Dependence of the rotational excitation upon permanent dipole moment μ (Debye) at n(H) = 2n(H₂) = 256    cm^-3, T_K= 35 K and x_e = 1.4 × 10^-4 as before. The horizontal axis is column density per unit velocity and vertical axes are line excitation temperatures (at left) and brightness temperatures (at right) above the cosmic background. The CS-like species at top has the same rotational energy spacing and H₂-excitation rates as CS, but the dipole moment is varied wherever it appears analytically in expressions for the line optical depth, brightness etc. and electron-neutral molecule excitation rates. Panels in the middle row are for the strongest hyperfine component of the J = 1−0 line of a neutral HCN analog, and the lowest two panels in each column pertain to the J = 1−0 line of a molecular ion analog of HCO⁺ (compare with Fig. 1). In each row one series of calculations is shown by red dotted lines; by coincidence those use values very close to the actual dipole moments of CS (1.96 D), HCN (2.98 D) and HCO⁺ (3.89 D). The dotted blue curve in each of the lowest two panels uses the correct dipole moment and the recent electron excitation rates of Faure et al. (2007a).