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

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: kswang@strw.leidenuniv.nl
² Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
³ SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD, 9712 Groningen, The Netherlands
⁴ Kapteyn Astronomical Institute, University of Groningen, 9712 Groningen, The Netherlands
⁵ Institute of Astronomy, ETH Zürich, 8093 Zürich, Switzerland
⁶ Ritter Observatory, MS-113, University of Toledo, 2801 W. Bancroft St., Toledo, OH 43606, USA
⁷ Naval Research Laboratory, Code 7210, Washington, DC 20375, USA

Received: 14 June 2013
Accepted: 21 August 2013

Abstract

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 (<8 M_⊙). The inferred densities are high, ≥10⁷ cm^-3, but the core masses are small, 0.2−0.6 M_⊙. The detected molecular emission reveals four different chemical zones. Abundant (X ~ few 10^-7 to 10^-6) SO and SO₂ are associated with SMM1 and SMM2, indicating active sulfur chemistry. A low abundance (5 × 10^-8) of CH₃OH concentrated on SMM3/4 suggest the presence of a hot core that is only just turning on, possibly by external feedback from SMM1/2. The gas kinematics are complex with contributions from a near pole-on outflow traced by CS, SO, and HCN; rotation in SO₂, and a jet in vibrationally excited HCN.

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 Ṁ ~ 10^-4M_⊙ yr^-1 by the massive young forming stars. Future mapping of the gas kinematics from large to small scales will determine whether large-scale gas inflow occurs and how the cluster members compete to accrete this material.