Beyond the main sequence: Binary evolution pathways to blue stragglers in the Gaia era

I. Galactic open and globular clusters

Institute of Astrophysics, Pontificia Universidad Católica de Chile, Avenida Vicuńa Mackenna 4860, 7820436 Macul, Santiago, Chile

^★ Corresponding authors: ffcarrasco@uc.cl; pnayak@astro.puc.cl; nayakphy@gmail.com

Received: 10 September 2024
Accepted: 26 March 2025

Abstract

Context. The study of blue straggler stars (BSSs) provides insight into the mechanisms of stellar mass exchange during binary stellar evolution and the complex gravitational interactions within dense stellar systems. In combination, they enhance our understanding of the possible life cycles of stars and the evolutionary pathways of star clusters.

Aims. We study the populations of BSSs in 41 globular clusters (GCs) and 42 open clusters (OCs) based on photometry, proper motion, and parallax from the Gaia Data Release 3 (DR3). We confirm their cluster membership. We find a total of 4399 BSSs: All GCs show BSSs (3965 or ∼ 90% of the sample), whereas only 42 out of 129 studied OCs show BSSs (434 or ∼ 10% of the sample). Clusters younger than ∼ 500 Myr do not host any BSSs.

Methods. We derived their astrophysical parameters such as effective temperature, surface gravity, and mass based on colortemperature relations, isochrone models, and Gaia DR3 spectroscopy (if available). We found values for T_eff=(6800 ± 585) K and (7570 ± 1400) K and an average mass of ⟨M_BSS⟩=(1.02 ± 0.1) M_⊙ and ⟨ M_BSS⟩=(1.75 ± 0.45) M_⊙ for GCs and OCs, respectively. We finally computed the difference of the BSS mass and the main-sequence turn-off (MSTO) mass of its respective cluster, normalized by the MSTO mass, for every identified BSS.

Results. Based on this parameter and on the BSSs ages derived from isochrone models, we find that (i) GC BSSs that are most likely to be formed through collisions show a boost for ages ∼ 1 – 2 Gyr. This agrees with the ages for core-collapse events in GCs that were reported in previous studies. We also find (ii) a double sequence for GC BSSs that might indicate a pre- or post-merger or close-binary scenario.