Testing the role of merging binaries in the formation of the split main sequence in young clusters

¹ Donostia International Physics Center (DIPC), Paseo Manuel de Lardizabal, 4, 20018, Donostia-San Sebastián, Guipuzkoa, Spain
² IKERBASQUE, Basque Foundation for Science, 48013 Bilbao, Spain
³ Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK
⁴ Leibniz-Institut für Astrophysik Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
⁵ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy

^★ Corresponding author: nate.bastian@dipc.org

Received: 2 May 2025
Accepted: 4 July 2025

Abstract

A number of theories have been put forward to explain the bimodal stellar rotational distribution observed in young massive clusters. These include stellar mergers and interactions induced in binary systems, and the role of angular momentum transfer between a star and its circumstellar disk in its early evolution. Each theory predicts unique rotation distributions in various locations of the color-magnitude diagram. Specifically, the stellar merger hypothesis posits that the upper end of the main sequence will host a significant number of slowly rotating merger products, i.e., the blue straggler stars are an extension of the blue main sequence. We used observations, a combination of HST photometry and VLT/MUSE spectroscopy, of three massive (∼10⁵ M_☉) young (100–300 Myr) clusters in the Large Magellanic Cloud. We show that in all three clusters, these bright blue stars have stellar rotational distributions that differ significantly from that measured on the blue main sequence. We conclude that stellar mergers do not play a significant role in the formation of the split main sequence or the bimodal rotational distribution. As a corollary, we show that blue straggler stars in these young massive clusters display a wide range of rotational velocities.