Oxygen 6156-8 angstrom triplet in chemically peculiar stars of the upper main sequence: Do HgMn stars show an oxygen anomaly?

An extensive spectrum-fitting analysis in the lambda similar to 6150 Pi reg ion 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 quant itatively establish the abundance of oxygen from the O-I 6156-8 triplet and to study its behavior/anomaly for each peculiarity group, where special at tention was paid to HgMn-type stars. Magnetic Ap variables (Si or SrCrEu ty pe) generally show a remarkably large oxygen depletion (by similar to -1.6 dex to similar to -0.4 dex relative to the solar abundance), and classical Am stars also show a clear underabundance (by similar to -0.6 dex on the av erage). In contrast, the oxygen abundance in HgMn stars, exhibiting only a mild deficiency relative to the Sun typically by similar to 0.3-0.4 dex, is not markedly different from the tendency of normal stars, also showing a s ubsolar abundance ([O/H] similar or equal to -0.2). In view of the recent o bservational implication that the present solar oxygen abundance is enriche d by similar to 0.2-0.3 dex relative to the interstellar gas (from which yo ung stars are formed), and thus unsuitable for the comparison standard, we concluded that the extent of the average O-deficiency in HgMn stars is appr eciably reduced down to only similar to 0.1-0.2 dex (i.e., not much differe nt from the initial composition), which contrasts with their well-known dra stically large N-depletion. Yet, as can be seen from the delicate and tight O vs. Fe anticorrelation, this marginal deficiency should be real, and thu s some physical process simultaneously producing O-depletion/Fe-enrichment must have actually worked in the atmosphere of HgMn stars.