Unbound Young Stellar Systems: Star Formation on the loose
  [abridged] Unbound young stellar systems, the loose ensembles of physically
related young bright stars, trace the typical regions of recent star formation
in galaxies. Their morphologies vary from small associations of stars to
enormous stellar complexes. Being associated with star-forming regions of
various sizes, they trace the regions where stars form at various scales, from
compact clusters to whole galactic disks. They have been, thus, the focus of
several studies with special interest on their demographics, classification,
and structural morphology. Their surveys demonstrate that the clear distinction
of these systems into well-defined classes is not straightforward, due to their
low densities, asymmetric shapes and variety in structural parameters. Unbound
stellar structures follow a hierarchical clustering pattern up to the scale of
a whole star-forming galaxy. This structural pattern, which is usually
characterized as self-similar or fractal, appears to be identical to that of
star-forming giant molecular clouds and interstellar gas, driven mainly by
turbulence cascade. In this short review, I make a concise compilation of our
understanding of unbound young stellar systems across various environments in
the local universe, as it is developed during the last 60 years. I present a
factual assessment of the clustering behavior of star formation, as revealed
from the assembling pattern of stars across loose stellar structures and its
relation to the interstellar medium and the environmental conditions. I also
provide a consistent account of the processes that possibly play important role
in the formation of unbound stellar systems, compiled from both theoretical and
observational investigations on the field.
