- 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 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 in the case of a hybrid plasma, since G is sensitive to . Figure 10 gives the temperature range where G is insensitive to and 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 3P0,1,2 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 ()
becomes possible, since
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'' . Note that the determination of is dependant of the shape of the incident continuum.
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