Carbon and oxygen abundances in stellar populations^⋆,⋆⋆

¹ Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, 100012 Beijing, PR China
² Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
e-mail: pen@phys.au.dk
³ Instituto de Astronomía, Universidad Nacional Autónoma de México, AP 70-264, 04510 México DF, Mexico
⁴ Observatorio Astronómico Nacional, Universidad Nacional Autónoma de México, Apartado Postal 877, CP 22800 Ensenada, B.C., Mexico

Received: 12 May 2014
Accepted: 17 June 2014

Abstract

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, O, and Fe are determined for F and G main-sequence stars 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 O abundances in stellar atmospheres and the occurrence of planets.

Methods. Carbon abundances are determined from the λλ 5052,5380 C i lines and oxygen abundances from the λ7774 O i triplet and the forbidden [O i] line at 6300 Å. MARCS model atmospheres are applied and non-LTE corrections for the O i 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_⊙) 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] ≃ + 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.