The Gaia-ESO Survey and CSI 2264: Substructures, disks, and sequential
  star formation in the young open cluster NGC 2264
  We explore the structure and star formation history of the open cluster NGC
2264 (~3 Myr). We combined spectroscopic data from the Gaia-ESO Survey (GES)
with multi-wavelength photometry from the Coordinated Synoptic Investigation of
NGC 2264 (CSI 2264). We examined a sample of 655 cluster members, including
both disk-bearing and disk-free young stars. We find a significant age spread
of 4-5 Myr among cluster members. Disk-bearing objects are statistically
associated with younger isochronal ages than disk-free sources. The cluster has
a hierarchical structure, with two main blocks. The northern half develops
around the O-type binary star S Mon; the southern half, close to the tip of the
Cone Nebula, contains the most embedded regions of NGC 2264, populated mainly
by objects with disks and ongoing accretion. The median ages of objects at
different locations within the cluster, and the spatial distribution of disked
and non-disked sources, suggest that star formation began in the north of the
cluster, over 5 Myr ago, and was ignited in its southern region a few Myr
later. Star formation is likely still ongoing in the most embedded regions of
the cluster, while the outer regions host a widespread population of more
evolved objects. We find a detectable lag between the typical age of
disk-bearing objects and that of accreting objects in the inner regions of NGC
2264: the first tend to be older than the second, but younger than disk-free
sources at similar locations within the cluster. This supports earlier findings
that the characteristic timescales of disk accretion are shorter than those of
disk dispersal, and smaller than the average age of NGC 2264 (i.e., < 3 Myr).
At the same time, disks in the north of the cluster tend to be shorter-lived
(~2.5 Myr) than elsewhere; this may reflect the impact of massive stars within
the region (notably S Mon), that trigger rapid disk dispersal.
