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

¹ Dipartimento di Fisica e Astronomia “Galileo Galilei”, Università Degli Studi di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
² Istituto Nazionale di Astrofisica – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
³ Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Roma, Via Frascati 33, 00077 Monte Porzio Catone, Italy
⁴ Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia
⁵ School of Physics and Astronomy, Sun Yat-sen University, Zhuhai 519082, China
⁶ CSST Science Center for the Guangdong–Hong Kong-Macau Greater Bay Area, Zhuhai 519082, PR China
⁷ Key Laboratory for Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, PR China
⁸ School of Astronomy and Space Science, University of Chinese Academy of Sciences, Beijing 100049, PR China
⁹ Department of Astronomy, School of Physics and Astronomy, China West Normal University, Nanchong 637002, PR China
¹⁰ South-Western Institute for Astronomy Research, Yunnan University, Kunming 650500, PR China

^★ Corresponding author; fabrizio.muratore@studenti.unipd.it

Received: 29 June 2024
Accepted: 30 October 2024

Abstract

Nearly all star clusters younger than ~600 Myr exhibit extended main sequence turnoffs and split main sequences (MSs) in their color-magnitude diagrams. Works based on both photometry and spectroscopy have clearly 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, the early evolution of pre-MS 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 used high-precision photometry from the Hubble Space Telescope to study the binaries of three young Magellanic star clusters exhibiting split MSs, 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 determined the fractions of binaries within the red and blue MS. We find that the fractions of binaries among the blue-MS stars 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.