Importance of the Initial Conditions for Star Formation - I. Cloud
  Evolution and Morphology
  We present a detailed parameter study of collapsing turbulent cloud cores,
varying the initial density profile and the initial turbulent velocity field.
We systematically investigate the influence of different initial conditions on
the star formation process, mainly focusing on the fragmentation, the number of
formed stars, and the resulting mass distributions. Our study compares four
different density profiles (uniform, Bonnor-Ebert type, $\rho\propto r^{-1.5}$,
and $\rho\propto r^{-2}$), combined with six different supersonic turbulent
velocity fields (compressive, mixed, and solenoidal, initialised with two
different random seeds each) in three-dimensional simulations using the
adaptive-mesh refinement, hydrodynamics code FLASH. The simulations show that
density profiles with flat cores produce hundreds of low-mass stars, either
distributed throughout the entire cloud or found in subclusters, depending on
the initial turbulence. Concentrated density profiles always lead to the
formation of one high-mass star in the centre of the cloud and, if at all,
low-mass stars surrounding the central one. In uniform and Bonnor-Ebert type
density distributions, compressive initial turbulence leads to local collapse
about 25% earlier than solenoidal turbulence. However, central collapse in the
steep power-law profiles is too fast for the turbulence to have any significant
influence. We conclude that (I) the initial density profile and turbulence
mainly determine the cloud evolution and the formation of clusters, (II) the
initial mass function (IMF) is not universal for all setups, and (III) that
massive stars are much less likely to form in flat density distributions. The
IMFs obtained in the uniform and Bonnor-Ebert type density profiles are more
consistent with the observed IMF, but shifted to lower masses.
