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.
|Titolo:||Relic Signatures of Reionization Sources|
|Data di pubblicazione:||19-ott-2006|
|Appare nelle tipologie:||8.1 PhD thesis|