Can surface oxygen abundances of red giants be explained by the canonical mixing theory?

Abstract

Abstract Extensive oxygen abundance determinations were carried out for 239 late-G/early-K giant stars of 1.5–5 M⊙ by applying the spectrum-fitting technique to O i 7771–5 and [O i] 6300/6363 lines based on the high-dispersion spectra in the red region newly obtained at the Okayama Astrophysical Observatory. Our main purpose was to clarify whether any significantly large (≲ 0.4–0.5 dex) O-deficit really exists in these evolved stars, which was once suspected by Takeda et al. (2008, PASJ, 60, 781) from the analysis of the [O i] 5577 line, since it (if real) cannot be explained by the current theory and may require the necessity of special non-canonical deep mixing in the envelope. We found, however, that the previous [O/H]5577 results (differential abundances relative to the Sun) were systematically underestimated compared to the more reliable [O/H]7773 (from O i 7771–5 triplet lines) or [O/H]6300 (from [O i] 6300 line) obtained in this study. Comparing the updated [O/Fe] ratios with the theoretically predicted surface abundance changes caused by mixing of nuclear-processed products dredged-up from the interior, we concluded that the oxygen deficiency in these red giants is insignificantly marginal (only by ≲ 0.1 dex), which does not contradict the expectation from the recent theoretical simulation. This consequence of reasonable consistency between theory and observation also applies to the extent of peculiarity in [C/Fe] and [Na/Fe], which were also examined by re-analyzing the previous equivalent-width data of C i 5052/5380 and Na i 6160 lines.