Core-helium-burning stars in young clusters in the Large Magellanic Cloud

Photometry of seven Large Magellanic Cloud clusters, presented elsewhere by Robertson, is compared with theoretical core-helium-burning evolutionary sequences. Six of the clusters have ages t 3 x 10 years; and the seventh cluster, NGC 1866, has t 7 x 10 years. The observed color-magnitude diagrams demonstrate the increase in the length of the "Cepheid loop," with increasing stellar mass, as predicted by the theoretical sequences. The observed spread in luminosity of the blue giants suggests that star formation occurred over a period At <% l0 years (although At may be as small as a few million years). The observed ratio of the number of giants to the number of main-sequence stars agrees with the theoretical ratio of core-helium-burning to core-hydrogen-burning lifetime. There is some observational evidence that the ratio tb of the number of blue giants to the number of red giants is a measure of the chemical composition, and the observed values of tb tentatively indicate low Z or high Y for the peculiar cluster NGC 2136 and possibly high Z or low Y for the outlying cluster NGC 2214. Several stars above the main sequence in the younger clusters may be binaries which have undergone mass exchange. The observational evidence for increases in the envelope opacities, presented earlier by Robertson, is reviewed and found to be inconclusive because of the new data for NGC 1866. Subject headings: abundances, stellar - interiors, stellar - Magellanic Clouds - open clusters