Dynamics of cluster-forming hub-filament systems: The case of the
  high-mass star-forming complex Monoceros R2
  High-mass stars and star clusters commonly form within hub-filament systems.
Monoceros R2, harbors one of the closest such systems, making it an excellent
target for case studies. We investigate the morphology, stability and dynamical
properties of the hub-filament system on basis of 13CO and C18O observations
obtained with the IRAM-30m telescope and H2 column density maps derived from
Herschel dust emission observations. We identified the filamentary network and
characterized the individual filaments as either main (converging into the hub)
or secondary (converging to a main filament) filaments. The main filaments have
line masses of 30-100 Msun/pc and show signs of fragmentation. The secondary
filaments have line masses of 12-60 Msun/pc and show fragmentation only
sporadically. In the context of Ostriker's hydrostatic filament model, the main
filaments are thermally super-critical. If non-thermal motions are included,
most of them are trans-critical. Most of the secondary filaments are roughly
trans-critical regardless of whether non-thermal motions are included or not.
From the main filaments, we estimate a mass accretion rate of 10(-4)-10(-3)
Msun/pc into the hub. The secondary filaments accrete into the main filaments
with a rate of 0.1-0.4x10(-4) Msun/pc. The main filaments extend into the hub.
Their velocity gradients increase towards the hub, suggesting acceleration of
the gas. We estimate that with the observed infall velocity, the mass-doubling
time of the hub is ~2.5 Myr, ten times larger than the free-fall time,
suggesting a dynamically old region. These timescales are comparable with the
chemical age of the HII region. Inside the hub, the main filaments show a ring-
or a spiral-like morphology that exhibits rotation and infall motions. One
possible explanation for the morphology is that gas is falling into the central
cluster following a spiral-like pattern.
