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

7. Lithium iso-electronic sequence

The emission spectrum of Lithium-like ions is dominated by the 2s ² S - 2p ² P transitions, the most well-known of which are the C IV 1548 Å and 1550 Å lines. In the SERTS-89 waveband, we find some of the weaker n=2 to n=3 transitions of C IV, together with the aforementioned doublet of the sulphur, argon and calcium ions.

7.1. C IV

Four lines of C IV are identified in the spectrum, all of which are density insensitive relative to each other. There are two lines predicted by CHIANTI at 419.71 Å and 419.52 Å, and related to each other by a branching ratio (the 419.52 Å line is a decay to the P level). Only the former transition is identified, and is found at 419.72 Å with intensity 12.4 erg cm^-2 s^-1 sr^-1. From the branching ratio we can estimate an intensity for the 419.52 Å line of 6 erg cm^-2 s^-1 sr^-1, making it potentially observable, yet no line is found in the spectrum at this wavelength.

The line at 384.17 Å is actually a blend of the reported P - D transition with the P - D transition, although the latter only contributes around 10% of the line intensity. We take three density insensitive ratios relative to this line and display the comparisons with theory in Table 2. Agreement within the error bars is found for all three. Note that the error bars on the theoretical data are due to temperature sensitivity of the ratios. Since the error bars on the observed data are larger than this temperature sensitivity, we can not use the lines to estimate a temperature for the plasma.

Table 2. Insensitive ratios for Li-like ions.

Keenan et al. (1993a) compared the C IV lines observed by SERTS-89 with theory and found the same results as those given here.

7.2. S XIV, Ar XVI, Ca XVIII

Only the S - P lines mentioned previously are seen for these ions, and they show no relative temperature or density sensitivity. Comparisons with observations are presented in Table 2.

Despite the strength of the two S XIV lines, the observed ratio disagrees marginally with theory. This may reflect an inaccuracy in the calibration of the instrument at the longest wavelengths, as is discussed further in Sect. 15.1.

The Ca XVIII ratio is found to be in agreement with the observations, while from the Ar XVI ratio we can estimate a Ar XVI contribution of 3.5 erg cm^-2 s^-1 sr^-1 to the 389.08 Å line's total of 12.8 erg cm^-2 s^-1 sr^-1. The other component is due to Fe XVII and is discussed in Sect. 13.9.

7.3. Summary

Very few lithium-like ion lines are seen in the SERTS-89 waveband and no density diagnostics are available. The observed lines can instead be used as a check on the atomic data adopted by the CHIANTI database. The features we have found are as follows:

• Agreement with theory is reasonable.
• The S XIV 445.66/417.64 ratio shows a mild discrepancy which may be due to calibration errors.

© European Southern Observatory (ESO) 1998

Online publication: November 24, 1997
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