Filament coalescence and hub structure in MonR2: Implications to massive
  star and cluster formation
  Here we study the MonR2 star forming region, which has a rich network of
filaments joining in a star cluster forming hub, aiming at understanding the
hub structure and to examine the mass fraction residing in the hub and in the
filaments, which is a key factor that influences massive star formation. We
conducted a multi-scale, multi-component analysis of the Herschel column
density maps (resolution of 18.2" or $\sim$0.07 pc at 830 pc) of the region
using a newly developed algorithm "getsf" to identify the structural
components, namely, extended cloud, filaments, and sources. We find that
cascades of lower column density filaments coalesce to form higher density
filaments eventually merging inside the hub (0.8 pc radius). As opposed to the
previous view of the hub as a massive clump with $\sim$1 pc radius, we find it
to be a network of short high-density filaments. The total mass reservoir in
the MonR2 HFS (5 pc $\times$ 5 pc) is split between filaments (54%), extended
cloud (37%) and sources (9%). The M/L of filaments increase from $\sim$ 10
Msun/pc at 1.5pc from the hub to $\sim$ 100 Msun/pc at its centre, while the
number of filaments per annulus of 0.2pc width decreases from 20 to 2 in the
same range. The observed radial column density structure of the HFS (filament
component only) displays a power-law dependence of $N_{\mathrm{H}_2} \propto
r^{-2.17}$ up to a radius of $\sim$2.5 pc from the central hub, resembling a
global collapse of the HFS. We present a scenario where the HFS can be
supported by magnetic fields which interact, merge and reorganize themselves as
the filaments coalesce. In the new view of the hub as a network of high-density
filaments, we suggest that only the stars located in the network can benefit
from the longitudinal flows of gas to become massive, which may explain the
reason for the formation of many low-mass stars in cluster centres.
