Supernova Driving. I. The Origin of Molecular Cloud Turbulence
  Turbulence is ubiquitous in molecular clouds (MCs), but its origin is still
unclear because MCs are usually assumed to live longer than the turbulence
dissipation time. Interstellar medium (ISM) turbulence is likely driven by SN
explosions, but it has never been demonstrated that SN explosions can establish
and maintain a turbulent cascade inside MCs consistent with the observations.
In this work, we carry out a simulation of SN-driven turbulence in a volume of
(250 pc)$^3$, specifically designed to test if SN driving alone can be
responsible for the observed turbulence inside MCs. We find that SN driving
establishes a velocity scaling consistent with the usual scaling laws of
supersonic turbulence, suggesting that previous idealized simulations of MC
turbulence, driven with a random, large-scale volume force, were correctly
adopted as appropriate models for MC turbulence, despite the artificial
driving. We also find that the same scaling laws extend to the interior of MCs,
and that the velocity-size relation of the MCs selected from our simulation is
consistent with that of MCs from the Outer-Galaxy Survey, the largest MC sample
available. The mass-size relation and the mass and size probability
distributions also compare successfully with those of the Outer Galaxy Survey.
Finally, we show that MC turbulence is super-Alfv\'{e}nic with respect to both
the mean and rms magnetic-field strength. We conclude that MC structure and
dynamics are the natural result of SN-driven turbulence.
