Carbon and oxygen abundances in stellar populations

Context. Carbon and oxygen abundances in stars are important in many fields of astrophysics including nucleosynthesis. stellar structure, evolution of galaxies, and formation of planetary systems, Still, our knowledge of the abundances of these elements in different stellar populations is uncertain because of difficulties in observing and analyzing atomic and molecular lines of C and O. Aims. Abundances of C, 0, and Fe are determined for F and G main sequence saws in the solar neighborhood with metallicities in the range -1.6 < [Fe/H] < +0.4 in order to study trends and possible systematic differences in the C/Fe, O/Fe, and C/O ratios for thin- and thick-disk stars as well as high- and low-alpha halo stars. In addition, we investigate if there is any connection between C and 0 abundances in stellar atmospheres and the occurrence of planets. Methods. Carbon abundances are determined from the lambda lambda 5052,5380 CI lines and oxygen abundances from the lambda 7774 OI triplet and the forbidden [OI] line at 6300 angstrom. MARCS model atmospheres are applied and non-LTE corrections for the OI triplet are included. Results. Systematic differences between high- and low-alpha halo stars and between thin- and thick-disk stars are seen in the trends of [C/Fe] and [O/Fe]. The two halo populations and thick-disk stars show the same trend of [C/O] versus [O/H], whereas the thin-disk stars are shifted to higher [C/O] values. Furthermore, we find some evidence of higher C/O and C/Fe ratios in stars hosting planets than in stars for which no planets have been detected. Conclusions, The results suggest that C and O in both high- and low-alpha halo stars and in thick-disk stars are made mainly in massive (M > 8 M-circle dot) stars, whereas thin-disk stars have an additional carbon contribution from low-mass AGB and massive stars of high metallicity causing a rising trend of the C/O ratio with increasing metallicity. However, at the highest metallicities investigated ([Fe/H] similar or equal to +0.4), C/O does not exceed 0.8, which seems to exclude formation of carbon planets if proto-planetary disks have the same composition as their parent stars.