In order to investigate the carbon-to-oxygen ratio of the young open cluster M 45 (Pleiades), the C and O abundances of 32 selected F–G type dwarfs (in the effective temperature range of Teff ∼ 5800–7600 K and projected rotational velocity range of vesin i ∼ 10–110 km s−1) 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-local thermodynamical equilibrium 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 vesin i. 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 that of oxygen was slightly supersolar ([O/H] ∼ +0.1). This leads to a conclusion that the [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 average. Such a difference in the C/O ratio between two star groups of distinctly different ages may be explained as a consequence of the orbit migration mechanism which Galactic stars may undergo over a long time.

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