The fractions of post-binary-interaction stars and evolved blue straggler stars on the red giant branch of globular clusters

TL;DR

This study uses binary population synthesis to estimate the fraction of red giant branch stars in globular clusters resulting from binary interactions, highlighting the significant contribution of evolved blue stragglers to the RGB population.

Contribution

It provides the first detailed quantification of the fraction of RGB stars formed via binary interactions and evolved blue stragglers in globular clusters, considering different initial conditions.

Findings

Approximately 50% of RGB stars experienced binary interactions.

Evolved blue stragglers constitute about 10% of RGB stars.

Orbital period and mass ratio distributions significantly influence the fraction of post-interaction giants.

Abstract

The red giant branch (RGB) of globular clusters (GCs) is home to some exotic stars, which may provide clues on the formation of multiple stellar populations in GCs. It is well known that binary interactions are responsible for many exotic stars. Thus, it is important to understand what fraction of stars on the RGB of GCs is the result of binary interactions. In this paper, we performed a binary population synthesis study to track the number of post-binary-interaction (post-BI) stars that appear on the RGB, with particular emphasis on the evolved blue straggler stars (E-BSSs). Assuming an initial binary fraction of nearly 50%, we find that about half of the objects on the RGB (called giants) underwent the binary interactions, and that E-BSSs account for around 10% of the giants in our standard simulation. We also compare the properties of post-BI giants that evolved from different channels. We find that the initial orbital period and mass ratio distributions significantly affect the fraction of post-BI giants. Our results imply that the non-standard stars from binary interactions provide a non-negligible contribution to the RGB stars in GCs, which should be considered in future investigations of the origin of multiple stellar populations.