Multicolor photometry provides many possibilities for determinations of metallicity and reddening, two of the most important cluster parameters. UBVRI photometry provides 10 different color indices, 4 of them independent, and hence 45 different two-color diagrams. Below some of them are discussed in detail.
|Figure 3: The diagram for NGC 6723 (all stars). The straight lines and different symbols are explained in the text|
The (B-V, V-R) diagram for NGC 6723 is shown in Fig.3. It is clearly seen that numerous faint stars, with comparatively large photometric errors, deviate from the well-defined central line of the distribution. The group of stars lying parallel to the main strip, but below it and to the left, is especially conspicuous. Such data points, with rather "blue'' (V-R) values, cannot correspond to any physical parameters of normal stars and supposedly are the result of photometric errors. All stars with this kind of error are located in the cluster's central part and, some of them, for reasons unclear to us, received, in our reductions, V magnitudes that were too bright. Because of this, their (B-V) values were found too "red'' and (V-R) values, too "blue''. Similarly, data points significantly deviating upward and to the right of the main cloud correspond to the stars for which the derived V magnitudes are too faint. Some of the data points to the right of the main cloud may belong to field stars; these stars do not show any noticeable concentration towards the cluster center. In the subsequent analysis, we did not take into account stars situated, in the (B-V, V-R) plane, to the left or to the right of the area delimited in the figure by two parallel lines. Such stars have been plotted in Fig.3 with smaller symbols.
|Figure 4: The diagram for different sequences: stars of the RGB (filled circles), AGB (open circles), and the region of blue stragglers (open squares). The line is the theoretical giant sequence for|
Figure 4 presents the (B-V, V-R) diagram for cluster stars of three sequences: the red-giant branch (RGB, filled circles), the asymptotic giant branch (AGB, open circles) and the region of blue stragglers (BSs, open squares). Most are among the cluster's brightest stars with best photometric accuracy. The straight line in the figure follows from computations of Buser & Kurucz (1992) and corresponds to the giant sequence with .The vertical separation between the RGB and the theoretical sequence, at (V-R)=+0.6, is .According to calculations in Grebel & Roberts (1995), based upon Kurucz's theoretical data, for moderately metal-deficient giants with K (i.e. in the range of color indices 1.59>(B-V)0>0.49), ratios of differential interstellar extinction are ,, ,. Using the above value for EV-R/EB-V, and taking into account the theoretical slope of the giant branch ,we find for NGC 6723 and .
According to Grebel & Roberts (1995), the decrease of metallicity from to changes the slope of the giant branch in the (B-V, V-R) diagram, RBV,VR, from 0.534 to 0.512. However, the observed value of this slope from our diagram is , somewhat outside the above range. This discrepancy does not significantly influence our determination of the differential interstellar extinction, but we cannot determine [Fe/H] from such slope.
|Figure 5: The diagram for RGB stars in NGC 6723. Dotted line, the mean ridge line for M 3 according to Ferraro et al. (1996). Solid line, the same ridge line shifted as described in the text|
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