From Clouds to Cores to Protostars and Disks:
New Insights from Numerical Simulations

Shantanu Basu
University of Western Ontario

Understanding the sequence of events that turns
turbulent, magnetized, partially ionized molecular
cloud gas into stars is an outstanding challenge
of modern astrophysics. Our numerical simulations
of various stages of the star formation process
reveal several new features of turbulence,
fragmentation, and disk accretion.

We find that turbulent motions, even when driven
from local sources, can quickly fill a cloud with
motions in which most of the power is present on
the largest scales. The densest regions of the clouds
contain subsonic or transonic turbulent motions
and there the core formation process may be driven
by gravity. When mediated by energetically significant
magnetic fields and ion-neutral friction, a unique
fragmentation process is discovered which can be
identified with the isolated mode of star formation.
When we follow the collapse of a nonaxisymmetric
disk down to the formation of a protostar and disk,
the mass accumulation of the protostar begins in a
well-known "smooth mode" of accretion, but then
proceeds through a newly discovered "burst mode"
that can explain the FU Ori phenomenon. Our models of
disk accretion also have have important implications
for the possibility of giant planet formation by
gravitational instability.