In order to investigate the carbon-to-oxygen ratio of the young open cluster M 45 ( Pleiades ) , the C and O abundances of selected 32 F–G type dwarfs ( in the effective temperature range of T _ { eff } \sim 5800–7600 K and projected rotational velocity range of v _ { e } \sin i \sim 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-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 T _ { eff } or v _ { e } \sin i .
We found , however , in spite of almost solar Fe abundance ( [ Fe/H ] \sim 0 ) , carbon turned out to be slightly subsolar ( [ C/H ] \sim - 0.1 ) while oxygen slightly supersolar ( [ O/H ] \sim + 0.1 ) .
This lead to a conclusion that [ C/O ] ratio was moderately subsolar ( \sim - 0.2 ) in the primordial gas from which these Pleiades stars were formed \sim 120–130 Myr ago .
Interestingly , similarly young B-type stars are reported to show just the same result ( [ C/O ] \sim - 0.2 ) , while rather aged ( \sim 1–10 Gyr ) field F–G stars of near-solar metallicity yield almost the solar value ( [ C/O ] \sim 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 .