## 3. Correction for differential reddeningIn order to correct the CMDs for differential reddening, we used a refined version of the method described by Grebel et al. (1995). The entire frame is divided into subframes. These are determined by covering the whole frame with a regular subgrid and dividing the grid cells further until the number of stars in one cell becomes too small to define the CMD structure. The CMDs will be shifted according to the reddening vector (described below) with respect to CMDs from neighbouring cells. The shift in colour supplies the differential reddening. If two neighbouring subframes have the same reddening, these subframes are merged. There are two problems with this method: First, one has to be careful to use the HB as a means for comparing two CMDs, as the HB may be intrinsically elongated. Useful results can only be achieved by comparing the RGBs and TOPs, as far as they are accessible. Second, the size of the subfields must be large enough to render meaningful CMDs. Fig. 22 shows the resulting extinction maps for the seven clusters. The smallest subfields have a size of about . But as some of them still showed differential reddening, the scale of the structures responsible for the differential reddening is be expected to be even smaller. The smallest scales we got from a comparison of coordinates of stars with different reddening amounted to values of . For the correction of the CMDs we need the extinction However, assuming a uniform reddening law led to CMDs which in some cases showed the corrected HBs having larger or smaller slopes than the uncorrected HBs. Moreover, as there is some uncertainty in the literature regarding , with values varying between (Savage & Mathis 1979) and (Grebel & Roberts 1995), we determined the slope of the reddening vector via the tilted HBs of our CMDs. This leads to reasonable results only if the HBs are intrinsically clumpy. This assumption is corroborated by the fact that the well dereddened CMDs (Fig. 23, 27, 29) have clumpy HBs indeed. Table 1 shows the slopes for each cluster. In Fig. 21, the slopes are plotted against the galactic longitude. These variations, although at the margin of the errors, confirm earlier observations by Meyer & Savage (1981) and Turner (1994). Meyer & Savage determined via two-color-diagrams the deviation of single stars in the extinction behaviour from the galactic mean extinction law. Turner demonstrated the inapplicability of a mean galactic reddening law for objects lying close to the galactic plane. To correct the diagrams for differential reddening, we referred all sub-CMDs of one cluster to the one with detected minimal reddening and we shifted all other sub-CMDs onto that. As we thus use the minimal absolute reddening as a point of reference, the absolute reddening determined later on will be smaller than value given in the literature. The differentially dereddened CMDs are shown in Figs. 7 through 19. As the correction led to a clearly improved appearance for all clusters, the corrected versions of the CMDs will be used for further investigation.
© European Southern Observatory (ESO) 1999 Online publication: June 30, 1999 |