6. Hydrogen iso-electronic sequence
6.1. He II
Checking the consistency of the four He II lines seen in the SERTS-89 spectrum is difficult as the line ratios are very sensitive to temperature. Furthermore, the 256.32/303.78 ratio is also strongly density sensitive, and 256.32 Å is a known blend with Si X. However, we can estimate the contribution of Si X to the blend with some confidence (see Sect. 9.4). Even taking that into account, the observed He II 256.32/303.78 ratio of lies close to the low density limit (0.028 at a density of 10 cm-3 ) for a temperature of (the of He II ) - a lower temperature of around is required to give higher, more realistic densities.
The 243.03/303.78 and 237.35/303.78 ratios are insensitive to density and we give comparisons with theory in Table 1. As the temperature sensitivity is large for He II, we only give the theoretical values of the ratios at the of the ion. The agreement of the 243.03/303.78 is excellent, but that of 237.35/303.78 less good, perhaps because the 237.35 Å line lies close to the low wavelength end of the spectrometer.
Table 1. Insensitive ratios for He II.
© European Southern Observatory (ESO) 1998
Online publication: November 24, 1997