Dependence of the [S II] 6716/6730 Å line ratio on the electron number density, n_e, and temperature, T_e. The grey-scale image and contours are the model predictions computed using PyNeb (Luridiana et al. 2015), with the corresponding line ratios denoted beside each contour. The blue (1.35) and red (1.40) contours are the median values measured in HB9 (Fig. 5c) and G159.2+3.3 (Fig. 6c), respectively. For high-density gas with n_e ≳ 10² cm⁻³, the [S II] 6716/6730 Å line ratio is highly density dependant so a good density tracer. On the other hand, at n_e ≲ 10² cm⁻³, [S II] 6716/6730 Å is less sensitive to density, but more affected by temperature.