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

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-circle dot 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((+770)(-385))M-circle dot 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. M-inf similar or equal to 2.5(+/- 1.0) x 10(-3) Mfi yr(-1) towards the central pc-size region of SDC335. This infall rate brings 750(+/- 300) Mfi of gas to the centre of the cloud per free-fall time (t(ff) = 3 x 10(5) yr). This is enough to double the mass already present in the central pc-size region in 3.5 (+2:2)(-1:0) x 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.