Mass segregation and sequential star formation in NGC 2264 revealed by
  Herschel
  The mass segregation of stellar clusters could be primordial rather than
dynamical. Despite the abundance of studies of mass segregation for stellar
clusters, those for stellar progenitors are still scarce, so the question on
the origin and evolution of mass segregation is still open. Our goal is to
characterize the structure of the NGC 2264 molecular cloud and compare the
populations of clumps and young stellar objects (YSOs) in this region whose
rich YSO population has shown evidence of sequential star formation. We
separated the Herschel column density map of NGC 2264 in three subregions and
compared their cloud power spectra using a multiscale segmentation technique.
We identified in the whole NGC 2264 cloud a population of 256 clumps with
typical sizes of ~0.1 pc and masses ranging from 0.08 Msun to 53 Msun. Although
clumps have been detected all over the cloud, the central subregion of NGC 2264
concentrates most of the massive, bound clumps. The local surface density and
the mass segregation ratio indeed indicate a strong degree of mass segregation
for the 15 most massive clumps, with a median $\Sigma_6$ three time that of the
whole clumps population and $\Lambda_{MSR}$ about 8. We showed that this
cluster of massive clumps is forming within a high-density cloud ridge, itself
formed and probably still fed by the high concentration of gas observed on
larger scales in the central subregion. The time sequence obtained from the
combined study of the clump and YSO populations in NGC 2264 suggests that the
star formation started in the northern subregion, that it is now actively
developing at the center and will soon start in the southern subregion. Taken
together, the cloud structure and the clump and YSO populations in NGC 2264
argue for a dynamical scenario of star formation.
