Gravity drives the evolution of infrared dark hubs: JVLA observations of
  SDC13
  Converging networks of interstellar filaments i.e. hubs, have been recently
linked to the formation of stellar clusters and massive stars. The goal is to
study the kinematic and density structure of the SDC13 hub at high angular
resolution to determine what drives its evolution and fragmentation. We have
mapped SDC13, a 1000Msun infrared dark hub, in NH3(1,1) and NH3(2,2) emission
lines, with both the JVLA and GBT down to 0.07pc. The mass-per-unit-lengths of
all four hub filaments are thermally super-critical, consistent with the
presence of tens of gravitationally bound cores along them. These cores exhibit
regular separation of 0.37 +/- 0.16 pc suggesting gravitational instabilities
running along these super-critical filaments are responsible for their
fragmentation. The observed local increase of the dense gas velocity dispersion
towards starless cores is believed to be a consequence of such fragmentation
process. We see velocity gradient peaks towards 63% of the cores as expected
during the early stages of filament fragmentation. The most massive cores are
located at the filament junctions, where the velocity dispersion is the
largest. We interpret this as the result of the hub morphology generating the
largest acceleration gradients near the hub centre. We propose a scenario for
the evolution of the SDC13 hub in which filaments first form as post-shock
structures in a supersonic turbulent flow. Then gravity takes over and starts
shaping the evolution of the hub, both fragmenting filaments and pulling the
gas towards the centre of the gravitational well. By doing so, gravitational
energy is converted into kinetic energy in both local (cores) and global (hub
centre) potential well minima. Furthermore, the generation of larger
gravitational acceleration gradients at the filament junctions promotes the
formation of more massive cores. [abridged]
