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We investigate the formation of CIII 4647-51-50 and CIII 5696 in theatmosphere of O stars to see if they can be reliably used for abundancedeterminations. We use atmosphere models computed with the code CMFGEN. The keyphysical ingredients explaining the formation of the CIII lines are extractedfrom comparisons of models with different stellar parameters and throughexamining rates controlling the level populations. The strength of CIII 5696critically depends on UV CIII lines at 386, 574 and 884 A. These lines controlthe CIII 5696 upper and lower level population. CIII 884 plays a key role inlate O stars where it drains the lower level of CIII 5696. CIII 386 and CIII574 are more important at early spectral types. The overlap of these UV lineswith FeIV 386.262, FeIV 574.232 and SV 884.531 influences the radiativetransfer at 386, 574 and 884 A, and consequently affects the strength of CIII5696. CIII 4650 is mainly controlled by the CIII 538 line which acts as a drainon its lower level. FeIV 538.057 interacts with CIII 538 and has an impact onthe CIII 4650 profile. Low temperature dielectronic recombinations have anegligible effect on the line profiles. Given our current understanding of thestellar and wind properties of O stars, and in view of the present results, thedetermination of accurate carbon abundances from CIII 4647-51-50 and CIII 5696is an extremely challenging task. Uncertainties lower than a factor of two onC/H determinations based only on these two sets of lines should be regarder ashighly doubtful. Our results provide a possible explanation of the variabilityof CIII 4650 in Of?p stars.

On the formation of CIII 4647-50-51 and CIII 5696 in O star atmospheres