Environmental effects on the globular cluster blue straggler population: a statistical approach .

Abstract

Blue stragglers stars (BSS) constitute nowadays an ubiquitous population of objects whose origin involves both dynamical and stellar evolution. In this work we analyze the properties of a catalogue of BSS extracted from an homogeneous sample of 56 Galactic globular clusters observed with WFPC2/HST and described in \citet{pio02}. The goal of this study is to reveal possible environmental dependencies of the number of BSS with other Globular Cluster parameters. Therefore we explore possible monovariate relations between the frequency of BSS (divided in different subsamples according to their location with respect to the parent cluster core radius and half mass radius) and the main parameters of their host GC. We also make use of the Principal Component Analysis to extract the main parent cluster parameters which characterize the BSS family. We find that any subpopulation of BSS strongly depends on the luminosity of the cluster, on the extension of the cluster horizontal branch and on the central velocity dispersion: more luminous clusters, clusters with a smaller central density, and a smaller central velocity dispersion have a higher BSS frequency. Moreover, we find that clusters having higher masses, higher central densities, and smaller core relaxation timescales possess on average more luminous BSS. Finally we point out a possible different behavior of BSS in clusters having different luminosities: while more massive clusters seem to host BSS mainly dependent on the collisional parameter, smaller ones are mostly influenced by the cluster luminosity and the dynamical time-scales.