Logarithmic plots of column densities. The error bars represent 20% uncertainties in N. Top: N(CN) vs. N(CH). The solid line presents our best fit. Bottom: N(CH⁺‐like CH) vs. N(CH⁺). The solid line is the best fit to components with ﹩\mathrm{log}\,N( \mathrm{CH}\,^{+}) \leq 12.6﹩ (see text for more detail). Here, and elsewhere, components toward Cep OB2 (asterisks), Cep OB3 (diamonds), and ρ Oph (triangles) are distinguished.