Star Cluster Formation from Turbulent Clumps. I. The Fast Formation
  Limit
  We investigate the formation and early evolution of star clusters assuming
that they form from a turbulent starless clump of given mass bounded inside a
parent self-gravitating molecular cloud characterized by a particular mass
surface density. As a first step we assume instantaneous star cluster formation
and gas expulsion. We draw our initial conditions from observed properties of
starless clumps. We follow the early evolution of the clusters up to 20 Myr,
investigating effects of different star formation efficiencies, primordial
binary fractions and eccentricities and primordial mass segregation levels. We
investigate clumps with initial masses of $M_{\rm cl}=3000\:{\rm M}_\odot$
embedded in ambient cloud environments with mass surface densities,
$\Sigma_{\rm cloud}=0.1$ and $1\:{\rm g\:cm^{-2}}$. We show that these models
of fast star cluster formation result, in the fiducial case, in clusters that
expand rapidly, even considering only the bound members. Clusters formed from
higher $\Sigma_{\rm cloud}$ environments tend to expand more quickly, so are
soon larger than clusters born from lower $\Sigma_{\rm cloud}$ conditions. To
form a young cluster of a given age, stellar mass and mass surface density,
these models need to assume a parent molecular clump that is many times denser,
which is unrealistic compared to observed systems. We also show that in these
models the initial binary properties are only slightly modified by
interactions, meaning that binary properties, e.g., at 20 Myr, are very similar
to those at birth. With this study we set up the basis of future work where we
will investigate more realistic models of star formation compared to this
instantaneous, baseline case.
