Left : map of the difference between the neutral pNH₂D and the ion N₂D⁺ centroid velocity (δV^shift = V^neutral – V^ion). The filled white circle shows the 23″.5 beam size (0.019 pc at the 170 pc distance of L1544) and the white vertical line indicates the 0.05 pc scale. Right : histogram of δ V^shift. The mean and standard deviation of the δV^shift distribution are 0.05 and 0.02 km/s, respectively, where the mean uncertainty on δV^shift is 0.006 km/s (cf. Appendix A).