High-velocity HCO(+) emission associated with the DR 21 molecular outflow

Abstract

The spatial and velocity distribution of HCO(+) J = 1-0 line emission from the DR 21 young stellar outflow is investigated with the Hat Creek millimeter interferometer. It is argued that the HCO(+) emission arises from two spatially distinct components: low-velocity clumps bordering the central DR 21 compact H II region, and extended high-velocity gas associated with the DR 21 outflow lobes. The high-velocity HCO(+) emission associated with the outflow lobes exhibits a remarkable spatial correlation with the distribution of shock-excited H2 line emission and is most likely formed by the compression and acceleration of ambient gas on interaction with a powerful young stellar wind. It is argued that the observed spatial correlation between HCO(+) and H2 line emission results from two interrelated effects: a small enhancement in the fractional abundance of HCO(+) in the shocked gas, and the more favorable conditions for excitation of the HCO(+) ion in the warm dense gas that comprises the outflow lobes. It is suggested that the DR 21 outflow source is one of the largest, most massive and energetic young stellar outflows discovered to date.