Three examples of infall/outflow signatures of the HCO⁺(1–0) line profile. Red marks both H¹³CO⁺(1–0), which is optically thin, and the systemic velocity of the dense gas. Compared to this the shift of the peak of the HCO⁺(1–0) emission, in black, becomes clear. The panels show infall (middle and bottom), outflow (bottom right), and a case of central self-absorption, where both peaks are equal (top). The profiles show various degrees of self-absorption.