HIGH-VELOCITY HCO+ EMISSION ASSOCIATED WITH THE DR-21 MOLECULAR OUTFLOW

We have used the Hat Creek millimeter interferometer to study the spatial and velocity distribution of HCO+ J = 1-0 line emission from the DR 21 young stellar outflow. The HCO+ emission appears to arise from two spatially distinct components: (1) low-velocity clumps bordering the central DR 21 compact H II region, and (2) extended high-velocity gas associated with the DR 21 outflow lobes. The low-velocity emission probably arises from preexisting clumps of ambient gas that have been swept up and compressed by the slowly expanding compact H II region. The high-velocity HCO+ emission associated with the outflow lobes exhibits a remarkable spatial correlation with the distribution of shock-excited H-2 line emission and is most likely formed by the compression and acceleration of ambient gas on interaction with a powerful young stellar wind. We argue that the observed spatial correlation between the HCO+ and H-2 line emission results from two interrelated effects: (1) a small enhancement in the fractional abundance of HCO+ in the shocked gas, and (2) the more favorable conditions for excitation of the HCO+ ion in the warm, dense gas that comprises the outflow lobes. The velocity field internal to the western outflow lobe suggests that collisions between the outflow and dense clumps in the ambient cloud medium play an important role in orchestrating the dynamical evolution of this particular outflow lobe. We estimate remarkably large values for both the mass (1100 --> 2670 M.) and kinetic energy (1 --> 2 x 10(48) ergs) of the DR 21 outflow, in good agreement with earlier estimates derived from measurements of CO line emission. Our observations therefore suggest that the DR 21 outflow source is one of the largest, most massive and energetic young stellar outflows discovered to date.