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5. Integrated colours of SSP

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 tex2html_wrap_inline2711, tex2html_wrap_inline2713, tex2html_wrap_inline2715, tex2html_wrap_inline2717, tex2html_wrap_inline2719, tex2html_wrap_inline2721, and tex2html_wrap_inline2723. Some colours have an ample dynamical range at varying age and metallicity. For instance, for ages older than 1 Gyr the colours tex2html_wrap_inline2725 and tex2html_wrap_inline2727 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 tex2html_wrap_inline2729, tex2html_wrap_inline2731 and tex2html_wrap_inline2733 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 tex2html_wrap_inline2739 versus tex2html_wrap_inline2741 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 tex2html_wrap_inline2745 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 tex2html_wrap_inline2749, tex2html_wrap_inline2751 and similar.

It is worth reminding the reader that the magnitudes of these SSP refer to the Salpeter (1955) initial mass function tex2html_wrap_inline2753 (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


equation568

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).


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