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

Context. Two competing models describe the formation of massive stars in objects like the Orion Trapezium. In the turbulent core accretion model, the resulting stellar masses are directly related to the mass distribution of the cloud condensations. In the competitive accretion model, the gravitational potential of the protocluster captures gas from the surrounding cloud for which the individual cluster members compete. Aims. With high resolution submillimeter observations of the structure, kinematics, and chemistry of the proto-Trapezium cluster W3 IRS5, we aim to determine which mode of star formation dominates. Methods. We present 354 GHz Submillimeter Array observations at resolutions of 1″–3″ (1800–5400 AU) of W3 IRS5. The dust continuum traces the compact source structure and masses of the individual cores, while molecular lines of CS, SO, SO2, HCN, H2CS, HNCO, and CH3OH (and isotopologues) reveal the gas kinematics, density, and temperature. Results. The observations show five emission peaks (SMM1–5). SMM1 and SMM2 contain massive embedded stars (~20 M⊙); SMM3–5 are starless or contain low-mass stars (