Unifying Main-sequence Morphologies in Young Open Clusters: Binary Evolution and the Emergence of Bridge-like Populations

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Abstract Most massive stars reside in binary systems, and binary interactions can profoundly alter the properties of both stellar components. In young open clusters, such interactions can produce multiple main-sequence populations, despite the coeval nature of the stellar population. In this work, we extend our previous studies on the role of binary evolution in shaping main-sequence morphologies by performing a more detailed and systematic analysis. We investigate the progenitor properties of different main-sequence populations from 10 to 100 Myr using detailed binary models computed with MESA, assuming SMC-like metallicity, and initial primary masses of 3–100 M ⊙ . We provide physical explanations for the features identified in our earlier work. We focus especially on post–Case A mass-transfer systems, which produce a prominent “bridge-like” feature connecting the reddest Be stars and the bluest blue stragglers. We show that the positions of these stars are sensitive to surface helium enrichment and rotation. We further perform quantitative comparisons with observed clusters aged 30–90 Myr whose turn-off masses are from approximately 9 to 5 M ⊙ . We find that our models broadly reproduce the observed stellar distributions. However, we underpredict the fraction of Be stars, suggesting that a larger fraction of binaries may undergo stable mass transfer. We also overpredict the number of stars on the redder side of the unresolved-binary sequence, indicating that the initial mass-ratio distribution may deviate from a flat distribution. Overall, this work provides a comprehensive analysis of how binary evolution produces diverse main-sequence populations and lays the foundation for future population-synthesis studies.

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