Context. The formation of massive stars is a highly complexprocess in which it is unclear whether the star-forming gas is inglobal gravitational collapse or an equilibrium state supported byturbulence and/or magnetic fields. In addition, magnetic fields mayplay a decisive role in the star-formation process since they influencethe efficiency of gas infall onto the protostar. Aims. By studying one of the most massive and dense star-formingregions in the Galaxy at a distance of less than 3 kpc, i.e. thefilament containing the well-known sources DR21 and DR21(OH), weattempt to obtain observational evidence to help us to discriminatebetween these two views. Methods. We use molecular line data from our 13CO ,CS ,and N2H+ survey of the Cygnus X region obtained with the FCRAO and high-angularresolution observations in isotopomeric lines of CO, CS, HCO+, N2H+, and H2CO,obtained with the IRAM 30m telescope, to investigate thedistribution of the different phases of molecular gas. Gravitationalinfall is identified by the presence of inverse P Cygni profiles thatare detected in optically thick lines, while the optically thinnerisotopomers are found to reach a peak in the self-absorption gap. Results. We observe a complex velocity field and velocitydispersion in the DR21 filament in which regions of the highestcolumn-density, i.e., dense cores, have a lower velocity dispersionthan the surrounding gas and velocity gradients that are not (only) dueto rotation. Infall signatures in optically thick line profiles of HCO+ and 12COare observed along and across the whole DR21 filament. By modelling theobserved spectra, we obtain a typical infall speed of 0.6kms-1 and mass accretion rates of the order of a few 10-3yr-1 for the two main clumps constituting the filament. These massive clumps (4900 and 3300at densities of around 105cm-3within 1pc diameter) are both gravitationally contracting (withfree-fall times much shorter than sound crossing times and low virialparameter ).The more massive of the clumps, DR21(OH), is connected to asub-filament, apparently ``falling'' onto the clump. This filament runsparallel to the magnetic field. Conclusions. All observed kinematic features in the DR21filament (velocity field, velocity dispersion, and infall), itsfilamentary morphology, and the existence of (a) sub-filament(s) can beexplained if the DR21 filament was formed by the convergence of flowson large scales and is now in a state of global gravitational collapse.Whether this convergence of flows originated from self-gravity onlarger scales or from other processes cannot be determined by thepresent study. The observed velocity field and velocity dispersion areconsistent with results from (magneto)-hydrodynamic simulations wherethe cores lie at the stagnation points of convergent turbulent flows. Key words: ISM: clouds - ISM: individual objects: Cygnus X - ISM: molecules - ISM: kinematics and dynamics - radio lines: ISM - submillimeter: ISM
展开▼
