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 ( \pm 800 ) M _ { \odot } 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 _ { \odot } 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 ( \pm 0.2 ) km/s , and a total mass infall rate \dot { M } _ { inf } \simeq 2.5 ( \pm 1.0 ) \times 10 ^ { -3 } M _ { \odot } yr ^ { -1 } towards the central pc-size region of SDC335 .
This infall rate brings 750 ( \pm 300 ) M _ { \odot } of gas to the centre of the cloud per free-fall time ( t _ { ff } = 3 \times 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 } \times 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 .