Convergent flows and low-velocity shocks in DR21(OH)
  DR21(OH) is a pc-scale massive, 7000 Msun clump hosting three massive dense
cores (MDCs) at an early stage of their evolution. We present a high
angular-resolution mosaic, covering 70" by 100", with the IRAM PdBI at 3 mm to
trace the dust continuum emission and the N2H+ (J=1-0) and CH3CN (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 towards
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 CH3CN at the position of the N2H+
velocity shears. We propose that this extended CH3CN 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 (sub-parsec) 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.
