2. Theoretical line ratios
The model ion for O IV consisted of the 8 energetically lowest LS states, namely 2s 22p 2P; 2s 2p 2 4P, 2D, 2S, 2P; 2p 3 4S, 2D and 2P, making a total of 15 fine-structure levels. Energies for all of these were obtained from Safronova et al. (1996).
Electron impact excitation rates for transitions in O IV were taken from Zhang et al. (1994). For Einstein A-coefficients, the calculations of Nussbaumer & Storey (1982), Brage et al. (1996) and Dankworth & Trefftz (1978) were adopted for the 2s 22p - 2s 22p , 2s 22p - 2s 2p 2 and 2s 2p 2 - 2p 3 transitions, respectively. As noted by, for example Seaton (1964), excitation by protons may be important for fine-structure transitions. In the present analysis we have used the proton rates of Foster et al. (1996, 1997) for transitions within 2s 22p 2P and 2s 2p 2 4P, respectively.
Using the atomic data discussed above in conjunction with the statistical equilibrium code of Dufton (1977), relative O IV level populations and hence emission line strengths were calculated for a range of electron temperatures and densities. Details of the procedures involved and approximations made may be found in Dufton (1977) and Dufton et al. (1978).
In Fig. 1 we plot the theoretical ratio R = I(1399.8 Å)/I(1401.2 Å) as a function of electron density at the electron temperature of maximum O IV fractional abundance in ionisation equilibrium, log Te = 5.2 (Mazzotta et al. 1998). Given errors of typically 10% in the adopted atomic data (see references above), we would expect the theoretical values of R to be accurate to better than 15%. We note that R is very insensitive to variations in the adopted electron temperature. For example, varying Te by 0.2 dex leads to a 1% change in the theoretical R ratio.
Also shown for comparison in Fig. 1 are the values of R obtained from the CHIANTI database (Landi et al. 1999). An inspection of the figure shows that the current diagnostic calculations are quite similar to those from CHIANTI. We therefore adopt the former in the present analysis, but note that use of the CHIANTI ratios would not significantly affect our results nor discussion.
© European Southern Observatory (ESO) 1999
Online publication: November 16, 1999