On the universality of interstellar filaments: theory meets simulations
  and observations
  Filaments are ubiquitous in the universe. Recent observations have revealed
that stars and star clusters form preferentially along dense filaments.
Understanding the formation and properties of filaments is therefore a crucial
step in understanding star formation. Here we perform three-dimensional
high-resolution magnetohydrodynamical simulations that follow the evolution of
molecular clouds and the formation of filaments and stars. We apply a filament
detection algorithm and compare simulations with different combinations of
physical ingredients: gravity, turbulence, magnetic fields and jet/outflow
feedback. We find that gravity-only simulations produce significantly narrower
filament profiles than observed, while simulations that include turbulence
produce realistic filament properties. For these turbulence simulations, we
find a remarkably universal filament width of 0.10 +/- 0.02 pc, which is
independent of the star formation history of the clouds. We derive a
theoretical model that provides a physical explanation for this characteristic
filament width, based on the sonic scale (lambda_sonic) of molecular cloud
turbulence. Our derivation provides lambda_sonic as a function of the cloud
diameter L, the velocity dispersion sigma_v, the gas sound speed c_s, and the
ratio of thermal to magnetic pressure, plasma beta. For typical cloud
conditions in the Milky Way spiral arms, we find lambda_sonic = 0.04-0.16 pc,
in excellent agreement with the filament width of 0.05-0.15 pc from
observations. Consistent with the theoretical model assumptions, we find that
the velocity dispersion inside the filaments is subsonic and supersonic
outside. We further explain the observed p=2 scaling of the filament density
profile, rho ~ r^(-p) with the collision of two planar shocks forming a
filament at their intersection.
