4 Practical use of the diagnostics

The physical parameters which could be inferred are numerous:

- Firstly, we can determine, which ionization processes occur in the medium, i.e. whether photoionization dominates or if an additional process competes (such as a collisional one). Indeed, in the case of a pure photoionized plasma, the intensity of the resonance line w, is weak compared to those of the intercombination x+y and forbidden z lines. On the contrary, if there is a strong w line, this means that collisional processes are not negligible and may even dominate. This combined with the relative intensity of the K$_{\alpha}$ line (H-like) can give an estimate of the ratio of the ionic abundance of H-like/He-like and according to Fig. 7, this can also give an indication of the electronic temperature $T_{\rm e}$ in the case of a hybrid plasma, since G is sensitive to $T_{\rm e}$. Figure 10 gives the temperature range where G is insensitive to $X_{{\rm ion}}$ and $T_{\rm e}$ for pure photoionized plasmas.

- Next, density diagnostics can be used. The ratio R=z/(x+y) changes rapidly over approximatively two decades of density, around the critical value, which is different for each He-like ion (see Fig. 10). In this narrow density range, when the density increases the 1s2s ³S₁ level (metastable) is depopulated by electron impact excitation to the 1s2p ³P_0,1,2 levels which imply that the intensity of the forbidden z line decreases while the intensity of the intercombination x+y lines increases (see Fig. 11). Outside this range, at the low density limit (intense z and a constant R value), R gives an upper limit for the value of the gas density producing the He-like ion. At higher densities (the forbidden z line disappears since the density value is greater than the critical density and hence R tends to zero), R gives a lower density limit. Thus if the physical parameters deduced from each He-like ion do not correspond, this could be the signature of stratification of the WA. - Once the density is determined from the ratio R, an estimate of the size of the medium ($\Delta r$) becomes possible, since $N_{{\rm H}}=n_{{\rm H}}~\Delta r$, where $N_{{\rm H}}$ is the column density of the WA.
- In addition, the distance r of the medium from the central ionizing source could be deduced, since the density and the distance are related by the "ionization parameter'' $\xi=L/n_{{\rm H}}\,r^{2}$. Note that the determination of $\xi$ is dependant of the shape of the incident continuum.