Position-velocity diagram for N₂H⁺(1–0) along the cut marked in Figs. 6b,c from south-east to north-west. The solid line marks the fitted velocity to highlight the shape of the curve. Contours are plotted in steps of 10% of the peak emission. The two dashed lines indicate (a) the velocity gradient attributed to solid body rotation of the core (with a measured slope of 4.3 km s^-1 pc^-1); and (b) the CAD12 N₂D⁺(3−2) gradient of 18 km s^-1 pc^-1 for comparison. See Sect. 5.2.2 for more details.