Global collapse of molecular clouds as a formation mechanism for the most massive stars

¹ Laboratoire AIM, CEA/DSM-CNRS-Universté Paris Diderot, IRFU/Service d’Astrophysique, C.E. Saclay France
² School of Physics and Astronomy, Cardiff University, Queens Buildings, The Parade, Cardiff CF24 3AA, UK
e-mail: Nicolas.Peretto@astro.cf.ac.uk
³ Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, University of Manchester, Manchester, M13 9PL, UK
⁴ UK ALMA Regional Centre node  
⁵ Université de Bordeaux, LAB, UMR 5804, 33270 Floirac, France
⁶ CNRS, LAB, UMR 5804, 33270 Floirac, France
⁷ European Southern Observatory (ESO), 85748 Garching, Germany
⁸ IFSI, INAF, Area di Recerca di Tor Vergata, via Fosso Cavaliere 100, 00133 Roma, Italy

Received: 19 February 2013
Accepted: 2 June 2013

Abstract

The relative importance of primordial molecular cloud fragmentation versus large-scale accretion still remains to be assessed in the context of massive core/star formation. Studying the kinematics of the dense gas surrounding massive-star progenitors can tell us the extent to which large-scale flow of material impacts the growth in mass of star-forming cores. Here we present a comprehensive dataset of the 5500(±800) M_⊙ infrared dark cloud SDC335.579-0.272 (hereafter SDC335), which exhibits a network of cold, dense, parsec-long filaments. Atacama Large Millimeter Array (ALMA) Cycle 0 observations reveal two massive star-forming cores, MM1 and MM2, sitting at the centre of SDC335 where the filaments intersect. With a gas mass of 545(_-385⁺⁷⁷⁰) M_⊙ contained within a source diameter of 0.05 pc, MM1 is one of the most massive, compact protostellar cores ever observed in the Galaxy. As a whole, SDC335 could potentially form an OB cluster similar to the Trapezium cluster in Orion. ALMA and Mopra single-dish observations of the SDC335 dense gas furthermore reveal that the kinematics of this hub-filament system are consistent with a global collapse of the cloud. These molecular-line data point towards an infall velocity V_inf = 0.7( ± 0.2) km s^-1, and a total mass infall rate Ṁ_inf ≃ 2.5(±1.0) × 10^-3 M_⊙ yr^-1 towards the central pc-size region of SDC335. This infall rate brings 750(±300) M_⊙ of gas to the centre of the cloud per free-fall time (t_ff = 3 × 10⁵ yr). This is enough to double the mass already present in the central pc-size region in 3.5_-1.0^+2.2 × t_ff. These values suggest that the global collapse of SDC335 over the past million year resulted in the formation of an early O-type star progenitor at the centre of the cloud’s gravitational potential well.