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Photospheric carbon and oxygen abundances of F-G type stars in the Pleiades cluster

In order to investigate the carbon-to-oxygen ratio of the young open cluster M45 (Pleiades), the C and O abundances of selected 32 F-G type dwarfs (in the effective temperature range of Teff~5800-7600K and projected rotational velocity range of vesini~10-110km/s) belonging to this cluster were determined by applying the synthetic spectrum-fitting technique to C I 5380 and O I 6156-8 lines. The non-LTE corrections for these C I and O I lines were found to be practically negligible (less than a few hundredths dex). The resulting C and O abundances (along with the Fe abundance) turned out nearly uniform without any systematic dependence upon Teff or vesini. We found, however, in spite of almost solar Fe abundance ([Fe/H]~0), carbon turned out to be slightly subsolar ([C/H]~-0.1) while oxygen slightly supersolar ([O/H]~+0.1). This lead to a conclusion that [C/O] ratio was moderately subsolar (~-0.2) in the primordial gas from which these Pleiades stars were formed ~120--130 Myr ago. Interestingly, similarly young B-type stars are reported to show just the same result ([C/O]~-0.2), while rather aged (~1-10 Gyr) field F-G stars of near-solar metallicity yield almost the solar value ([C/O]~0) on the average. Such a difference in the C/O ratio between two star groups of distinctly different ages may be explained as a consequence of orbit migration mechanism which Galactic stars may undergo over a long time.