rather than turbulence. The required ``cold'' initial conditions may result from rapid compression by an external trigger. We speculate that NGC 2264-C is not an isolated case but may point to key features of the initial phases of high-mass star formation in protoclusters.