The Average Magnetic Field Strength in Molecular Clouds: New Evidence of Super-Alfvnic Turbulence

The magnetic field strength in molecular clouds is a fundamental quantity for theories of star formation. It is estimated by Zeeman splitting measurements in a few dense molecular cores, but its volume--averaged value within large molecular clouds (over several parsecs) is still uncertain. In this work we provide a new method to constrain the average magnetic field strength in molecular clouds. We compare the power spectrum of gas density of molecular clouds with that of two 350³ numerical simulations of supersonic MHD turbulence. The numerical simulation with approximate equipartition of kinetic and magnetic energies (model A) yields the column density power spectrum P(k)∝ k-2.25± 0.01, the super--Alfv\'{e}nic simulation (model B) P(k)∝ k-2.71± 0.01. The column density power spectrum of the Perseus, Taurus and Rosetta molecular cloud complexes is found to be well approximated by a power law, Pₒ(k)∝ k⁻ᵃ, with a=2.74± 0.07, 2.74± 0.08 and 2.76± 0.08 respectively. We conclude that the observations are consistent with the presence of super--Alfv\'{e}nic turbulence in molecular clouds (model B) while model A is inconsistent (more than 99% confidence) with the observations.