Heliocentric distance versus the FWHM of (a) N₂H⁺ and (b) N₂D⁺. Orange and magenta lines represent the median FWHM values of N₂H⁺ and N₂H⁺, respectively, within the quiescent (solid), the protostellar (dotted) and the YSO (dashed) groups. Panel (c) compares the FWHMs of the two molecular tracers, with colours indicating the different evolutionary stages (dark blue: quiescent; green: protostellar; yellow: YSO). The uncertainties are shown as black bars.