Blue straggler stars in open clusters using Gaia: dependence on cluster parameters and possible formation pathways

Abstract

Blue straggler stars (BSSs) are the most massive stars in a cluster formed via binary or higher-order stellar interactions. Though the exact nature of such formation scenarios is difficult to pin down, we provide observational constraints on the different possible mechanism. In this quest, we first produce a catalogue of BSSs using Gaia DR2 data. Among the 670 clusters older than 300 Myr, we identified 868 BSSs in 228 clusters and 500 BSS candidates in 208 clusters. In general, all clusters older than 1 Gyr and massive than 1000 M⊙ have BSSs. The average number of BSSs increases with cluster age and mass, and there is a power-law relation between the cluster mass and the maximum number of BSSs in the cluster. We introduce the term fractional mass excess ( $\mathcal {M}_{e}$) for BSSs. We find that at least 54 per cent of BSSs have $\mathcal {M}_{e}$ < 0.5 (likely to have gained mass through a binary mass transfer (MT)), 30 per cent in the 1.0 < $\mathcal {M}_{e}$ <0.5 range (likely to have gained mass through a merger) and up to 16 per cent with $\mathcal {M}_{e}$ > 1.0 (likely from multiple mergers/MT). We also find that the percentage of low $\mathcal {M}_{e}$ BSSs increases with age, beyond 1–2 Gyr, suggesting an increase in formation through MT in older clusters. The BSSs are radially segregated, and the extent of segregation depends on the dynamical relaxation of the cluster. The statistics and trends presented here are expected to constrain the BSS formation models in open clusters.

Investigating the UV-excess in star clusters with $N$-body simulations: predictions for future CSST observations

“…The BSS in our simulations are 2-3 mag bluer in F U V − N U V color and 3 mag brighter in the F 555W band than the MSTO stars (see Figures 5 and 6). They generate a minor peak in the FUV spectra (Figure A.4) and more binary stars, consistent with the observations that BSs are more numerous in higher-mass and older clusters (Jadhav & Subramaniam 2021).…”

Investigating kinematics and dynamics of three open clusters towards Galactic anticentre

“…We have also calculated the values of colour excess ( − ), ( − ) and ( − ) as 0.26, 0.16 and 0.20 mag respectively. The stars which are bluer and brighter than the turn-off stars in the CMD of a star cluster are known as Blue straggler Stars (BSS), and evolved counterparts of these stars are known as Yellow Straggler stars (YSS) (Jadhav & Subramaniam 2021b). Based on their positions in the CMD in Gaia filters, we found seventeen blue straggler stars with membership probability ≥ 95 % and seven yellow straggler stars with membership probability ≥ 88 % in this cluster, as shown in blue and yellow, respectively.…”

A Fresh Look at AGB Stars in Galactic Open Clusters with Gaia: Impact on Stellar Models and the Initial–Final Mass Relation

“…Among the various BSS formation pathways proposed in literature, we mention mass transfer from a binary companion (McCrea 1964), mergers of close binary systems, and collisions of single stars (Hills & Day 1976;Leonard 1989). In principle, all these mechanisms can contribute to the production of BSSs: the most recent studies indicate that in open clusters the major fraction (55%-75%) of BSSs is produced through mass transfer in an interacting binary system, while a smaller role is to be attributed to collisional (15%) or merging processes (10%-30%) (e.g., Mathieu & Geller 2009;Geller et al 2013;Gosnell et al 2015;Jadhav & Subramaniam 2021;Leiner & Geller 2021).…”