x_e = n(e⁻) /n_H as a function of R_D. Values from the analytical method (G77) in Sect. 5.1 are given at T = 20 K (dashed line). Results from our calculation (see Sect. 5.2) for temperatures between 7 K and 20 K are contained within the solid lines. ζ values are given for n_H = 10⁴ cm^-3, although the model only depends on the ζ/n_H ratio (see text). R_D values or upper limits as derived from observations are indicated by the red symbols; see Sect. 6.1.