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Precise determinations of the chemical composition in early B-type starsconsitute fundamental observational constraints on stellar and galactochemicalevolution. Carbon is one of the most abundant metals in the Universe butanalyses in early-type stars show inconclusive results, like largediscrepancies between analyses of different lines in C II, a failure toestablish the C II/III ionization balance and the derivation of systematicallylower abundances than from other objects. We present a comprehensive and robustC II/III/IV model for non-LTE line-formation calculations based on carefullyselected atomic data. The model is calibrated with high-S/N spectra of sixapparently slow-rotating early B-type dwarfs and giants, which cover a wideparameter range and are randomly distributed in the solar neighbourhood. Aself-consistent quantitative spectrum analysis is performed using an extensiveiteration scheme to determine stellar atmospheric parameters and to select theappropriate atomic data used for the derivation of chemical abundances. Weestablish the carbon ionization balance for all sample stars based on a uniqueset of input atomic data, achieving consistency for all modelled lines. Highlyaccurate atmospheric parameters and a homogeneous carbon abundance with reducedsystematic errors are derived. This results in a present-day stellar carbonabundance in the solar neighbourhood, which is in good agreement with recentdeterminations of the solar value and with the gas-phase abundance of the OrionH II region. The homogeneous present-day carbon abundance also conforms withpredictions of chemical-evolution models for the Galaxy. The present approachallows us to constrain the effects of systematic errors on fundamentalparameters and abundances. (abridged)

Carbon abundances of early B-type stars in the solar vicinity. Non-LTE line-formation for C II/III/IV and self-consistent atmospheric parameters