Hubble Space Telescope survey of Magellanic Cloud star clusters. Binaries among the split main sequences of NGC 1818, NGC 1850, and NGC 2164

TL;DR

This study uses Hubble Space Telescope data to analyze binary star populations in three Magellanic Cloud star clusters with split main sequences, revealing higher binary fractions among blue MS stars, which informs theories on their formation.

Contribution

It provides the first detailed comparison of binary fractions between blue and red main sequence stars in these clusters, constraining formation scenarios of split MSs.

Findings

Higher binary fractions among blue MS stars in all three clusters.

Binary fraction ratios vary significantly among the clusters.

Results support certain formation models over others for split MSs.

Abstract

Nearly all star clusters younger than ~600 Myr exhibit extended main sequence turn offs and split main sequences (MSs) in their color-magnitude diagrams. Works based on both photometry and spectroscopy have firmly demonstrated that the red MS is composed of fast-rotating stars, whereas blue MS stars are slow rotators. Nevertheless, the mechanism responsible for the formation of stellar populations with varying rotation rates remains a topic of debate. Potential mechanisms proposed for the split MS include binary interactions, early evolution of pre-main sequence stars, and the merging of binary systems, but a general consensus has yet to be reached. These formation scenarios predict different fractions of binaries among blue- and red-MS stars. Therefore, studying the binary populations can provide valuable constraints that may help clarify the origins of the split MSs. We use high-precision photometry from the Hubble Space Telescope (HST) to study the binaries of three young Magellanic star clusters exhibiting split MS, namely NGC 1818, NGC 1850, and NGC 2164. By analyzing the photometry in the F225W, F275W, F336W, and F814W filters for observed binaries and comparing it to a large sample of simulated binaries, we determine the fractions of binaries within the red and the blue MS. We find that the fractions of binaries among the blue MS are higher than those of red-MS stars by a factor of ~1.5, 4.6, and ~1.9 for NGC 1818, NGC 1850, and NGC 2164, respectively. We discuss these results in the context of the formation scenarios of the split MS.