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The formation of massive stars is a highly complex process in which it is notclear whether the star-forming gas is in global gravitational collapse or in anequilibrium state, supported by turbulence. By studying one of the most massiveand 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 tofind 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 ofthe Cygnus X region obtained with the FCRAO and high-angular resolutionobservations of CO, CS, HCO+, N2H+, and H2CO, obtained with the IRAM 30mtelescope. We observe a complex velocity field and velocity dispersion in theDR21 filament in which regions of highest column-density, i.e. dense cores,have a lower velocity dispersion than the surrounding gas and velocitygradients that are not (only) due to rotation. Infall signatures in opticallythick line profiles of HCO+ and 12CO are observed along and across the wholeDR21 filament. From modelling the observed spectra, we obtain a typical infallspeed of 0.6 km/s and mass accretion rates of the order of a few 10^-3 Msun/yrfor the two main clumps constituting the filament. These massive (4900 and 3300Msun) clumps are both gravitationally contracting. All observed kinematicfeatures in the DR21 filament can be explained if it is formed by theconvergence of flows at large scales and is now in a state of globalgravitational collapse. Whether this convergence of flows originated fromself-gravity at larger scales or from other processes can not be settled withthe present study. The observed velocity field and velocity dispersion areconsistent with results from (magneto)-hydrodynamic simulations where the coreslie at the stagnation points of convergent turbulent flows.

Dynamic star formation in the massive DR21 filament