¹³CO and HCO⁺ spectra observed toward DR21, same as those shown in figure 2a. There is a dip in the HCO⁺ spectrum at the velocity of the 9 km s⁻¹ cloud. The HCO⁺ spectrum is scaled up by a factor of 2, and is offset by −20 K. V_char is the characteristic velocity of the outflow defined as the maximum velocity separation of the high-velocity wing(s) from the systemic velocity of DR21 (about −2.4 km s⁻¹, see the radial velocities of ¹³CO and C¹⁸O in table 2).