We present the analysis of an extensive set of new and literature highquality data concerning Fe, C, N, O, Na, and Mg, exploiting the Teff scaledetermined in Gratton et al. (1996), and the non-LTE abundance correctionscomputed in Gratton et al. (1999). Results obtained with various abundanceindices are discussed and compared. Our non-LTE analysis yields the same Oabundances from both permitted and forbidden lines for stars with Teff>4600 K,in agreement with King (1993), but not with other studies using a lowerTeff-scale for subdwarfs. However we obtain slightly smaller O abundances forthe most luminous metal poor field stars than for fainter stars of similarmetallicities, an effect attributed to inadequacies of the adopted modelatmospheres (Kurucz 1992, with overshooting) for cool stars. We find a nearlyconstant O overundance in metal poor stars ([Fe/H]<-0.8), at a mean value of0.46+-0.02 dex (sigma=0.12, 32 stars), with only a gentle slope with [Fe/H] (~-0.1); this result is different from the steeper slope recently obtained usingOH band in the near UV. If only 'bonafide' unmixed stars are considered, Cabundances scale with Fe ones (i.e. [C/Fe]~ 0) down to [Fe/H]~ -2.5. Due to ouradoption of a different Teff-scale, we do not confirm the slight C excess inthe most metal poor disk dwarfs (-0.8<[Fe/H]<-0.4) found in previous studies.Na abundances scale as Fe ones in the high metallicity regime, while metal-poorstars present a Na underabundance. None of the field stars analyzed belong tothe group of O-poor and Na-rich stars observed in globular clusters. Na isdeficient with respect to Mg in halo and thick disk stars; within thesepopulations, Na deficiency may be a slow function of [Mg/H]. Solar [Na/Mg]ratios are obtained for thin disk stars
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