Oxygen λ7771-5 triplet in Hyades A-type stars: A quest for the key to the Am phenomenon

Abstract

An extensive profile-fitting analysis of the O I 7771-5 triplet (along with the nearby Fe I line) was carried out for thirty-one A-type stars, eighteen of them belonging to the Hyades group, in order to clarify the key factor which plays the decisive role in the appearance of abundance anomalies of metallic-line stars (Am), by examining whether any meaningful relation exists between the microturbulence (ξ), the oxygen abundance (log ∈O), the iron abundance (log ∈Fe), the projected rotational velocity (ve sin i), and the stellar atmospheric parameters. It was clearly found based on the Hyades results that the abundance of O/Fe, both being good indicators of the Am phenomenon, because of the negative/positive correlation with the metallicity estimated from colors, progressively increases/decreases (i.e., toward the direction of disappearance of anomaly) with an increase in ve sin i as well as with a decrease in log g. It was thus concluded that the most important driver of the Am phenomenon, producing a significant deficiency/excess of O/Fe, is the rotational velocity which appears to be responsible for not only the triggering, but also the extent of the peculiarity. Meanwhile, the observed log g-dependence of the abundance anomalies is considered to be nothing but superficial, which may be interpreted as being due to the correlation between log g and ve sin i, i.e., rotation-induced lowering of the effective gravity.