Deep, multi-band photometry of low-mass stars to reveal young clusters:
  a blind study of the NGC 2264 region
  We aim to test a purely photometric approach to statistically identify a
young clustered population embedded in a large population of field stars, with
no prior knowledge on the nature of stars in the field. We conducted our blind
test study on the NGC 2264 region, which hosts a well-known young cluster. We
selected a large (4 sq. deg.) area around the NGC 2264 cluster, and assembled
an extensive r,i,J catalog of the field from pre-existing large-scale surveys.
We then mapped the stellar color locus on the (i-J, r-i) diagram to select
M-type stars, which: i) comprise a significant fraction of the Galactic stellar
population; ii) exhibit the strongest luminosity evolution from the PMS to the
MS; iii) have r,i,J color properties that provide a direct and empirical
estimate of A_V. A comparative analysis of the photometric and spatial
properties of M-type stars as a function of A_V enabled us to probe the
structure and stellar content of our field. We could identify two distinct
populations: a diffuse field population and a clustered population. The
presence of occulting material, spatially associated with the clustered
population, allowed us to derive an estimate of its distance (800-900 pc) and
age (~0.5-5 Myr), consistent with the literature parameters for the NGC 2264
star-forming region. The extracted clustered population exhibits a hierarchical
structure, in excellent agreement with the NGC 2264 subregions reported in the
literature. Our selection of clustered members is coherent with the literature
census of the NGC 2264 cluster for about 95% of the objects in the inner
regions of the field, where the contamination rate by field stars in our sample
is only 2%. The method tested here can be readily applied to surveys like
Pan-STARRS and the future LSST to undertake a first complete census of
low-mass, young star populations down to distances of several kpc across the
Galactic plane.
