Formation and Evolution of Blue Stragglers in 47 Tucanae

TL;DR

This study investigates the origin, evolution, and properties of blue straggler stars in 47 Tucanae, revealing different formation channels for bright and faint BSS and their evolutionary impacts on the cluster.

Contribution

It provides a detailed analysis of BSS formation mechanisms, radial distributions, and evolutionary stages using UV data and stellar models, highlighting differences between bright and faint BSS.

Findings

Bright BSS are more centrally concentrated, indicating a possible origin from multiple stellar systems.

Faint BSS have a distribution similar to main sequence binaries, suggesting a different formation pathway.

Evolved BSS contribute to the excess of asymptotic giant branch stars in 47 Tuc.

Abstract

Blue stragglers (BSS) are stars whose position in the Color-Magnitude Diagram (CMD) places them above the main sequence turn-off (TO) point of a star cluster. Using data from the core of 47 Tuc in the ultraviolet (UV), we have identified various stellar populations in the CMD, and used their radial distributions to study the evolution and origin of BSS, and obtain a dynamical estimate of the mass of BSS systems. When we separate the BSS into two samples by their magnitude, we find that the bright BSS show a much more centrally concentrated radial distribution and thus higher mass estimate (over twice the TO mass for these BSS systems), suggesting an origin involving triple or multiple stellar systems. In contrast, the faint BSS are less concentrated, with a radial distribution similar to the main sequence (MS) binaries, pointing to the MS binaries as the likely progenitors of these BSS. Putting our data together with available photometric data in the visible and using MESA evolutionary models, we calculate the expected number of stars in each evolutionary stage for the normal evolution of stars and the number of stars coming from the evolution of BSS. The results indicate that BSS have a post-MS evolution comparable to that of a normal star of the same mass and a MS BSS lifetime of about 200-300 Myr. We also find that the excess population of asymptotic giant branch (AGB) stars in 47 Tuc is due to evolved BSS.