14. The nickel ions
Lines from nickel ions Ni XIV-XVIII are seen in the SERTS-89 spectrum, although the CHIANTI database presently includes only Ni XV and Ni XVIII. As a general rule, the nickel ion lines show similar behaviour to their iron counterparts, apart from density sensitivity where the nickel line ratios are sensitive to slightly higher density ranges.
14.1. Ni XIV
Two lines are identified in the SERTS-89 spectrum, and we note that they are the iso-electronic analogues to Fe XII 352.11 Å and 364.47 Å which form a density insensitive pair (see Sect. 13.4). If we assume that the atomic data for Ni XIV will be approximately the same as for Fe XII, then we would expect the Ni XIV ratio to also be density insensitive with a value similar to the Fe XII ratio. We thus give the Ni XIV ratio in Table 21, with the theoretical value simply being that of Fe XII. As can be seen, the observed value strongly disagrees with the theoretical value, and indicating that some other ion must be responsible for most of the intensity seen at 302.29 Å.
Table 21. Insensitive ratios for the nickel ions.
14.2. Ni XV
Three lines are identified in the spectrum and two are blended. From the analysis of the Mg VIII lines in Sect. 9.2, we can estimate a Ni XV contribution of 31 erg cm-2 s-1 sr-1 to the 311.79 Å line. The 311.79/298.12 ratio is strongly density sensitive, providing a density value of 10 cm-3.
The contribution of Ni XV to the blend at 319.02 Å is difficult to estimate as the Ni XV line is density sensitive relative to both the 298.12 Å and 311.79 Å lines. The 319.02/311.79 ratio is least sensitive and at the predicted density of 10 cm-3 it takes the value 0.8, allowing us to deduce contributions of 25 erg cm-2 s-1 sr-1 and 51 erg cm-2 s-1 sr-1 to the 319.02 Å line from Ni XV and Mg VII respectively. The Mg VII value is used in Sect. 10.3.
The three lines seen are analogous to the 348.18 Å, 359.64 Å and 368.16 Å lines of Fe XIII.
14.3. Ni XVIII
Three lines are identified for Ni XVIII and they are all density insensitive relative to each other. We give comparisons with theory in Table 21. A large discrepancy can be seen for the 236.34/291.99 ratio suggesting another ion must be responsible for the 236.34 Å line.
Table 22. Density sensitive ratios for the nickel ions
© European Southern Observatory (ESO) 1998
Online publication: November 24, 1997