A tale of three cities

OmegaCAM discovers multiple sequences in the color-magnitude diagram of the Orion Nebula Cluster

¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
e-mail: gbeccari@eso.org
² INAF–Osservatorio Astrofisico di Arcetri, L.go E. Fermi 5, 50125 Firenze, Italy
³ Dipartimento di Fisica e Astronomia Galileo Galilei, Vicolo Osservatorio 3, 35122 Padova, Italy
⁴ Science Support Office, Directorate of Science, European Space Research and Technology Centre (ESA/ESTEC), Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
⁵ European Southern Observatory, Alonso de Córdova 3107, 19001 Casilla Santiago, Chile
⁶ School of Physics, Astronomy and Mathematics, University of Hertfordshire, College Lane Campus, Hatfield, AL10 9AB, UK
⁷ Departamento de Astronomía, Universidad de Chile, Casilla 36-D Correo Central, Santiago, Chile
⁸ CSIC-INTA Centro de Astrobiologia Carretera Ajalvir km 4, 28550 Madrid, Spain
⁹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹⁰ Armagh Observatory, College Hill, Armagh BT61 9DG, UK
¹¹ Astrophysics Group, Keele University, Keele ST5 5BG, UK
¹² Excellence Cluster Universe, Garching bei München, Germany

Received: 13 January 2017
Accepted: 26 May 2017

Abstract

As part of the accretion discs in Hα with OmegaCAM (ADHOC) survey, we imaged a region of 12 × 8 square degrees around the Orion Nebula Cluster in r, i and Hα. 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.