Ultraviolet spectra of planetary nebulae – I. The abundance of carbon in IC 418

IUE observations of IC 418 give fluxes in the lines C III] λ 1908, C II] λ 2326, [O II] λ 2470 and Mg II λ 2800. Fluxes on the scale of F(Hβ) = 100 are obtained using a computed [O II] branching ratio $$F(\lambda \lambda \,7320,\,30)/F(\lambda \,2470)$$ and an observed ratio $$F(\lambda \lambda \,7320,\,30)/F(H\beta )$$. The deduced carbon abundance is log {C} = 8.85 ± 0.2 compared with log {C} = 8.67 ± 0.1 for the Sun. A critical discussion is given on methods used previously to determine carbon abundances in planetary nebulae.