Accretion-Driven Turbulence as Universal Process: Galaxies, Molecular
  Clouds, and Protostellar Disks
  Complex turbulent motions are ubiquitously observed in many astrophysical
systems. Their origin, however, is still poorly understood. When cosmic
structures form, they grow in mass via accretion from the surrounding
environment. We propose that this accretion is able to drive internal turbulent
motions in a wide range of astrophysical objects and study this process in the
case of galaxies, molecular clouds and protoplanetary disks. We use a
combination of numerical simulations and analytical arguments to predict the
level of turbulence as a function of the accretion rate, the dissipation scale,
and the density contrast, and compare with observational data. We find that in
Milky Way type galaxies the observed level of turbulence in the interstellar
medium can be explained by accretion, provided that the galaxies gain mass at a
rate comparable to the rate at which they form stars. This process is
particularly relevant in the extended outer disks beyond the star-forming
radius. We also calculate the rate at which molecular clouds grow in mass when
they build up from the atomic component of the galactic gas and find that their
internal turbulence is likely to be driven by accretion as well. It is the very
process of cloud formation that excites turbulent motions on small scales by
establishing the turbulent cascade. In the case of T Tauri disks, we show that
accretion can drive subsonic turbulence at the observed level if the rate at
which gas falls onto the disk is comparable to the rate at which disk material
accretes onto the central star. This also explains the observed relation of
accretion rate and stellar mass, dM/dt ~ M^1.8. The efficiency required to
convert infall motion into turbulence is of the order of a few percent in all
three cases. We conclude that accretion-driven turbulence is a universal
concept with far-reaching implications for a wide range of astrophysical
objects.
