In Fig. 15 (click here) we present integrated colors as a function of the age
for the SSP with solar composition [Y=0.28, Z=0.020].
The left panel shows some colours of the classical
Johnson system, whereas the right panel shows some colours of the
HST-WFCP2 and HST-FOC systems. For these latter the colours on display are
,
,
,
,
,
,
and
. Some colours have an ample dynamical range at varying
age and metallicity. For instance, for ages older than 1 Gyr
the colours
and
have a linear age dependence
with amplitude of about 3 mag. The dependence on metallicity is best
shown by the left panel of Fig. 16 (click here) in which the evolution
of the colours
,
and
for the
SSPs with Z=0.0004 and Z=0.02 is displayed.
At given age, the dynamical range of these colours is about 3.5 mag.
In the two color plane
versus
shown in the right panel of Fig. 16 (click here)
the SSPs of different metallicity are located on a unique almost
linear relation along which both the age and the metallicity vary.
Even if the well known age-metallicity degeneracy is still there,
the above colours are useful to confine ages and metallicity.
An interesting point to be noticed is that all colours but the extreme blue ones
like (U-B) and show evidence of the onset in the SSP of
the AGB stars at the age of about 0.1 Gyr that make them significantly redder.
The effect is not the same in all colours, being more pronounced in (V-K) or
,
and similar.
It is worth reminding the reader that the magnitudes of these SSP refer to
the Salpeter (1955)
initial mass function (in number)
with the normalization constant C=1. To be
applied to real stellar populations (for instance clusters of assigned total
mass and/or number of stars) these magnitudes must be shifted by the quantity
where C is the real normalization constant (see Chiosi et al. 1988, 1989; Bertelli et al. 1994 for details). It goes without saying that these data cannot be used to model the integrated magnitudes and colours of a SSP with a different initial mass function. In such a case new isochrones and integrated quantities must be calculated. Nevertheless, despite the limitations in the initial mass function, these results give a realistic idea of how the integrated properties of SSP vary with time and metallicity (cf. Girardi et al. 1995).