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5. Comparison with other peculiarity criteria

This section compares the efficiency of tex2html_wrap_inline1701 with the criteria introduced by former authors when using the tex2html_wrap_inline1475 system or other photometric systems.

The lower efficiency of p in the intermediate and late regions is paralleled by previous authors. Maitzen (1975) showed using spectrophotometry how the depth of the tex2html_wrap_inline1515 feature decreases towards the cooler, i.e.
Sr stars. tex2html_wrap_inline1533 and tex2html_wrap_inline1855 were found to be correlated except for the Sr stars. Maitzen & Vogt (1983) also found
that the correlation between the Geneva's parameter tex2html_wrap_inline1857 and the tex2html_wrap_inline1533 index was lower for the Sr stars than
for the Si and Cr stars. In particular, the criterion of
tex2html_wrap_inline1861 mag in detecting CP2 stars is only able to classify 36% of the Sr stars sample used by the authors. In our case, the smaller size of CP2 samples (only 93 and 70 CP2 stars for the intermediate and late regions, respectively), hindered the analysis.

So, the following comparison is restricted to the stars in the early region.

5.1. The tex2html_wrap_inline1475 system

Figure 6: [m1] - [u-b] diagram for the stars in the early region. Dots represent normal stars and full circles CP2 stars. The solid line is the adopted standard relation for normal stars and the dashed line indicates the threshold above which 50% of the CP2 stars are placed

A study of the location of CP2 stars in a colour-colour diagram was carried out by Cameron (1967). On a c1-m1 diagram, certain regions corresponding to high m1 values are mainly occupied by Ap stars. Maitzen (1976) and Adelman et al. (1995) showed that there is a relation between tex2html_wrap_inline1871, using (b-y) as free parameter, and the strength of the tex2html_wrap_inline1515 feature. Figures 6 (click here) and 7 (click here) show our samples on the [m1]-[u-b] and [c1]-[u-b] diagrams. In the same way as with the p coordinate, we fitted standard relations to normal stars in each diagram, we considered peculiarity thresholds for which 50% of CP2 stars are separated and we computed the percentage of contamination of normal stars. The contamination ratios are of 16% and 17%. So, these diagrams are less effective than the p-[u-b] diagram.

Figure 7: The same as Fig. 6 but for the [c1] - [u-b] diagram

On the other hand, a star with an tex2html_wrap_inline1887 mag, computed by using the calibrations for normal stars (Crawford 1978), was considered as peculiar by Philip et al. (1976). In our CP2 sample, there are only 16 stars (3%) with such a colour excess, while all normal stars have a reddening higher than this. These 16 stars have a tex2html_wrap_inline1889 so they are also classified as peculiar with our peculiarity level of tex2html_wrap_inline1785.

5.2. tex2html_wrap_inline1533 photometry

Maitzen (1976) introduced a photometric index (the a index) to measure the intensity of the depression at tex2html_wrap_inline1515, by comparison of the flux at the center with the mean of the flux at each side of the depression. The tex2html_wrap_inline1533 index is the difference between the measured a index for a given star and the a index of the normality line for the same temperature. For the CP2 stars this tex2html_wrap_inline1533 index is significantly positive and all the stars with tex2html_wrap_inline1911 mag are considered as peculiar stars.

We compiled the published a photometry in the literature: Maitzen 1976; Maitzen & Vogt 1983; and the series of papers Photometric Search for CP2 Stars in Open Clusters (Maitzen 1993; Maitzen et al. 1988 and references therein) and built a sample of 1703 stars with measured tex2html_wrap_inline1533, 930 having complete tex2html_wrap_inline1475 photometry, too. The stars were treated with the same classification algorithm and their colour excess was computed. There are 577 stars belonging to the early region and 453 of them have tex2html_wrap_inline1727 mag.

Comparison of the tex2html_wrap_inline1701 defined by the Eqs. (1) and (2) with the measured tex2html_wrap_inline1533 shows a high correlation between both indices (Fig. 8 (click here)a). The tex2html_wrap_inline1715 threshold is able to detect 60% of the stars with tex2html_wrap_inline1911 mag with a contamination of about 5% of stars with tex2html_wrap_inline1929 mag. Although the tex2html_wrap_inline1533 index is more efficient than the tex2html_wrap_inline1701 index, the correlation between them demonstrates that tex2html_wrap_inline1701 is also a good indicator of peculiarity.

On the other hand, the correlation between tex2html_wrap_inline1533 and tex2html_wrap_inline1701 is better than the correlation between tex2html_wrap_inline1533 and tex2html_wrap_inline1943 or tex2html_wrap_inline1945, (Figs. 8 (click here)b and 8 (click here)c). The contaminations in these cases are three times higher.

Figure 8: a) Correlation between tex2html_wrap_inline1533 and tex2html_wrap_inline1701 defined by Eqs. (1) and (2). b) Correlation between tex2html_wrap_inline1533 and tex2html_wrap_inline1943. c) Correlation between tex2html_wrap_inline1533 and tex2html_wrap_inline1945. Horizontal and vertical dashed lines correspond to the threshold values of either index for the detection of CP2 stars

5.3. The Geneva system

The Geneva photometric system is capable of detecting CP2 stars through the tex2html_wrap_inline1515 depression (North & Cramer 1981). Two indices, tex2html_wrap_inline1857 and tex2html_wrap_inline1965, were defined with this purpose. tex2html_wrap_inline1857 is defined as the measured (V1-G) for a given star less the (V1-G) for a normal star with the same (B2-V1) colour. tex2html_wrap_inline1965 is a combination of the filters U, B1, B2, V1 and G (Cramer & Maeder 1979, 1980) and has the advantage of being reddening free, although it seems that it is slightly sensitive to certain systematic effects which are not clearly understood (North & Cramer 1981).

Figure 9 (click here) shows the comparison between the tex2html_wrap_inline1857 and the tex2html_wrap_inline1701. The photometric data in the Geneva system were extracted from Rufener (1988). There are 1235 normal stars and 396 CP2 stars with measurements in both the Geneva and the tex2html_wrap_inline1475 photometric systems. The stars with tex2html_wrap_inline1861 mag are considered peculiar stars. This criterion recognizes 71% of CP2 stars with a contamination of only 1%. So, the Geneva system is more efficient than the Strömgren-Crawford system, since it gives a direct measurement of the flux depression at tex2html_wrap_inline1515. tex2html_wrap_inline1857 is not a reddening free index either and the very reddened CP2 stars are not detected
(Hauck & North 1982).

Figure 9: Correlation between tex2html_wrap_inline1701 defined by Eqs. (1) and (2) and tex2html_wrap_inline1857. Dots are normal stars and full circles are CP2 stars from Renson et al. (1991) catalogue. Horizontal and vertical dashed lines correspond to the threshold values of both indices to recognize CP2 stars

For an analysis of the correlation between tex2html_wrap_inline1533 and tex2html_wrap_inline1857 peculiarity indices see Maitzen & Vogt (1983).

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