We have exploited the recent determination of the radial distribution of BSS in
four GCs, in order to investigate which mechanism of BSS formation prevails in these stellar
systems. Our conclusion is that the two main formation paths proposed so far, i.e. masstransfer
in PBs and merging of MS stars due to collisions in the cluster core, must coexist
and have similar efficiency both in a low density cluster (M3) and in much denser clusters,
like 47 Tuc and NGC 6752.
In particular, in M3, 47 Tuc, and NGC 6752 the COL-BSS sum to around 50 - 60% of
the total and mostly reside in the central region of the cluster. The MT-BSS are slightly
less abundant than the COL-BSS, but populate all the GC. The density of BSS reaches a minimum in a so-called zone of avoidance, which separates the portion of the GC mostly
occupied by COL-BSS from the cluster outskirts, where the MT-BSS dominate. The location
of the zone of avoidance is explained by accounting for the effects of the dynamical
friction on the PBs which were massive enough for generating the observed BSS.
The picture described above can also be applied to ! Cen; but in this case the lack
of a central peak in the BSS radial distribution requires that the large majority of the BSS
derive from PBs. The very low rate of production of COL-BSS could be in turn attributed
to the fact that mass segregation has not yet driven a sizeable number of PBs to the central
region of the cluster to produce BSS.
A very interesting further development of this research will be to perform a comparison
between the location of a significant sample of BSS in a GC and their spectroscopic
properties. According to the findings of this work, the position in the GC might represent
a strong dynamical clue for the formation mechanism of a given BSS. If it is located outside
the zone of avoidance, the BSS almost certainly results from evolution of a PB; if it
is harbored in the cluster core, the BSS has most likely a collisional origin. On the other
hand, indication about the origin of the same BSS can be independently obtained from high
resolution spectroscopy. Indeed the chemical signature of the MT-BSS formation process
has been recently discovered in 47 Tuc (Ferraro et al. 2006b). The acquisition of similar
sets of data in clusters with different structural parameters and/or in different regions of
the same cluster will provide an unprecedented tool for conforming the scenario presented
here and to finally address the BSS formation processes and their complex interplay with
the dynamical evolution of the cluster.
