  Context: Star formation takes place in giant molecular clouds, resulting in
mass-segregated young stellar clusters composed of Sun-like stars, brown
dwarves, and massive O-type(50-100\msun) stars. Aims: To identify candidate
hub-filament systems (HFS) in the Milky-Way and examine their role in the
formation of the highest mass stars and star clusters. Methods: Filaments
around ~35000 HiGAL clumps that are detected using the DisPerSE algorithm. Hub
is defined as a junction of three or more filaments. Column density maps were
masked by the filament skeletons and averaged for HFS and non-HFS samples to
compute the radial profile along the filaments into the clumps. Results:
~3700~(11\%) are candidate HFS of which, ~2150~(60\%) are pre-stellar,
~1400~(40\%) are proto-stellar. All clumps with L>10^4 Lsun and L>10^5 Lsun at
distances respectively within 2kpc and 5kpc are located in the hubs of HFS. The
column-densities of hubs are found to be enhanced by a factor of ~2
(pre-stellar sources) up to ~10 (proto-stellar sources). Conclusions: All
high-mass stars preferentially form in the density enhanced hubs of HFS. This
amplification can drive the observed longitudinal flows along filaments
providing further mass accretion. Radiation pressure and feedback can escape
into the inter-filamentary voids. We propose a "filaments to clusters" unified
paradigm for star formation, with the following salient features: a)
low-intermediate mass stars form in the filaments slowly (10^6yr) and massive
stars quickly (10^5yr) in the hub, b) the initial mass function is the sum of
stars continuously created in the HFS with all massive stars formed in the hub,
c) Feedback dissiption and mass segregation arise naturally due to HFS
properties, and c) explain age spreads within bound clusters and formation of
isolated OB associations.
