6 The color-position diagram

Another useful tool revealing the differences discussed in two previous sections is the color-position diagram (CPD). Figure 6 presents the plot of the color B-V versus position (X, in pixels) of stars along the major axis of NGC 1978. Positive and negative values of X correspond to the NWH and the SEH of the cluster, respectively. We have excluded stars which lie, on the CMD, above the adopted upper luminosity limit of the HB stars and to the right of the sequence of field stars (I<17.8 and B-V>0.2). Two reference lines are shown in the figure. The vertical one is the same as in Fig. 5; the horizontal one, at X=0, separates the stars of the cluster's NWH and SEH. It is apparent that, in the color range 0.80<B-V<1.05, most of the HB and GB stars with X<0 are to the left of the vertical reference line, in contrast to the position of such stars having X>0. As it follows from Figs. 4a, 5, and 6, the maximum difference in B-V color between both HB stars and lower giants in the cluster's NWH and their counterparts in the cluster's SEH may be as large as $0\hbox{$.\!\!^{\rm m}$}05-0\hbox{$.\!\!^{\rm m}$}07$. Similar behavior of stars of the cluster turnoff region may also be suspected in the CPD.

 

Figure 6: The color-position diagram for stars (see text for details) from the cluster field shown in Fig. 1. It presents the plot of color B-V versus position (X, in pixels) of stars along the major axis of NGC 1978. Positive and negative values of X correspond to NWH and SEH of the cluster. Stars near the vertical reference line (B-V=0.925) are the HB stars and the lower giants