Oxygen lambda 7771-5 triplet in Hyades A-type stars: A quest for the key to the Am phenomenon

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 (xi), the oxygen abundance (log epsilon(0)), the iron abundance (log epsilon(Fe)), the projected rotational velocity (v(e) 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 v(e) 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 v(e) sin i, i.e., rotation-induced lowering of the effective gravity.