The Contribution of Primordial Binaries to the Blue Straggler Population in 47 Tucanae

Michela Mapelli,Francesco R Ferraro,Andrea Possenti,Giacomo Beccari,Monica Colpi,Steinn Sigurdsson,Alison Sills,Robert T Rood

doi:10.1086/386370

Abstract

The recent observation (Ferraro et al. 2003b) of the blue str aggler population in 47 Tucanae gives the first detailed characterization of their spatial distribution i n the cluster over its entire volume. Relative to the light distribution, blue stragglers appear to be overabundant in the core and at large radii. The observed surface density profile shows a central peak, a zone of avoidance and a rise bey ond twenty core radii. In light of these findings we explored the evolution of blue stragglers mimicking their dynamics in a multi-mass King model for 47 Tucanae. We find that the observed spatial distribution can not be expl ained within a purely collisional scenario in which blue stragglers are generated exclusively in the core throu gh direct mergers. An excellent fit is obtained if we require that a sizable fraction of blue stragglers is generate d in the peripheral regions of the cluster inside primordial binaries that evolve in isolation experiencing mass-trans fer. Subject headings: stars: blue stragglers - binaries: general - globular clust ers: individual (47 Tuc)

Highlights

Blue Stragglers (BSs), first discovered by Sandage in the globular cluster M3, are stars lying above and blue-ward of the turn-off point in a cluster color-magnitude diagram
We find that the observed spatial distribution can not be explained within a purely collisional scenario in which blue stragglers are generated exclusively in the core through direct mergers
We observed that for recoil velocities greater than 3.5 σ BSs are expelled from the cluster, while for recoil velocities lower than 3 σ dynamical friction drags all of the BSs into the core in ∼ 108 years

Summary

INTRODUCTION

Blue Stragglers (BSs), first discovered by Sandage in the globular cluster M3, are stars lying above and blue-ward of the turn-off point in a cluster color-magnitude diagram. The second, or mass-transfer scenario, suggests that BSs are generated in primordial binaries (hereafter PB) that evolve mainly in isolation or harden gently by long-distance gravitational encounters until they reach contact, leading to (unstable) masstransfer and final coalescence (Carney et al 2001). In both these mechanisms BSs are formed with a mass exceeding the turn-off mass of the cluster and can stay on the main sequence through the mixing of the hydrogen-rich surface layers of its two progenitor stars. In this paper we study, in the light of the most recent data (Ferraro et al 2003b), the case of 47 Tuc, comparing the observed bimodal distribution of BSs with a series of simulations carried on using a new version of the dynamical code described in Sigurdsson & Phinney (1995)

DESCRIPTION OF THE DYNAMICAL CODE

RESULTS

THE ZONE OF AVOIDANCE

SUMMARY