Cloud-cloud collision as origin of the G31.41+0.31 massive protocluster
  The G31.41+0.31 (G31) hot molecular core (HMC) is a high-mass protocluster
showing accelerated infall and rotational spin-up that is well studied at
high-angular resolution. To complement the accurate view of the small scale in
G31, we have traced the kinematics of the large-scale material by carrying out
N$_2$H$^+$\,(1--0) observations with the IRAM 30m telescope of an area of
$\sim$6$\times6$\,arcmin$^2$ around the HMC. The N$_2$H$^+$ observations have
revealed a large-scale (5\,pc) hub-filament system (HFS) composed by at least
four filamentary arms and a NNE--SSW velocity gradient ($\sim$0.4\,km/s/pc)
between the northern and southern filaments. The linewidth increases towards
the hub at the center of the HFS reaching values of 2.5--3\,km\,s$^{-1}$ in the
central 1\,pc. The origin of the large-scale velocity gradient is likely
cloud-cloud collision. In this scenario, the filaments in G31 would have formed
by compression resulting from the collision and the rotation of the HMC
observed at scales of 1000\,au would have been induced by shear caused by the
cloud-cloud collision at scales of a few pc. We conclude that G31 represents a
HFS in a compressed layer with an orthogonal orientation to the plane of the
sky, and represents a benchmark for the filaments-to-clusters paradigm of star
formation.
