Appendix A: blue stragglers in M55
As mentioned in Sect. 3.1, a population of blue straggler stars (BSS) is visible in the CMD of M55 in the area above the turn-off point. The existence of BSS in M55 and their central concentration has already been shown in detailed analyses by Mandushev et al. (1997) and Zaggia et al. (1998).
At present there is much evidence that BSS are close or merged binary stars (e.g. see Mandushev et al. 1997 and references therein). This theory is supported by the finding that the BSS are often more centrally concentrated than other stellar types within globular clusters, which might be indicative for a dynamical mass segregation. In the CMD the BSS sequence ranges up to 2.5 magnitudes above the TOP (see Fig. A1). From our data sample of M55 we selected 56 BSS candidates from the inner for further investigations, shown in Fig. A1 as filled circles.
We compared the spatial distribution of the 56 BSS with that of 1377 sub-giant branch (SGB) stars chosen from the same magnitude interval to ensure that incompleteness does not influence the comparison. In Fig. A1, the cumulative distributions of the SGB stars and the BSS are plotted. The BSS are clearly more concentrated than the SGB stars within the selected radius. Running a Kolmogorov-Smirnov test we obtain a 92 percent probability that the BSS and the SGB stars are taken from different distributions, a value that is similar to the one found by Mandushev et al. (1997) for M55. These results favour mass segregation and further support that BSS have on the average larger masses than subgiants and thus are merged stars or binaries.
© European Southern Observatory (ESO) 1999
Online publication: October 4, 1999