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Astron. Astrophys. 327, 1230-1241 (1997)

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4. Conclusions

The results of the above discussion are summarized in Fig. 10.

[FIGURE] Fig. 9. Results of the method for Fe XV. See comments in the text
[FIGURE] Fig. 10. a (left) The L-functions versus temperature solutions for each ion. b (right) The density versus temperature solutions

In Fig. 10a, the L-function resulting from the analysis of both density sensitive and density insensitive ions of Sect. 3 is plotted versus the effective temperature for each ion. The L-function is directly related to the d.e.m. as shown in Sect. 2.

All Si ions but SiXI are lower about twice then the Fe lines at the same temperature; this could suggest that the adopted silicon abundance is too high.

In Fig. 10b, the density is plotted versus the effective temperature. For several ions only lower limits are put by the observations. No definite suggestion occurs either in favor of constant density or of constant pressure hypothesis.

Neither a constant density or a constant pressure solution can satisfy all the data; in particular the electron density provided by Ne V is very small, though it is necessary to remind that the Ne V value is due to only one line and that there are no other lines formed at similar temperatures that can provide a density diagnostic. Of further interest is the fact that also Fe XII and Si X give different measurements of electron density though their temperatures [FORMULA] and [FORMULA] are very similar.

Another important result is given by the systematic low intensity of lines whose wavelength is around 430 Å , as seen in Ne VI, Mg VII, Mg VIII and S XIV which causes the L functions of these transitions to be lower than all the other of the same ion. The suggested correction factor is around 2. Of further interest is the fact that we found evidence for problems on both element abundance and ionization equilibrium for Chromium, moreover also S XIII and S XIV do not agree with each other, while Ni XVIII needs a correction of a factor 1.5 to the [FORMULA] of both the observed lines. It is not possible to understand if this correction is due to element abundance or ionization equilibrium problems.

Using the d.e.m. it is also possible to calculate a synthetic spectrum and compare the results with all the observed intensities. The most important conclusions have been confirmed by this procedure: in particular the Silicon abundance adopted in the calculation of the synthetic spectrum (Feldman et al. 1992) is higher than observations indicate by a factor around 3, and there is a discrepancy between the adopted ion abundances of Si VIII, IX, X and the Si XI one.

To summarize the results, we note that the method described in this paper supplies detailed information for:

  • model temperature and density of the source along the line of sight
  • verify the chemical composition assumed to compare observations and theory
  • check the ionization equilibrium values for each ion
  • verify the intensity calibration at different wavelength
  • suggest incorrect line identifications and blending
  • select lines where atomic physics needs to be improved
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© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998