Homogeneous photospheric parameters and C abundances in G and K nearby stars with and without planets

Aims. We present a determination of photospheric parameters and carbon abundances for a sample of 172 G and K dwarf, subgiant, and giant stars with and without detected planets in the solar neighbourhood. The analysis was based on high signal-to-noise ratio and high resolution spectra observed with the ELODIE spectrograph (Haute Provence Observatory, France) and for which the observational data were publicly available. We intend to contribute precise and homogeneous C abundances in studies that compare the behaviour of light elements in stars with and without planets. This will bring new arguments to the discussion of possible anomalies that have been suggested and will contribute to a better understanding of different planetary formation process. Methods. The photospheric parameters were computed through the excitation potential, equivalent widths, and ionisation equilibrium of iron lines selected in the spectra. Carbon abundances were derived from spectral synthesis applied to prominent molecular head bands of C-2 Swan (lambda 5128 and lambda 5165) and to a C atomic line (lambda 5380.3). Careful attention was drawn to carry out this homogeneous procedure and to compute the internal uncertainties. Results. The distribution of [C/Fe] as a function of [Fe/H] shows no difference in the behaviour of planet-host stars in comparison with stars for which no planet was detected, for both dwarf and giant subsamples. This result agrees with the hypothesis of a primordial origin for the chemical abundances presently observed instead of self-enrichment during the planetary system formation and evolution. Additionally, giant stars are clearly depleted in [C/Fe] (by about 0.14 dex) when compared with dwarfs, which is probably related to evolution-induced mixing of H-burning products in the envelope of evolved stars. Subgiant stars, although in small number, seem to follow the same C abundance distribution as dwarfs. We also analysed the kinematics of the sample stars that in their majority are members of the Galaxy's thin disc. Finally, comparisons with other analogue studies were performed and showed good agreement within the uncertainties.