Ultraviolet spectra of planetary nebulae. IV. The CIII 2297 dielectronic recombination line and dust absorption in the CIV 1549 resonance doublet.

Calculations by Storey confirm, as suggested in Paper I of the present series, that dielectronic recombination via low-lying auto-ionizing states is important for the calculation of the ionization equilibria of carbon ions. These processes also produce various spectrum lines, of which one is C III 2p^{2 1}D → 2s 2p ¹P λ2297. Observations of this line are reported for NGC 7009 and 7662. Using C³⁺ abundances deduced from λ 2297, it is shown that the fluxes in the C IV λ 1549 resonance doublet are attenuated by dust absorption. The C/H abundance ratios deduced from C III λ2297, C III] λ1908 and C II] λ2326 are 4 × 10^-4 for NGC 7009 and NGC 7662.