Astron. Astrophys. 329, 291-314 (1998)

12. Oxygen iso-electronic sequence

In terms of the 2s - 2p transitions, the Oxygen-like ions are very simple: the 2s² 2p^{4 3}P - 2s2p^{5 3}P transitions give rise to 6 closely spaced lines which, for the higher ions, show some relative density sensitivity. For the lower ions 2s² 2p^{4 1}D - 2s2p^{5 1}P is usefully density sensitive relative to the ³P - ³P transitions.

12.1. Ne III

The six P - P transitions lie at 488-491 Å and so fall outside the SERTS-89 bandpass. Of the four lines in the spectrum identified as Ne III, two involve transitions that originate from the 2p³ 3s configuration for which electron collisional excitation data are unavailable.

The remaining two transitions are related by branching ratio, as shown in Table 15. Clearly, the predicted Ne III line at 427.84 Å is far too weak to explain the intensity measured at this location, and we suggest this line belongs to another ion.

Table 15. Branching ratios for the O-like ions.

12.2. Mg V

There are six transitions identified, but the 376.63 Å line cannot be the Mg V 2s2p⁵ P - 2p⁶ S transition since it is theoretically predicted to be around five orders of magnitude weaker than the 353.08 Å line. The remaining lines show a very weak relative density sensitivity, and are of no use as diagnostics. Tables 15 and 16 show that there seem to be problems with the 353.08 Å line, even taking into account the large error bars of the data. The intensity of this line seems to be too low compared with theory. The remaining lines are consistent.

Table 16. Insensitive ratios for the O-like ions.

For the 353.29 Å blend with Na VII, we can use Table 15 to estimate the Mg V portion as around 7 erg cm^-2 s^-1 sr^-1 of the total 10 erg cm^-2 s^-1 sr^-1.

Dwivedi et al. (1997) have identified an additional Mg V line at 276.600 Å and we give the fit and transition information in Table 24. This line is the ¹D - ¹P transition mentioned earlier and shows density sensitivity relative to the ³P - ³P transitions. We give in Table 17 the density predicted from the observed 276.60/351.12 ratio.

Table 17. Density sensitive line ratios for the O-like ions.

Another Mg V line identified by Dwivedi et al. (1997) is at 352.213 Å and the details of the fit and transition are given in Table 24. This line is insensitive relative to the 351.12 Å line, but the observed intensity is lower than expected as can be seen in Table 16.

12.3. Si VII

Only one transition of the ³P - ³P multiplet is seen at 275.38 Å, but another five weaker lines are expected in the 270-280 Å region. Two of these lines are predicted to provide components to observed lines at 276.85 Å and 278.41 Å (identified as Si VIII and Mg VII lines, respectively), and the Si VII components can be estimated from the 275.38 Å intensity of 105 erg cm^-2 s^-1 sr^-1 using the branching ratio and density insensitive ratio provided in Tables 15 and 16.

Of the remaining three transitions, the line expected at 274.17 Å is density dependent below 10 cm^-3 relative to the 275.38 Å line, but unfortunately it is masked by the far stronger Fe XIV 274.21 Å and contributes no more than 3% to that feature. Dwivedi et al. (1997) give new identifications for the remaining two lines which CHIANTI predicts to be at 272.64 Å and 275.67 Å. The fits to these lines are given in Table 24, where we note that the wavelengths differ significantly from the CHIANTI values. The two lines are related to each other by the branching ratio given in Table 15, but the large error bars on the data make comparisons with theory meaningless. The 272.64/275.35 ratio shows mild density sensitivity, increasing from 0.20 to 0.29 over the density regime 10 - 10 cm^-3, with the observed value being , agreeing reasonably with theory.

12.4. Summary

The O-like ions give rise to weak lines in the EUV, which makes them difficult to use for diagnostic work.

• The 2p - 3s transitions of Ne III can not be modelled at present as no collisional data are available for these transitions.
• The potentially useful Mg V 276.58 Å line is expected to be quite weak, requiring a very high sensitivity instrument for detection. All of the observed Mg V lines are consistent with theory, with the exceptions of 353.08 Å which is weaker than predicted and 376.63 Å which must have been mis-identified.
• Two unidentified Si VII lines are suggested to be blended with observed lines. No density diagnostics are available for this ion.

© European Southern Observatory (ESO) 1998

Online publication: November 24, 1997
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