An ALMA study of hub-filament systems - I. On the clump mass concentration within the most massive cores

The physical processes behind the transfer of mass from parsec-scale clumps to massive star-forming cores remain elusive. We investigate the relation between the clump morphology and the mass fraction that ends up in its most massive core (MMC) as a function of infrared brightness, i.e. a clump evolutionary tracer. Using Atacama Large Millimeter/submillimeter Array (ALMA) 12 m and Atacama Compact Array, we surveyed six infrared dark hubs in 2.9 mm continuum at ∼3 arcsec resolution. To put our sample into context, we also re-analysed published ALMA data from a sample of 29 high-mass surface density ATLASGAL sources. We characterize the size, mass, morphology, and infrared brightness of the clumps using Herschel and Spitzer data. Within the six newly observed hubs, we identify 67 cores, and find that the MMCs have masses between 15 and 911 M⊙ within a radius of 0.018-0.156 pc. The MMC of each hub contains 3-24 per cent of the clump mass (fMMC), becoming 5-36 per cent once core masses are normalized to the median core radius. Across the 35 clumps, we find no significant difference in the median fMMC values of hub and non-hub systems, likely the consequence of a sample bias. However, we find that fMMC is ∼7.9 times larger for infrared dark clumps compared to infrared bright ones. This factor increases up to ∼14.5 when comparing our sample of six infrared dark hubs to infrared bright clumps. We speculate that hub-filament systems efficiently concentrate mass within their MMC early on during its evolution. As clumps evolve, they grow in mass, but such growth does not lead to the formation of more massive MMCs.