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[Context] Two competing models describe the formation of massive stars inobjects like the Orion Trapezium. In the turbulent core accretion model, theresulting stellar masses are directly related to the mass distribution of thecloud condensations. In the competitive accretion model, the gravitationalpotential of the protocluster captures gas from the surrounding cloud for whichthe individual cluster members compete. [Aims] With high resolutionsubmillimeter observations of the structure, kinematics, and chemistry of theproto-Trapezium cluster W3 IRS5, we aim to determine which mode of starformation dominates. [Methods] We present 354 GHz Submillimeter Arrayobservations at resolutions of 1"-3" (1800-5400 AU) of W3 IRS5. [Results] The observations show five emission peaks (SMM1-5). SMM1 and SMM2contain massive embedded stars (~20 Msun); SMM3-5 are starless or containlow-mass stars (<8 Msun). The inferred densities are high, >= 10^7 cm^-3, butthe core masses are small, 0.2-0.6 Msun. The detected molecular emissionreveals four different chemical zones.  [Conclusions] The proto-Trapeziumcluster W3 IRS5 is an ideal test case to discriminate between models of massivestar formation. Either the massive stars accrete locally from their localcores; in this case the small core masses imply that W3 IRS5 is at the very endstages (1000 yr) of infall and accretion, or the stars are accreting from theglobal collapse of a massive, cluster forming core. We find that the observedmasses, densities and line widths observed toward W3 IRS 5 and the surroundingcluster forming core are consistent with the competitive accretion of gas atrates of Macc~10^-4 Msun yr^-1 by the massive young forming stars.

Dense molecular cocoons in the massive protocluster W3 IRS5: a test case for models of massive star formation