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arXiv · 2607.18681

Examining the stellar-merger origin of the blue main sequence in the open cluster NGC\,3532 with N-body simulations

Khushboo K. Rao·Sambaran Banerjee
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Abstract

Extended main-sequence turnoffs, extended main sequences, and split main sequences observed in the colour-magnitude diagrams of young and intermediate-age star clusters are now widely interpreted as the consequence of a distribution of stellar rotation rates among their intermediate-mass (1.5-1.8~M$_\odot$) members. However, the origin of the slowly rotating population that occupies the blue main sequence (bMS) remains uncertain, and stellar mergers have been proposed as one possible pathway. We investigate whether stellar mergers can account for the observed bMS population in the 330-Myr Galactic open cluster NGC 3532. We perform fourteen direct NBODY7 simulations spanning different initial cluster masses, radii, binary fractions, and binary orbital distributions. The simulations are selected to reproduce the present-day properties of NGC 3532 within the observational uncertainties, allowing us to estimate the expected number of merger products among its intermediate-mass main-sequence (MS) members. The cluster hosts $\approx 37\%$ bMS members among its intermediate-mass MS population, which are predominantly slow rotators. Despite this large bMS population, our simulations produce only a handful of MS-MS merger products, due to the cluster's low density and substantial mass loss during its evolution. Stellar mergers are unlikely to be the dominant formation channel for the observed slowly rotating bMS population in NGC 3532 and other disperse open clusters. Our results instead favour angular-momentum loss mechanisms operating before or shortly after the zero-age main sequence, such as pre-main-sequence star-disk interactions or tidal synchronization in low-mass ratio binaries.

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Khushboo K. Rao, Sambaran Banerjee. 2026-07-21. Examining the stellar-merger origin of the blue main sequence in the open cluster NGC\,3532 with N-body simulations. https://arxiv.org/abs/2607.18681

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