K-Line Photometry of Stars in Population i Clusters

Abstract

Phptoelectric photometry of the K-line of calcium has been performed for the A stars of five open clusters (Hyades, Pleiades, IC 2391, IC 2602, and NGC 6475) and one association (Orion). For NGC 6475 (M7), as we previously reported, the relation between k and (b - Y)o is a unique one, implying that abundance variations of calcium (and, presumably, other metals as well) in the cluster are less than about 125 percent star to star, and that the original interstellar cloud from which it formed was similarly homogeneous. The Hyades appear metal-rich in comparison with the other clusters, the remainder of which have calcium abundances similar to those of field stars. The Hyades are also found to exhibit less variation in calcium abundance than the field stars, but the relationship in k versus b - y is not as tight as for NGC 6475. Uncertain individual reddening corrections are the most serious limitation for the interpretation of data for reddened clusters other than NGC 6475. A number of stars in these clusters appear to exhibit previously unsuspected spectral peculiarities, as revealed by K4ine and uvby-H photometry. In particular, the young cluster IC 2391 may contain more weakly metallic stars than heretofore recognized. From discussion of the observations in these two papers, evidence supporting three constraints on the metallicity anomaly has emerged. (i) Metallicism does not appear as a discrete phenomenon in the (dk, d[m1])-plane, and a relatively smooth transition between normal and metallic-line stars is suggested by the observations. (li) Possible support for the suggestion that metallicism is moderated by mild evolution off the xero-age main sequence may be inferred from the data. (iii) Evidence is found for the existence of weak Am stars in clusters younger than the Hyades.