The clustered nature of star formation. Pre--main-sequence clusters in
  the star-forming region NGC 602/N90 in the Small Magellanic Cloud
  Located at the tip of the wing of the Small Magellanic Cloud (SMC), the
star-forming region NGC602/N90 is characterized by the HII nebular ring N90 and
the young cluster of pre--main-sequence (PMS) and early-type main sequence
stars NGC602. We present a thorough cluster analysis of the stellar sample
identified with HST/ACS camera in the region. We show that apart from the
central cluster, low-mass PMS stars are congregated in thirteen additional
small compact sub-clusters at the periphery of NGC602. We find that the spatial
distribution of the PMS stars is bimodal, with an unusually large fraction
(~60%) of the total population being clustered, while the remaining is
diffusely distributed in the inter-cluster area. From the corresponding
color-magnitude diagrams we disentangle an age-difference of ~2.5Myr between
NGC602 and the compact sub-clusters which appear younger. The diffuse PMS
population appears to host stars as old as those in NGC602. Almost all detected
PMS sub-clusters appear to be centrally concentrated. When the complete PMS
stellar sample, including both clustered and diffused stars, is considered in
our cluster analysis, it appears as a single centrally concentrated stellar
agglomeration, covering the whole central area of the region. Considering also
the hot massive stars of the system, we find evidence that this agglomeration
is hierarchically structured. Based on our findings we propose a scenario,
according to which the region NGC602/N90 experiences an active clustered star
formation for the last ~5Myr. The central cluster NGC602 was formed first and
rapidly started dissolving into its immediate ambient environment, possibly
ejecting also massive stars found away from its center. Star formation
continued in sub-clusters of a larger stellar agglomeration, introducing an
age-spread of the order of 2.5Myr among the PMS populations.
