2003ApJ...596..344C

The DR 21 massive star-forming region contains two cometary H II regions, aligned nearly perpendicular to each other on the sky. This offers a unique opportunity to discriminate among models of cometary H II regions. We present hydrogen recombination and ammonia line observations of DR 21 made with the Very Large Array. The velocity of the molecular gas, measured from NH₃ emission and absorption lines, is constant to within ±1 km/s across the region. However, the radial velocity of the ionized material, measured from hydrogen recombination lines, differs by ~9 km/s between the ``heads'' of the two cometary H II regions and by up to ~7 km/s from that of the molecular gas. These findings indicate a supersonic velocity difference between the compact heads of the cometary regions and between each head and the ambient molecular material. This suggests that the observed cometary morphologies are created largely by the motion of wind-blowing, ionizing stars through the molecular cloud, as in a bow shock model.