The Two States of Starforming Clouds
Abstract
We examine the effects of selfgravity and magnetic fields on supersonic turbulence in isothermal molecular clouds with highresolution simulations and adaptive mesh refinement. These simulations use large root grids (512^{3}) to capture turbulence and four levels of refinement to follow the collapse to high densities, for an effective resolution of 8192^{3}. Three Mach 9 simulations are performed, two superAlfvénic and one transAlfvénic. We find that gravity splits the clouds into two populations, one lowdensity turbulent state and one highdensity collapsing state. The lowdensity state exhibits properties similar to nonselfgravitating in this regime, and we examine the effects of varied magnetic field strength on statistical properties: the density probability distribution function is approximately lognormal, the velocity power spectral slopes decrease with decreasing mean field strength, the alignment between velocity and magnetic field increases with the field, and the magnetic field probability distribution can be fitted to a stretched exponential. The highdensity state is well characterized by selfsimilar spheres: the density probability distribution is a power law, collapse rate decreases with increasing mean field, density power spectra have positive slopes, P(ρ, k)vpropk, thermaltomagnetic pressure ratios are roughly unity for all mean field strengths, dynamictomagnetic pressure ratios are larger than unity for all mean field strengths, the magnetic field distribution follows a powerlaw distribution. The high Alfvén Mach numbers in collapsing regions explain the recent observations of magnetic influence decreasing with density. We also find that the highdensity state is typically found in filaments formed by converging flows, consistent with recent Herschel observations. Possible modifications to existing star formation theories are explored. The overall transAlfvénic nature of starforming clouds is discussed.
 Publication:

The Astrophysical Journal
 Pub Date:
 May 2012
 DOI:
 10.1088/0004637X/750/1/13
 arXiv:
 arXiv:1202.2594
 Bibcode:
 2012ApJ...750...13C
 Keywords:

 magnetohydrodynamics: MHD;
 stars: formation;
 Astrophysics  Solar and Stellar Astrophysics
 EPrint:
 19 pages, 20 figures