Annu. Rev. Astron. Astrophys. 1996. 34: 511-550
Copyright © 1996 by . All rights reserved

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3. THE INTERMEDIATE-AGE POPULATIONS OF THE MAGELLANIC CLOUDS

3.1. Definitions

In this section, we consider field stars and clusters with ages between approximately 1 Gyr and 10-12 Gyr (using the age scale adopted in Section 2.1) to represent the "intermediate-age" populations of the Clouds. For comparison, Westerlund (1990) adopted the age range 0.2-7 Gyr for this purpose. Our definition of intermediate-age has the advantage of being empirically easy to apply. The upper age limit corresponds to the epoch of RR Lyr formation that was discussed in Section 2.2, while the younger age limit corresponds to the earliest appearance of the red giant branch. The precise ages of these limits remain uncertain because different treatments of convective overshoot significantly affect the ages of clusters younger than 1-2 Gyr (e.g. Mateo & Hodge 1987, Seggewiss & Richtler 1989, Bomans et al 1995). Moreover, the RGB-turnon age seems to exhibit an intrinsic spread of 250-500 Myr (Ferraro et al 1995).