Oxygen 6156–8 Å Triplet in Chemically Peculiar Stars of t he Upper Main Sequence: Do HgMn Stars Show an Oxygen Anomaly?

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An extensive spectrum-fitting analysis in the λ ∼ 6150 Å region was performed for forty late-B and A chemically peculiar (HgMn, Ap, Am, weak-lined) and normal stars of the upper main sequence, in order to quantitatively establish the abundance of oxygen from the O I 6156-8 triplet and to study its behavior/anomaly for each peculiarity group, where special attention was paid to HgMn-type stars. Magnetic Ap variables (Si or SrCrEu type) generally show a remarkably large oxygen depletion (by ∼ −1.6 dex to ∼ −0.4 dex relative to the solar abundance), and classical Am stars also show a clear underabundance (by ∼ −0.6 dex on the average). In contrast, the oxygen abundance in HgMn stars, exhibiting only a mild deficiency relative to the Sun typically by ∼ 0.3−0.4 dex, is not markedly different from the tendency of normal stars, also showing a subsolar abundance ([O/H] ⋍ −0.2). In view of the recent observational implication that the present solar oxygen abundance is enriched by ∼ 0.2−0.3 dex relative to the interstellar gas (from which young stars are formed), and thus unsuitable for the comparison standard, we concluded that the extent of the average O-deficiency in HgMn stars is appreciably reduced down to only ∼ 0.1−0.2 dex (i.e., not much different from the initial composition), which contrasts with their well-known drastically large N-depletion. Yet, as can be seen from the delicate and tight O vs. Fe anticorrelation, this marginal deficiency should be real, and thus some physical process simultaneously producing O-depletion/Fe-enrichment must have actually worked in the atmosphere of HgMn stars.