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

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

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Author

Beccari, G.

Petr-Gotzens, M.G.

Boffin, H.M.J.

Romaniello, M.

Fedele, D.

Carraro, G.

Marchi, G. De

de Wit, W.J.

Drew, J. E.

Kalari, V. M.

Manara, Carlo Felice

Martin, Eduardo L.

Mieske, S.

Panagia, N.

Testi, L.

Vink, Jorick S.

Walsh, J. R.

Wright, N. J.

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.