Star clusters have long been considered to comprise a simple stellar population, but this paradigm is being challenged, since in addition to multiple populations in Galactic globular clusters1, a number of younger star clusters exhibit a significant colour spread at the main sequence turn-off2,3,4,5,6,7,8,9. A sequential evolution of multiple generations of stars formed over 100–200 Myr is a natural explanation of this colour spread10. Another approach to explain this feature is to introduce the effect of stellar rotation11. However, its effectiveness has not yet been proven due to the lack of direct measurements of rotational velocities. Here, we report the distribution of projected rotational velocities (Veqsin i) of stars in the Galactic open cluster M11, measured by Fourier transform analysis. Cluster members display a broad Veqsin i distribution, and fast rotators, including Be stars, have redder colours than slow rotators. Monte Carlo simulations infer that cluster members have highly aligned spin axes and a broad distribution of equatorial velocities biased towards high velocities. Our synthetic cluster simulation further demonstrates how stellar rotation affects the colours of cluster members, suggesting that the colour spread observed in populous clusters can be understood in the context of stellar evolution without introducing multiple stellar populations.
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In this paper, we use publicly available data: photometry from ref. 16, evolutionary tracks from the Geneva Stellar Evolution group (ref. 25; https://www.unige.ch/sciences/astro/evolution/en/?lang=en), astrometry from Gaia Data Release 2 (https://www.cosmos.esa.int/gaia) and spectra from the Gaia–ESO Survey (https://www.gaia-eso.eu/data-products/public-data-releases). New MMT Hα spectra (shown in Supplementary Fig. 1) are available for download at http://hdl.handle.net/2268/228255. The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.
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The authors thank M. Bessell, S. Ekström and D. Gray for helpful comments, as well as P. Berlind, M. Calkins, C. Ly, S. Kattner and N. Caldwell for assisting with the Hectochelle observations. B.L. is grateful for assistance from S. Kim in running the simulation codes. This research was supported by the Basic Science Research Program through the National Research Foundation (NRF) of Korea (grant number NRF-2017R1A6A3A03006413), and the BK21 plus programme through the NRF funded by the Ministry of Education of Korea. This uses data obtained under the K-GMT Science Program (PID: MMT-2017A-1) funded through the K-GMT Project operated by the Korea Astronomy and Space Science Institute (KASI), and from the European Space Agency mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the Gaia Data Processing and Analysis Consortium has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. Y.N. (an FNRS research associate) and G.R. acknowledge support from the FNRS and the PRODEX contract, Belgium. H.S. acknowledges support from the NRF of Korea (grant number NRF-2015R1D1A1A01058444). N.H. acknowledges support from the Large Optical Telescope Project operated by KASI. B.-G.P. acknowledges support from the K-GMT Project operated by KASI.