5 Giant branch

 

Figure 5: The V-(B-V) diagrams for the northwestern half [panel a)] and for the southeastern half [panel b)] of the cluster field. The dashed reference line corresponds to B-V=0.925

If the stars of NGC 1978 really have inhomogeneous parameters (age or chemical abundance), the inhomogeneity should reveal itself not only for the horizontal branch. We cannot be sure, however, that the differences can be noticed for given photometric accuracy, sample size, etc. Figs. 5a and 5b present the V-(B-V) diagrams for two parts of the symmetrical NGC 1978 field described in the previous section, separated by the cluster's minor axis: the northwestern half (NWH) and the southeastern half (SEH), respectively. To facilitate comparison, we show, in both figures, the vertical dashed reference line at B-V=0.925. A somewhat different appearance of the two diagrams can be suspected. First of all this concerns a slightly different visible position of the giant branch (GB) at the level of HB stars and below it, down to $V\sim20.5$(in the most probable color range of the GB, 0.80<B-V<1.05). Most GB stars in these magnitude and color ranges in the CMD of the cluster's SEH are located to the left of the reference line as opposed to those in the CMD of the cluster's NWH. Also, there is a hint that in the turnoff region (V>20.0 and 0.2<B-V<0.6) the NWH stars are systematically redder than the SEH ones. Again, the differences for lower giants are apparent for colors formed using B magnitudes, but are practically absent for V-I. Note that the described difference between visible CMD positions of the GB stars in the cluster's NWH (which includes quadrant A, see the previous section) and SEH (which includes quadrant B) resembles qualitatively that between color distributions of HB stars in CMDs of the two quadrants.