Cosmic-Lab: Chemical and kinematical properties of Blue Straggler stars in Galactic Globular Clusters

TL;DR

This study analyzes the chemical and rotational properties of over 130 Blue Straggler Stars across four Galactic Globular Clusters, revealing insights into their formation mechanisms and physical characteristics.

Contribution

It presents the largest spectroscopic survey of BSSs in multiple GCs, identifying CO-depletion and rapid rotation fractions, and highlights the effects of radiative levitation on spectral analysis.

Findings

Few CO-depleted BSSs suggest a collisional origin or transient depletion.

High fractions of fast-rotating BSSs observed in M4 and omega Centauri.

First evidence of radiative levitation in BSSs hotter than 8200 K.

Abstract

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.