HCO⁺ (black) and H¹³CO⁺ (green) spectral lines extracted over an area with a diameter of 2 × θ_beam toward pronounced bright knot features marked on the velocity channel map at 12.5 km s⁻¹ in Fig. 10. The blue dashed-vertical lines indicate the systemic velocity of DR18, and the red dashed lines have a velocity of 12.5 km s⁻¹. The gray filled areas indicate ±1σ_rms (= 7.15 mJy beam⁻¹) for HCO⁺.