Cosmic-Lab: Chemical and kinematical properties of Blue Straggler stars
  in Galactic Globular Clusters
  Blue straggler stars (BSSs) are brighter and bluer than the main-sequence
(MS) turnoff and more massive than MS stars.Two scenarios for their formation
have been proposed: collision-induced stellar mergers (COL-BSSs),or
mass-transfer in binary systems (MT-BSSs).Depleted surface abundances of C and
O are expected for MT-BSSs,whereas no chemical anomalies are predicted for
COL-BSSs. Both MT and COL-BSSs should rotate fast,but braking mechanisms may
intervene with efficiencies and time-scales unknown,thus preventing a clear
prediction of the expected rotational velocities. In this context,an extensive
survey is ongoing by using FLAMES@VLT, with the aim to obtain abundance
patterns and rotational velocities for representative samples of BSSs in
Galactic GCs.A sub-population of CO-depleted BSSs has been identified in 47
Tuc,with only one fast rotating star detected (Ferraro et al.2006). For this
PhD Thesis work I analyzed FLAMES spectra of more than 130 BSSs in 4 GCs:M4,NGC
6397,M30 and {\omega}Centauri.This is the largest BSS sample spectroscopically
investigated so far.Hints of CO-depletion have been observed in only 4-5 cases
(in M30 and {\omega}Centauri),suggesting either that the majority of BSSs have
a collisional origin, or that CO- depletion is a transient
phenomenon.Unfortunately,no conclusions in terms of formation mechanism could
be drawn in a large number of cases,because of the effects of radiative
levitation.Remarkably,this is the first time that evidence of radiative
levitation is found in BSSs hotter than 8200 K.Finally,we also discovered the
largest fractions of fast rotating BSSs ever observed in any GCs:40% in M4 and
30% in {\omega}Centauri. While not solving the problem of BSS formation,these
results provide invaluable information about the BSS physical properties,which
is crucial to build realistic models of their evolution.
