Dynamic star formation in the massive DR21 filament
  The formation of massive stars is a highly complex process in which it is not
clear whether the star-forming gas is in global gravitational collapse or in an
equilibrium state, supported by turbulence. By studying one of the most massive
and dense star-forming regions in the Galaxy at a distance of less than 3 kpc,
the filament containing the well-known sources DR21 and DR21(OH), we expect to
find observational signatures that allow to discriminate between the two views.
We use molecular line data from our 13CO 1-0, CS 2-1, and N2H+ 1-0 survey of
the Cygnus X region obtained with the FCRAO and high-angular resolution
observations of CO, CS, HCO+, N2H+, and H2CO, obtained with the IRAM 30m
telescope. We observe a complex velocity field and velocity dispersion in the
DR21 filament in which regions of highest column-density, i.e. dense cores,
have a lower velocity dispersion than the surrounding gas and velocity
gradients that are not (only) due to rotation. Infall signatures in optically
thick line profiles of HCO+ and 12CO are observed along and across the whole
DR21 filament. From modelling the observed spectra, we obtain a typical infall
speed of 0.6 km/s and mass accretion rates of the order of a few 10^-3 Msun/yr
for the two main clumps constituting the filament. These massive (4900 and 3300
Msun) clumps are both gravitationally contracting. All observed kinematic
features in the DR21 filament can be explained if it is formed by the
convergence of flows at large scales and is now in a state of global
gravitational collapse. Whether this convergence of flows originated from
self-gravity at larger scales or from other processes can not be settled with
the present study. The observed velocity field and velocity dispersion are
consistent with results from (magneto)-hydrodynamic simulations where the cores
lie at the stagnation points of convergent turbulent flows.
