Deuterated fraction, D_frac= N(N₂D⁺)/N(N₂H⁺), as a function of several parameters: kinetic temperature a), N(N₂H⁺) b), N₂H⁺ (3–2) line width c) and N₂D⁺ (2–1) line width d). The symbols have the same meaning as in Fig. 1. For some sources, the errorbars are not visible because they are smaller than the symbol size. In panel a), only the sources with temperature derived from VLA ammonia observations are plotted.