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