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

摘要

In order to investigate the carbon-to-oxygen ratio of the young open cluster M45 (Pleiades), the C and O abundances of 32 selected F-G type dwarfs (in the effective temperature range of T-eff similar to 5800-7600 K and projected rotational velocity range of v(e)sin i similar to 10-110kms(-1)) belonging to this cluster were determined by applying the synthetic spectrum-fitting technique to C I 5380 and OI 6156-8 lines. The non-local thermodynamical equilibrium corrections for these C I and OI 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] similar to 0), carbon turned out to be slightly subsolar ([C/H] similar to -0.1) while that of oxygen was slightly supersolar ([O/H] similar to +0.1). This leads to a conclusion that the [C/O] ratio was moderately subsolar (similar to -0.2) in the primordial gas from which these Pleiades stars were formed similar to 120-130 Myr ago. Interestingly, similarly young B-type stars are reported to show just the same result ([C/O] similar to -0.2), while rather aged (similar to 1-10 Gyr) field F-G stars of near-solar metallicity yield almost the solar value ([C/O] similar to 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.