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In order to investigate the carbon-to-oxygen ratio of the young open clusterM45 (Pleiades), the C and O abundances of selected 32 F-G type dwarfs (in theeffective temperature range of Teff~5800-7600K and projected rotationalvelocity range of vesini~10-110km/s) belonging to this cluster were determinedby applying the synthetic spectrum-fitting technique to C I 5380 and O I 6156-8lines. The non-LTE corrections for these C I and O I lines were found to bepractically negligible (less than a few hundredths dex). The resulting C and Oabundances (along with the Fe abundance) turned out nearly uniform without anysystematic dependence upon Teff or vesini. We found, however, in spite ofalmost 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 aconclusion that [C/O] ratio was moderately subsolar (~-0.2) in the primordialgas from which these Pleiades stars were formed ~120--130 Myr ago.Interestingly, similarly young B-type stars are reported to show just the sameresult ([C/O]~-0.2), while rather aged (~1-10 Gyr) field F-G stars ofnear-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 distinctlydifferent ages may be explained as a consequence of orbit migration mechanismwhich Galactic stars may undergo over a long time.

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