Effects of heavy-element abundances on the color-magnitude diagrams of young clusters in the Galaxy and the Magellanic Clouds

Abstract

Evolutionary sequences have been calculated for stars of 3.5, 4.5, 6.0, and 9.0 M/sub sun/ with Z=0.01 and 0.04, holding Y/X fixed at 0.40. Synthetic clusters have been produced at ages of tauapprox. =3 x 10/sup 7/ yr and approx. =5 x 10/sup 7/ yr for both a fixed age and a finite time scale of star formation equal to a spread of +- 0.1 in log tau. We find that differences in the distributions of the evolved stars for the two compositions (in part) mimic differences observed between young clusters in the Galaxy and in the Magellanic Clouds. In all cases, the clusters with a spread in star formation time scales fit the observations noticeably better than the fixed-age models. Such comparisons also verify the existence of blue helium-burning loops and their sensitivity to heavy-element abundance. We also suggest that the position of the red-giants stars in the LMC clusters requires L/H/sub p/>1.0. (AIP)