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6. Reddening


The (b-y) and c1 indices are smaller for a peculiar than for a normal star. So in the c1 vs. (b-y) diagram used by Crawford (1978) to establish the standard relation for early normal stars, the peculiar stars are often below this standard relation and yield negative reddenings. The criterion by Philip et al. (1976) to separate peculiar stars is based on such negative reddenings. So, the excess is often underestimated if the peculiar stars are treated as normal stars.

Figure 10: Difference in colour excess computed from Johnson photometry by the Q-method and from tex2html_wrap_inline1475 photometry for normal stars belonging to the early region

Figure 11: Correlation between the difference of colour excess due to peculiarity for CP2 stars belonging to the early region

Since Johnson filters are wider than tex2html_wrap_inline1475 filters, the Johnson colour indices are not greatly affected by the peculiarity. So, when treating the peculiar stars as if they were normal stars, the colour excess computed from Johnson photometry is more realistic than when computing from Strömgren-Crawford photometry. We compared the colour excesses computed from both photometries for CP2 (284) and normal (844) stars of the early region.

To compute E(B-V), we applied the Q-method using E(U-B)/E(B-V)=0.72 (Crawford & Mandwewala 1976) and the luminosity class V calibration by Schmidt-Kaler (1982) for the relation (U-B) vs. (B-V). The E(B-V) was converted to E(b-y)Q by the factor 3/4.27. The E(b-y) was computed from Crawford's (1978) calibration for normal main-sequence stars. Differences tex2html_wrap_inline2035 in the sense E(b-y)Q-E(b-y) are plotted vs. [u-b] index in Fig. 10 (click here) for the normal stars. This tex2html_wrap_inline2035 reflects the intrinsic differences of the two methods when computing reddening, i.e. the differences between the standard calibrations used. However, the tex2html_wrap_inline2035 of the CP2 stars includes the difference due to the calibrations and the difference induced by the peculiarity.

For normal stars tex2html_wrap_inline2035 can be fitted by:

Subtracting this difference from the tex2html_wrap_inline2035 of CP2 stars, the new tex2html_wrap_inline2035 will only be due to the peculiarity of the star. The correlation with tex2html_wrap_inline1701 is shown in Fig. 11 (click here) and is fitted by:

Maitzen & Vogt (1983) performed an analysis of the colour excess, computed from Crawford's calibration, of CP2 stars located in an almost reddening-free sky region. They deduced a moderate correlation between the negative reddening values and the peculiarity index tex2html_wrap_inline1533. Our correlation is stronger than theirs. With this tex2html_wrap_inline2055 the number of negative excesses in our sample is reduced considerably.

Since without a spectrum we cannot know a priori if a star is peculiar, the best solution is to use the tex2html_wrap_inline1701 parameter given by Eqs. (1) and (2) and to correct the excess a posteriori with the preceding expression if the star is classified as CP2.

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