Indication of the half-power beam width of Herschel/HIFI, represented by black circles: 42′′ at 500 GHz (left) and 19′′ at 1100 GHz (right). The contour map represents HCO⁺ 4–3 emission toward AFGL 2591 measured by the HARP-B instrument at JCMT. The blue- and redshifted wings are shown in solid and dashed contours, respectively, and are defined as in Van der Wiel et al. (2011). The lowest contour and spacing between consecutive contours is 5 K km s^-1 for both components.