Infalling streamline model for the HL Tau streamer. Top: Streamline model, plotted in green, on top of the moment-1 map of HCO⁺, spatially matching the observed streamer emission extending to the south (outlined in the green polygon). The model predicts that the streamer should extend beyond the HCO⁺ emission, into a region traced by CS (see Fig. 3). Bottom: Velocity components of HCO⁺ (black) andCS (orange) taken from inside the green polygon plotted as a function of projected distance from the central young stellar object, with varying levels of kernel density estimation of the velocity plotted as filled contours and contours calculated as in Fig. B.1. The streamline model is shown in green, and highlights that both HCO⁺ and CS trace different parts of the same infalling streamer.