7 Interstellar reddening

In Fig. 5 we show the $\beta$ index as a function of the b-y index for the observed stars in NGC 6134 and NGC 3680. As can be seen the open cluster NGC 6134 suffers from significant interstellar reddening.

Figure 5: The relation of b-y and $\beta$ for stars with well-determined indices in NGC 6134 and NGC 3680

To determine the interstellar reddening we have used the calibrations of Crawford (1975a) (A stars), (1979) (F stars) with a small correction which was noted by Nissen (1988). Histograms of the interstellar reddening for the stars in two open clusters are shown in Fig. 6.

The mean value of each cluster after removing 3$\sigma$ outliers are: NGC 6134: $E(b-y)=0.263\pm0.003$ and NGC 3680: $E(b-y)=0.046\pm0.011$, cf. Table 3.

These results agree reasonably well with previous results obtained for both clusters: For NGC 3680 Nissen (1988) found $E(b-y)=0.034\pm0.005$ when using the cluster membership assignment by Nordström et al. (1997). Kozhurina-Platais et al. (1997) found a somewhat higher value E(b-y)=0.053 from Johnson BV photometry, when using only stars that were probable cluster members according to proper motion measurements. They made a fit of overshoot isochrones to the observed CMD with E(b-y) and distance modulus as free parameters. Kjeldsen et al. (1991) found $E(b-y)=0.32\pm0.02$ for NGC 6134 from Johnson UBV photometry. From revised DDO photometry of giant stars Twarog et al. (1997) estimate that E(b-y)=0.27, which is in better agreement with our results. As discussed by Twarog et al. (1997 the reason for the somewhat higher reddening found by Kjeldsen et al. (1991) may be due to the fact that they assumed solar metallicity.

Figure 6: Histograms of the interstellar reddening of stars in NGC 6134 and NGC 3680. In the top plot the dashed histogram is for all stars, while the solid curve is for the stars that are confirmed cluster members