Less wrong: a more realistic initial condition for simulations of
  turbulent molecular clouds
  Simulations of isolated giant molecular clouds (GMCs) are an important tool
for studying the dynamics of star formation, but their turbulent initial
conditions (ICs) are uncertain. Most simulations have either initialized a
velocity field with a prescribed power spectrum on a smooth density field
(failing to model the full structure of turbulence) or "stirred" turbulence
with periodic boundary conditions (which may not model real GMC boundary
conditions). We develop and test a new GMC simulation setup (called TURBSPHERE)
that combines advantages of both approaches: we continuously stir an isolated
cloud to model the energy cascade from larger scales, and use a static
potential to confine the gas. The resulting cloud and surrounding envelope
achieve a quasi-equilibrium state with the desired hallmarks of supersonic ISM
turbulence (e.g. density PDF and a $\sim k^{-2}$ velocity power spectrum),
whose bulk properties can be tuned as desired. We use the final stirred state
as initial conditions for star formation simulations with self-gravity, both
with and without continued driving and protostellar jet feedback, respectively.
We then disentangle the respective effects of the turbulent cascade, simulation
geometry, external driving, and gravity/MHD boundary conditions on the
resulting star formation. Without external driving, the new setup obtains
results similar to previous simple spherical cloud setups, but external driving
can suppress star formation considerably in the new setup. Periodic box
simulations with the same dimensions and turbulence parameters form stars
significantly slower, highlighting the importance of boundary conditions and
the presence or absence of a global collapse mode in the results of star
formation calculations.
