Oxygen Abundances in Late-B through F Supergiants

Abstract

A spectral-synthesis analysis was performed for twenty-six late-B through F supergiants (including Cepheids) in order to determine their photospheric oxygen abundances from the O I 6156–8 feature, while taking into account the non-LTE effect, which is progressively Teff-dependent from ≲ 0.1 dex for F-type supergiants to ∼ 0.4 dex for late-B ones in terms of the non-LTE abundance correction. The resulting oxygen abundances show a moderate underabundance relative to the Sun $([\textrm O/\textrm H]{\sim -0.3} $) along with a remarkably small scatter (within ± 0.1 dex) over this wide temperature range. Considering the recent evidence that the solar oxygen abundance is mildly enhanced by 0.2−0.3 dex relative to that of the galactic gas which forms young stars, we conclude that the original O-composition in the atmosphere of these supergiants had suffered almost no appreciable alteration during their past evolutionary history, even though signs of significant mixing of CN-cycle or NeNa-cycle products are evident. This excludes the possibility of any global-mixing mechanism, such that producing a significant reduction of oxygen due to a deep dredge-up of considerable ON-processed material, whatever the details of the envelope-mixing process for these relatively massive stars may be.