The cometary HII regions of DR 21: Bow shocks or champagne flows or
  both?
  We present deep Very Large Array H66$\alpha$ radio recombination line (RRL)
observations of the two cometary HII regions in DR 21. With these sensitive
data, we test the "hybrid" bow shock/champagne flow model previously proposed
for the DR 21 HII regions. The ionized gas down the tail of the southern HII
region is redshifted by up to ~30 km/s with respect to the ambient molecular
gas, as expected in the hybrid scenario. The RRL velocity structure, however,
reveals the presence of two velocity components in both the northern and
southern HII regions. This suggests that the ionized gas is flowing along
cone-like shells, swept-up by stellar winds. The observed velocity structure of
the well-resolved southern HII region is most consistent with a picture that
combines a stellar wind with stellar motion (as in bow shock models) along a
density gradient (as in champagne flow models). The direction of the implied
density gradient is consistent with that suggested by maps of dust continuum
and molecular line emission in the DR 21 region.
