Dynamo effect in unstirred self-gravitating turbulence
  In many astrophysical environments, self-gravity can generate kinetic energy,
which, in principle, is available for driving dynamo action. Using direct
numerical simulations, we show that in unstirred self-gravitating subsonic
turbulence with helicity and a magnetic Prandtl number of unity, there is a
critical magnetic Reynolds number of about 25 above which the work done against
the Lorentz force exceeds the Ohmic dissipation. The collapse itself drives
predominantly irrotational motions that cannot be responsible for dynamo
action. We find that, with a weak magnetic field, one-third of the work done by
the gravitational force goes into compressional heating and the remaining
two-thirds go first into kinetic energy of the turbulence before a fraction of
it is converted further into magnetic and finally thermal energies. Close to
the collapse, however, these fractions change toward 1/4 and 3/4 for
compressional heating and kinetic energy, respectively. When the magnetic field
is strong, the compressional heating fraction is unchanged. Out of the
remaining kinetic energy, one quarter goes directly into magnetic energy via
work against the Lorentz force. The fraction of vortical motions diminishes in
favor of compressive motions that are almost exclusively driven by the Jeans
instability. For an initially uniform magnetic field, field amplification at
scales larger than those of the initial turbulence are driven by tangling.
