THE TWO STATES OF STAR-FORMING CLOUDS

Abstract

We examine the effects of self-gravity and magnetic fields on supersonic\nturbulence in isothermal molecular clouds with high resolution simulations and\nadaptive mesh refinement. These simulations use large root grids (512^3) to\ncapture turbulence and four levels of refinement to capture high density, for\nan effective resolution of 8,196^3. Three Mach 9 simulations are performed, two\nsuper-Alfv\\'enic and one trans-Alfv\\'enic. We find that gravity splits the\nclouds into two populations, one low density turbulent state and one high\ndensity collapsing state. The low density state exhibits properties similar to\nnon-self-gravitating in this regime, and we examine the effects of varied\nmagnetic field strength on statistical properties: the density probability\ndistribution function is approximately lognormal; velocity power spectral\nslopes decrease with field strength; alignment between velocity and magnetic\nfield increases with field; the magnetic field probability distribution can be\nfit to a stretched exponential. The high density state is characterized by\nself-similar spheres; the density PDF is a power-law; collapse rate decreases\nwith increasing mean field; density power spectra have positive slopes,\nP({\\rho},k) \\propto k; thermal-to-magnetic pressure ratios are unity for all\nsimulations; dynamic-to-magnetic pressure ratios are larger than unity for all\nsimulations; magnetic field distribution is a power-law. The high Alfv\\'en Mach\nnumbers in collapsing regions explain recent observations of magnetic influence\ndecreasing with density. We also find that the high density state is found in\nfilaments formed by converging flows, consistent with recent Herschel\nobservations. Possible modifications to existing star formation theories are\nexplored.\n