When the tale comes true: multiple populations and wide binaries in the
  Orion Nebula Cluster
  The high-quality OmegaCAM photometry of the 3x3 deg around the Orion Nebula
Cluster (ONC) in r, and i filters by Beccari et al.(2017) revealed three
well-separated pre-main sequences in the color-magnitude diagram (CMD). The
objects belonging to the individual sequences are concentrated towards the
center of the ONC. The authors concluded that there are two competitive
scenarios: a population of unresolved binaries and triples with an exotic mass
ratio distribution, or three stellar populations with different ages. We use
Gaia DR2 in combination with the photometric OmegaCAM catalog to test and
confirm the presence of the putative three stellar populations. We also study
multiple stellar systems in the ONC for the first time using Gaia DR2. We
confirm that the second and third sequence members are more centrally
concentrated towards the center of the ONC. In addition we find an indication
that the parallax and proper motion distributions are different among the
members of the stellar sequences. The age difference among stellar populations
is estimated to be 1-2 Myr. We use Gaia measurements to identify and remove as
many unresolved multiple system candidates as possible. Nevertheless we are
still able to recover two well-separated sequences with evidence for the third
one, supporting the existence of the three stellar populations. We were able to
identify a substantial number of wide binary objects (separation between
1000-3000 au). This challenges previously inferred values that suggested no
wide binary stars exist in the ONC. Our inferred wide-binary fraction is approx
5%. We confirm the three populations correspond to three separated episodes of
star formation. Based on this result, we conclude that star formation is not
happening in a single burst in this region. (abridged)
