Top panels: degree of deuterium fractionation ([N₂D⁺]∕[N₂H⁺]) against o-H₂D⁺ abundance, CO depletion factor, and NH₃-based gas kinetic temperature. Bottom panels: o-H₂D⁺ abundance against CO depletion timescale; CO depletion factor against gas temperature. The arrows pointing left and down indicate upper limits. In the top middle panel, the prestellar core SMM 6 has two data points that represent our two different target positions within the source (see Table 6).