On the properties and implications of collapse-driven MHD turbulence

Abstract

We numerically investigate the driving of MHD turbulence by gravitational contraction using simulations of an initially spherical, magnetically supercritical cloud core with initially transonic and trans-Alfvénic turbulence. We perform a Helmholtz decomposition of the velocity field, and investigate the evolution of its solenoidal and compressible parts, as well as of the velocity component along the gravitational acceleration vector, a proxy for the infall component of the velocity field. We find that: 1) In spite of being supercritical, the core first contracts to a sheet perpendicular to the mean field, and the sheet itself collapses. 2) The solenoidal component of the turbulence remains at roughly its initial level throughout the simulation, while the compressible component increases continuously. This implies that turbulence does {\it not} dissipate towards the center of the core. 3) The distribution of simulation cells in the $B$-$ρ$ plane occupies a wide triangular region at low densities, bounded below by the expected trend for fast MHD waves ($B \propto ρ$, applicable for high local Alfvénic Mach number $\Ma$) and above by the trend expected for slow waves ($B \sim$ constant, applicable for low local $\Ma$). At high densities, the distribution follows a single trend $B \propto ρ^{\gamef}$, with $1/2 < \gamef < 2/3$, as expected for gravitational compression. 4) The measured mass-to-magnetic flux ratio $λ$ increases with radius $r$, due to the different scalings of the mass and magnetic flux with $r$. At a fixed radius, $λ$ increases with time due to the accretion of material along field lines. 5) The solenoidal energy fraction is much smaller than the total turbulent component, indicating that the collapse drives the turbulence mainly compressibly, even in directions orthogonal to that of the collapse.