Correlation between HF and CH column densities towards W49 N, W51 e2, W3 IRS5 and W3(OH) (clockwise from top left). Column densities are for 1 km s ⁻¹ wide velocity intervals (Fig. 2). Ordinate offsets and slopes from the weighted regression (parameters a and b in Eq. (2)) are given in the lower-right corners. The red lines show the corresponding fits, the blue lines a ratio of HF/CH = 0.4 (Shefferet al. 2008; Sonnentrucker et al. 2015). Double-logarithmic scales illustrate the bimodal distribution at low column densities towards W49 N and W51 e2. Black markers indicate the absorption around, or close to, the location of the continuum source (W49 N: υ_lsr < 27km s⁻¹ ; W51 e2: υ_lsr > 30km s⁻¹ ; W3(OH), W3 IRS5: υ_lsr < −30km s⁻¹ ). Green markers indicate: for W49 N, 27 km s^-1 < υ_lsr < 47 km s^-1, for W51 e2, υ_lsr < 30km s⁻¹ , and for W3(OH), W3 IRS5 − 30 km s^-1 < υ_lsr < −15 km s^-1. Remaining velocities are shown in cyan.