On the oxygen abundances of M 67 stars from the turn-off point through the red-giant branch

TL;DR

This study investigates whether surface oxygen abundances in M 67 stars change during stellar evolution from the turn-off point to the red giant branch, finding no significant differences that support current stellar evolution models.

Contribution

It provides observational evidence that oxygen abundance remains largely unchanged during stellar evolution in low-mass stars of M 67, supporting the first dredge-up theory.

Findings

No significant difference in oxygen abundance between non-giant and giant stars.

Oxygen content appears unaffected by mixing processes in red giants.

Results are consistent with standard stellar evolution predictions.

Abstract

With an aim to examine whether the surface oxygen composition suffers any appreciable change due to evolution-induced mixing of nuclear-processed material in the envelope of red giants, abundance determinations for O/Fe/Ni based on the synthetic spectrum-fitting method were performed by using the moderate-dispersion spectra in the 7770-7792A region (comprising O I 7771-5, Fe I 7780, and Ni I 7788 lines) for 16 stars of the old open cluster M 67 in various evolutionary stages from the turn-off point through the red giant branch. We could not find any meaningful difference in the oxygen abundances between the non-giant group (T_eff > 5000 K) and the red-giant group (T_eff < 5000 K), which are almost consistent with each other on the average (despite that both have rather large dispersions of a few tenths dex caused by insufficient data quality), though only one giant star (S 1054) appears to show an exceptionally low O abundance and thus needs a more detailed study. This result may suggest that oxygen content in the stellar envelope is hardly affected (or changes are insignificant) by the mixing of H-burning products in the red-giant phase, as far as M 67 stars of low mass (~1.3 M_sun) are concerned, which is consistent with the prediction from the conventional stellar evolution theory of first dredge-up.