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. ......
[Results] The observations show five emission peaks (SMM1-5). SMM1 and SMM2
contain massive embedded stars (~20 Msun); SMM3-5 are starless or contain
low-mass stars (<8 Msun). The inferred densities are high, >= 10^7 cm^-3, but
the core masses are small, 0.2-0.6 Msun. The detected molecular emission
reveals four different chemical zones. ...... [Conclusions] The proto-Trapezium
cluster W3 IRS5 is an ideal test case to discriminate between models of massive
star formation. Either the massive stars accrete locally from their local
cores; in this case the small core masses imply that W3 IRS5 is at the very end
stages (1000 yr) of infall and accretion, or the stars are accreting from the
global collapse of a massive, cluster forming core. We find that the observed
masses, densities and line widths observed toward W3 IRS 5 and the surrounding
cluster forming core are consistent with the competitive accretion of gas at
rates of Macc~10^-4 Msun yr^-1 by the massive young forming stars. ......
