Stellar mergers as the origin of the blue main-sequence band in young star clusters

TL;DR

This study suggests that blue main-sequence stars in young clusters originate from stellar mergers, explaining their slow rotation and distinct position in the color-magnitude diagram, and highlights dual formation pathways for cluster stars.

Contribution

It introduces a merger-based origin for blue MS stars, supported by mass function analysis and merger timing, contrasting with traditional disk accretion models.

Findings

Blue MS stars are likely slow rotators from stellar mergers.

Mass function of blue MS stars is nearly flat.

Merger activity peaked early and persisted for tens of Myr.

Abstract

Recent high-quality Hubble Space Telescope (HST) photometry shows that the main sequences (MS) stars of young star clusters form two discrete components in the color-magnitude diagram (CMD). Based on their distribution in the CMD, we show that stars of the blue MS component can be understood as slow rotators originating from stellar mergers. We derive the masses of the blue MS stars, and find that they follow a nearly flat mass function, which supports their unusual formation path. Our results imply that the cluster stars gain their mass in two different ways, by disk accretion leading to rapid rotation, contributing to the red MS, or by binary merger leading to slow rotation and populating the blue MS. We also derive the approximate merger time of the individual stars of the blue MS component, and find a strong early peak in the merger rate, with a lower level merger activity prevailing for tens of Myr. This supports recent binary formation models, and explains new velocity dispersion measurements for members of young star clusters. Our findings shed new light on the origin of the bi-modal mass, spin, and magnetic field distributions of main-sequence stars.