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, C.F. Manara, E.L. Martin, S. Mieske, N. Panagia, L. Testi, J.S. Vink, J.R. Walsh, N.J. Wright. 2017. A tale of three cities OmegaCAM discovers multiple sequences in the color-magnitude diagram of the Orion Nebula Cluster . Astronomy and Astrophysics 604, DOI: 10.1051/0004-6361/201730432
As part of the accretion discs in H alpha with OmegaCAM (ADHOC) survey, we imaged a region of 12×8 square degrees around the Orion Nebula Cluster in r, i and H alpha. 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 a revised look at the formation mode and early evolution of stars in clusters.