# A Tale of Three Cities: OmegaCAM discovers multiple sequences in the color-magnitude diagram of the Orion Nebula Cluster

G. Beccari, M.G. Petr-Gotzens, H.M.J. Boffin, M. Romaniello, D. Fedele, G. Carraro, G. De Marchi, W.J. de Wit, J. E. Drew, V. M. Kalari, Carlo Felice Manara, Eduardo L. Martin, S. Mieske, N. Panagia, L. Testi, Jorick S. Vink, J. R. Walsh, N. J. Wright

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## Abstract

As part of the Accretion Discs in H$\alpha$ with OmegaCAM (ADHOC) survey, we imaged in r, i and H-alpha a region of 12x8 square degrees around the Orion Nebula Cluster. Thanks to the high-quality photometry obtained, we discovered three well-separated pre-main sequences in the color-magnitude diagram. The populations are all concentrated towards the cluster's center. Although several explanations can be invoked to explain these sequences we are left with two competitive, but intriguing, scenarios: a population of unresolved binaries with an exotic mass ratio distribution or three populations with different ages. Independent high-resolution spectroscopy supports the presence of discrete episodes of star formation, each separated by about a million years. The stars from the two putative youngest populations rotate faster than the older ones, in agreement with the evolution of stellar rotation observed in pre-main sequence stars younger than 4 Myr in several star forming regions. Whatever the final explanation, our results prompt for a revised look at the formation mode and early evolution of stars in clusters.
Original language English A22 8 Astronomy & Astrophysics 604 A&A 27 Jul 2017 https://doi.org/10.1051/0004-6361/201730432 E-pub ahead of print - 27 Jul 2017

• astro-ph.SR
• astro-ph.GA

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