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