Phase-space structures and stellar populations in the star-forming
  region NGC~ 2264
  In this work we analyse the structure of a subspace of the phase space of the
star-forming region NGC~ 2264 using the Spectrum of Kinematic Groupings (SKG).
We show that the SKG can be used to process a collection of star data to find
substructure at different scales. We have found structure associated with the
NGC~ 2264 region and also with the background area. In the NGC~ 2264 region, a
hierarchical analysis shows substructure compatible with that found in previous
specific studies of the area but with an objective, compact methodology that
allows us to homogeneously compare the structure of different clusters and
star-forming regions. Moreover, this structure is compatible with the different
ages of the main NGC~ 2264 star-forming populations. The structure found in the
field can be roughly associated with giant stars far in the background,
dynamically decoupled from NGC~ 2264, which could be related either with the
Outer Arm or Monoceros Ring. The results in this paper confirm the relationship
between structure in the RV phase-space subspace and different kinds of
populations, defined by other variables not necessarily analysed with the SKG,
such as age or distance, showing the importance of detecting phase-space
substructure in order to trace stellar populations in the broadest sense of the
word.
