Orion Revisited - I. The massive cluster in front of the Orion Nebula
  Cluster
  The aim of this work is to characterize the stellar population between Earth
and the Orion A molecular cloud where the well known star formation benchmark
Orion Nebula Cluster (ONC) is embedded. We use the denser regions the Orion A
cloud to block optical background light, effectively isolating the stellar
population in front of it. We then use a multi-wavelength observational
approach to characterize the cloud's foreground stellar population. We find
that there is a rich stellar population in front of the Orion A cloud, from
B-stars to M-stars, with a distinct 1) spatial distribution, 2) luminosity
function, and 3) velocity dispersion from the reddened population inside the
Orion A cloud. The spatial distribution of this population peaks strongly
around NGC 1980 (iota Ori) and is, in all likelihood, the extended stellar
content of this poorly studied cluster. We infer an age of ~4-5 Myr for NGC
1980 and estimate a cluster population of the order of 2000 stars, which makes
it one of the most massive clusters in the entire Orion complex. What is
currently taken in the literature as the ONC is then a mix of several
intrinsically different populations, namely: 1) the youngest population,
including the Trapezium cluster and ongoing star formation in the dense gas
inside the nebula, 2) the foreground population, dominated by the NGC 1980
cluster, and 3) the poorly constrained population of foreground and background
Galactic field stars. Our results support a scenario where the ONC and L1641N
are not directly associated with NGC 1980, i.e., they are not the same
population emerging from its parental cloud, but are instead distinct
overlapping populations. This result calls for a revision of most of the
observables in the benchmark ONC region (e.g., ages, age spread, cluster size,
mass function, disk frequency, etc.). (abridged)
