How can young massive clusters reach their present-day sizes?
  The classic question that how young massive star clusters attain their shapes
and sizes, as we find them today, remains to be a challenge. Both observational
and computational studies of star-forming massive molecular gas clouds infer
that massive cluster formation is primarily triggered along the small-scale
($\lesssim0.3$ pc) filamentary substructures within the clouds. The present
study is intended to investigate the possible ways in which a
filament-like-compact, massive star cluster (effective radius 0.1-0.3 pc) can
expand $\gtrsim10$ times, still remaining massive enough ($\gtrsim10^4
M_\odot$), to become a young massive star cluster, as we observe today. To that
end, model massive clusters (of initially $10^4 M_\odot-10^5 M_\odot$) are
evolved using Sverre Aarseth's state-of-the-art N-body code NBODY7. All the
computed clusters expand with time, whose sizes (effective radii) are compared
with those observed for young massive clusters, of age $\lesssim100$ Myr, in
the Milky Way and other nearby galaxies. It is found that beginning from the
above compact sizes, a star cluster cannot expand by its own, i.e., due to
two-body relaxation, stellar-evolutionary mass loss, dynamical heating by
primordial binaries and stellar-mass black holes, up to the observed sizes of
young massive clusters; they always remain much more compact compared to the
observed ones. This calls for additional mechanisms that can boost the
expansion of a massive cluster after its assembly. Using further N-body
calculations, it is shown that a substantial residual gas expulsion, with
$\approx30$% star formation efficiency, can indeed swell the newborn embedded
cluster adequately. The limitations of the present calculations and their
consequences are discussed.
