  Although most young massive stars appear to be part of multiple systems, it
is poorly understood how this multiplicity influences the formation of massive
stars. The high-mass star-forming region W3 IRS5 is a prime example of a young
massive cluster where the cluster center is resolved into multiple subsources
at cm and infrared wavelengths, a potential proto-Trapezium system. The region
W3 IRS5 was mapped with the PdBI at 1.4mm and 3.4mm in the AB configurations,
observing shock-tracing SiO and SO_2 emission. In the continuum we detect five
sources, one of them for the first time, while counterparts were detected in
the NIR, MIR or at radio wavelengths for the remaining four sources. Three of
the detected sources are within the inner 2100AU, where the protostellar number
density exceeds 10^6 protostars pc^-3 assuming spherical symmetry. Lower limits
for the circumstellar masses of the detected sources were calculated, although
they were strongly affected by the spatial filtering of the interferometer.
However, the projected separations of the sources ranging between ~750 and
~4700AU indicate a multiple, Trapezium-like system. We detected five molecular
outflows in SiO, two of them nearly in the line of sight direction, which
allowed us to see the collapsing protostars in the NIR through the cavities
carved by the outflows. The SO_2 velocity structure indicates a rotating, bound
system, and we find tentative signatures of converging flows as predicted by
the gravoturbulent star formation and converging flow theories. The obtained
data strongly indicate that the clustered environment has a major influence on
the formation of high-mass stars; however, our data do not clearly allow us to
distinguish whether the ongoing star-forming process follows a monolithic
collapse or a competitive accretion mechanism.
