Chemical composition of Galactic OB stars I. CNO abundances in O9 stars

Abstract

We present NLTE abundances of CNO for a sample of four O9 stars in the Galaxy, together with new determinations of their stellar parameters, $T_{\rm eff}$, $\log g$, $ε$(He) and microturbulence. These new analyses take into account the effect of {\it line--blocking} in the spectral synthesis with our classical NLTE, plane--parallel and hydrostatic model atmospheres. The sample includes three O9 He normal stars: two dwarfs, HD 214680 and HD 34078, and one supergiant, HD 209975, and one fast rotating giant with a preliminary high He overabundance, HD 191423 with $ε$(He)=0.20. We find first that the consideration of microturbulence in the spectral synthesis for the fast rotator leads to a considerably lower He abundance, $ε$(He)=0.12. The CNO abundances of the three He normal stars are in good agreement with the values in the literature for Galactic B dwarfs with no evidence of mixing, and show that they all have the same chemical composition. We also discuss however the possible CNO contamination of the supergiant HD 209975. For the fast rotator we find that the abundances show the trend of the CNO contamination: a N overabundance together with C and O depletion. The N/C and N/O ratios of our stars as a function of their projected rotational velocities are consistent with the predictions of the recent evolutionary models of Meynet & Maeder [Mey&Mae00].