Density effects on C-like, B-like, and Be-like Fe. Panel a: normalized population of the three low-lying excited levels of C-like, B-like, and Be-like Fe, as a function of electron density. The population of the ground states is not plotted. The density effect on the model spectra for the C-like, B-like, and Be-like Fe is shown in panels b, c, and d, respectively. The spectra of low density (1 cm⁻³) and intermediate density (10¹⁴ cm⁻³) are plotted in black and red. The temperatures are the same as in Fig. 7.