2011ApJ...740L...5C

DR21(OH) is a pc-scale massive, ∼7000 M_☉ clump hosting three massive dense cores (MDCs) at an early stage of their evolution. We present a high angular resolution mosaic, covering ∼70''x100'', with the IRAM Plateau de Bure Interferometer at 3 mm to trace the dust continuum emission and the N₂H⁺ (J = 1-0) and CH₃ CN (J = 5-4) molecular emission. The cold, dense gas traced by the compact emission in N₂H⁺ is associated with the three MDCs and shows several velocity components toward each MDC. These velocity components reveal local shears in the velocity fields which are best interpreted as convergent flows. Moreover, we report the detection of weak extended emission from CH₃ CN at the position of the N₂H⁺ velocity shears. We propose that this extended CH₃ CN emission is tracing warm gas associated with the low-velocity shocks expected at the location of convergence of the flows where velocity shears are observed. This is the first detection of low-velocity shocks associated with small (subparsec) scale convergent flows which are proposed to be at the origin of the densest structures and of the formation of (high-mass) stars. In addition, we propose that MDCs may be active sites of star formation for more than a crossing time as they continuously receive material from larger scale flows as suggested by the global picture of dynamical, gravity-driven evolution of massive clumps which is favored by the present observations.