Colour-Magnitude Diagrams for Nine Globular-Like Clusters in the Magellanic Clouds

Abstract

Colour–magnitude diagrams have been obtained for nine globular-like clusters in the Magellanic Clouds, from plates taken with the 74 in. telescope and calibrated with photoelectric sequences observed on the 50 in. A separate sequence was observed for each cluster, in favourable circumstances to a limiting magnitude of about V = 19.5. The clusters fall into three groups: (i) NGC 1466, 1841 and 2257 are similar to galactic halo clusters. Their moduli can be determined, from giant branch, horizontal branch and RR Lyrae fitting, and confirm the moduli determined from the cepheids in the Clouds to within the errors of measurement, (ii) Lindsay 1, Kron 3 and NGC 339, all members of the SMC, differ from (i) in having no blue horizontal branch stars, and very blue integrated colours as compared with their giant branches. They are tentatively identified with the intermediate-age clusters in the galaxy. Possibly five of the six SMC clusters so far studied are in this group, in which case they must have been born within a fairly short time interval, and about 10 9 years ago. (iii) NGC 2209, 2231 and Hodge 11, all in the LMC, differ again. Their giant branches are weak, with few stars brighter than MV ∼ −1.5, they have strong concentrations of faint blue stars, and Hodge 11 has a number of very blue stars at MV ∼ 0 well to the left of the main sequence. These clusters appear to have no counterparts in the galaxy.