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 ALMA 12 m and ACA we surveyed 6 infrared-dark hubs
in 2.9mm continuum at $\sim$3" 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 characterise the size, mass, morphology, and
infrared brightness of the clumps using Herschel and Spitzer data. Within the 6
newly observed hubs, we identify 67 cores, and find that the MMCs have masses
between 15-911 $\mathrm{M}_{\odot}$ within a radius of 0.018-0.156 pc. The MMC
of each hub contains 3-24% of the clump mass ($f_\mathrm{MMC}$), becoming 5-36%
once core masses are normalised to the median core radius. Across the 35
clumps, we find no significant difference in the median $f_\mathrm{MMC}$ values
of hub and non-hub systems, likely the consequence of a sample bias. However,
we find that $f_\mathrm{MMC}$ is $\sim$7.9 times larger for infrared-dark
clumps compared to infrared-bright ones. This factor increases up to $\sim$14.5
when comparing our sample of 6 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.
