CNO behaviour in planet-harbouring stars. I Nitrogen abundances in stars with planets

Context. Carbon, nitrogen, and oxygen (CNO) are key elements in stellar formation and evolution, and their abundances should also have a significant impact on planetary formation and evolution. Aims. We present a detailed spectroscopic analysis of 74 solar-type stars, 42 of which are known to harbour planets. We determine the nitrogen abundances of these stars and investigate a possible connection between N and the presence of planetary companions. Methods. We used VLT/UVES to obtain high-resolution near-UV spectra of our targets. Spectral synthesis of the NH band at 3360 A was performed with the spectral synthesis codes MOOG and FITTING. Results. We identify several spectral windows from which accurate N abundance can be obtained. Nitrogen distributions for stars with and without planets show that planet hosts are nitrogen-rich when compared to single stars. However, given the linear trend between [N/Fe] vs. [Fe/H], this fact can be explained as being due to the metal-rich nature of planet hosts. Conclusions. We conclude that reliable N abundances can be derived for metal-rich solar type stars from the near UV molecular band at 3360 A. We confirm a linear trend between [N/Fe] and metallicity expected from standard models of Galactic chemical evolution.