Recombination emission, high density

Figure 15 shows a comparison of the emission spectrum at densities of 10$^4$ cm$^{-3}$ (panel a) and 10$^{17}$ cm$^{-3}$ (panel b) due to oxygen in the 0.5-1 keV energy range. The ionization parameter is log($\xi$)=2 and the temperature is 10$^5$K. In the high density case the ratio of continuum to line emission is reduced, and the ratios of the He-like lines is changed from the familiar low-density case in which the forbidden/intercombination line ratio is large at low density to values $\sim$1 at high density.