Gas dynamics in massive dense cores in Cygnus-X

Context. The physical conditions in massive dense cores (MDCs) that form massive stars and clusters, are not well constrained. Few observations have been made to confront the theories. An extensive study has started of the most massive and youngest cores in the Cygnus-X molecular complex, whose first results have uncovered exceptional fragmentation properties in a sample of five cores where individual massive protostars have been recognized. Aims. We study the kinematic properties of dense gas surrounding massive protostars in these five cores to investigate whether turbulent support plays a major role in stabilizing the whole core against a rapid fragmentation into Jeans-mass objects. The observed kinematics could indicate a high level of dynamics suggesting that the cores are actually not in equilibrium and dynamical processes could be the main driver of the build up of the final stellar masses. Methods. We present IRAM 30m single-dish ((HCO+)-C-13 and HCO+) data and IRAM Plateau de Bure Interferometer high angular-resolution observations of dense gas tracers ((HCO)-C-13+ and (HCN)-C-13) to reveal the kinematics of molecular gas on scales from 0.03 to 0.1 pc. Results. Using radiative transfer modeling, we show that the (HCO+)-C-13 abundance drops within the envelopes of massive protostars and traces the bulk of material surrounding the protostars instead of their inner envelopes. H13CN shows a better correspondence with the peak of the continuum emission, possibly because of abundance anomalies and specific chemistry in the close vicinity of massive protostars. Analyzing the line-widths, we show that the observed line-dispersion of H13CO+ on the scale of MDCs is smaller than expected from the quasi-static, turbulent-core model. On large-scales, global organized bulk motions are identified for three of the MDCs. On small-scales, several spectral components are identified in all MDCs showing filamentary structures and intrinsic velocity gradients across the continuum peaks. The dynamics of these flows show diversity across the sample, which we link to the specific fragmentation properties of the MDCs. Altogether this is indicative of different initial conditions in CygX-N3 and -N63 compared to CygX-N12, -N48 and -N53, which may represent different evolutionary stages. Conclusions. No clear evidence is found of a turbulence-regulated, equilibrium scenario within the sample of MDCs. We propose a picture in which MDCs are not in equilibrium and their dynamics is governed by small-scale converging flows, which may initiate star-formation via their shears. We suggest that dynamical processes are linked to the formation of proto-clusters and high-mass protostars.