3. The photometric data
A whole quadrant of M 55 was mapped (from the center out to , with as in Trager et al. 1995), on the night of July 5 1992 with 18 EMMI-NTT fields () in the V and I bands. Fig. 1 shows the field positions on the sky. For each field a V and a I band image were taken in succession, with exposure times of 40 and 30 seconds respectively. The night was not photometric and the observing conditions improved as we moved from the outer fields to the internal ones. Information on the various fields and on all the technicalities of the reduction and analysis are reported in the appendix of the paper.
The V vs. color magnitude diagram for a total of 33615 stars of M 55 is shown in Figs. 2 and 3. In total we detected 36800 objects of the cluster field; of them were eliminated after having applied a selection in the DAOPHOT II PSF interpolation parameters as in Piotto et al. (1990a). Although the exposure time was relatively short, the brightest stars of the red giant branch and of the asymptotic giant branch are saturated, though they can be still used for the radial star counts. We have omitted them from the final CMD.
In the following we will analyze the data using a division into three radial subsamples: inner (), intermediate (), and outer (). The core radius is , as found from the radial density profile analysis (cf. Sect. 5). In Fig. 3 we show the brightest part of the CMD of M 55, divided in the three radial subsamples. A large population of blue straggler stars (BS) is clearly visible, particularly in the inner part of the cluster where the background/foreground star contamination is low. In the intermediate zone, the BS population is better defined, and the sequence seems to reach brighter magnitudes. The BS sequence of the inner part appears to be broader in color than the sequence of the intermediate radial range. Part of this broadening can be attributed to the photometric errors that are larger in the inner region than in the intermediate one. The rest of the broadening is probably natural and could be connected to the two formation mechanisms of BS stars: the outer BS stars might mainly come from merger events, while the inner BS might be the final products of collisions (see Bailyn 1995).
We compared the distribution of the 95 BS with that of the 1669 sub-giant branch (SGB) stars selected in the same magnitude interval. In order to minimize the background star contamination along the SGB (very low indeed in the inner part of the cluster), we chose only the stars inside (where is the standard deviation in the mean color) from the mean position of the SGB. We subtracted the background stellar contamination estimated from the star counts in the radial zone . The BS seem to be more concentrated than the corresponding SGB stars only in the inner (Fig. 4). At larger distances, the BS distribution becomes less concentrated than the comparison SGB stars. We run a 2-population Kolmogorov-Smirnov test. The test does not give a particularly high statistical significance to the result: the probability that the BS and SGB stars are not taken from the the same distribution is 96%. However, this possibility cannot be excluded: see Piotto et al. (1990b) and Djorgovski & Piotto (1993) for a discussion on the limits in applying this statistical test for checking population gradients. Another way to look into the same problem is to investigate the radial trend of the ratio BS/SGB, as plotted in Fig. 4 (lower panel). Also in this case the bimodal trend is quite evident. The relative number of BS stars decreases from the center of the cluster to reach a minimum at arcsec (), and then it rises again. Again, the statistical significance is questionable, in view of the small number of BS at arcsec (6 stars). Nevertheless, this possible bimodality is noteworthy. Indeed, there is a growing body of evidence that the radial distribution of BS stars in GCs might be bimodal, as shown by Ferraro et al. (1997) for M 3 or Saviane et al. (1997) and Piotto et al. (1997) for NGC 1851. What makes our result for M 55 of some interest is that this distribution has been interpreted in terms of environmental effects on the production of BS stars. However, the fact that M 55 has a very low concentration (c=0.8), while M 3 and NGC 1851 are high concentration clusters (c=1.85 and c=2.24 respectively, Djorgovski 1993), might make this conclusion at least questionable.
© European Southern Observatory (ESO) 1997
Online publication: April 6, 1998