  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. 2006). 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. Our Plateau de Bure
observations reveal the presence of a new compact source (C-MM13) located only
\~ 10000 AU away, but separated by ~ 1.1 km/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 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., < 10^5 yr). 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.
