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3. General properties of the photometric systems

The dependence of the tex2html_wrap_inline2607 on the tex2html_wrap_inline2609 (at given gravity and chemical composition) governs the appearance of a star in the CMD. This is best shown by the colour-tex2html_wrap_inline2611 relation for any pair of pass-bands. The colour is simply given by


 equation468

with tex2html_wrap_inline2613 and tex2html_wrap_inline2615 17W, 21W, 25W, 30W, 33W, 43W, 45W, 55W, 60W, 70W, 81W, 85LW for the WFPC; tex2html_wrap_inline2617 and tex2html_wrap_inline2619 15F, 17F, 22F, 33F for the FOC, with obvious meaning of the symbols. In all cases tex2html_wrap_inline2621.

  table476
Table 2: Model Galaxies: tex2html_wrap_inline2623, tex2html_wrap_inline2625 and Age are in Gyr, the galaxy mass is in tex2html_wrap_inline2627

The relations for each group are shown in the series of Figs. 3 (click here) through  10 (click here) limited to the case of solar composition and gravity typical of main sequence stars (log g=5). It is apparent that colours involving one or two UV pass-bands do not possess a monotonic relation with tex2html_wrap_inline2647. With the standard Johnson system, it is natural to see colours becoming redder as tex2html_wrap_inline2649 decrease, as long as the colours are defined as bluer magnitudes minus redder magnitudes. With the HST system the colour becomes bluer again when the temperature is cooler than a certain value that depends on the pass-band (hereafter the colour turnover). The turnover temperature somewhat depends on the gravity and chemical composition because the spectral energy distribution varies with them. Finally, the colour turnover does no longer occur for the visible-red pass-bands.

The turnover is caused by the combination of two effects. First the UV pass-bands are affected by the the so-called visible/red-leak problem, i.e. long tails of significant transmittance at wavelengths much longer than the nominal peak value (cf. Figs. 1 (click here) and 2 (click here)). For very hot stars, the amount of flux gathered in the visible/red-leak region is small and the effect of this on the colour is negligible. As the star gets cooler the amount of flux falling into the visible/red-leak region becomes more and more important and so its effect on the colour. The other effect is that the various UV filters have visible/red-leaks that intersect each other. For instance the visible/red-leak of the filter 15W is higher than the visible/red-leak of the filters 21W and 25W, whereas the visible/red-leak of the filter F30W is higher than the previous ones. This anomalous behaviour tends to disappear going to filters of longer and longer peak wavelength. Analogous considerations hold for the UV filters of the FOC.

The obvious consequence of the colour turnover is that one cannot determine a unique value of the temperature from the colours at least as long as some UV bands are involved. Although the combination of a UV pass-band with one, whose peak is at much longer wavelength can improve upon this point of ambiguity, there are cases in which the effect is still there (see for instance the case of tex2html_wrap_inline2651 or tex2html_wrap_inline2653).

The effect of the colour turnover on the observed CMD is easy to foresee and is discussed in the next section by means of select isochrones.

Finally, we notice that colours like tex2html_wrap_inline2655, tex2html_wrap_inline2657, tex2html_wrap_inline2659, and tex2html_wrap_inline2661 with tex2html_wrap_inline2663, respectively, show pronounced dips at tex2html_wrap_inline2665 which are caused by the secondary peaks in the functions tex2html_wrap_inline2667 pointed out in the previous sections.

There is a final remark to be made. The colour-tex2html_wrap_inline2669 relations for the WFPC2 system obey the basic condition that at tex2html_wrap_inline2671 all the colours are zero or nearly zero (see Figs. 3 (click here) through  8 (click here)), which is typical of the ground based photometric systems (Johnson for instance). This means that the calibration of the tex2html_wrap_inline2673 has been properly made. In the case of the FOC system, the above condition is no longer always satisfied, cf. Figs. 9 (click here) and  10 (click here). At tex2html_wrap_inline2675 the colours may be significantly different from zero. Similar behaviour is also visible in the FOC colours calculated by Yi et al. (1995). This implies that the corresponding tex2html_wrap_inline2677 requires further calibration. Although this point can be easily fixed up by adding suitable shifts to the magnitudes or suitably correcting the tex2html_wrap_inline2679, we prefer to present results for the original tex2html_wrap_inline2681.


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