The Average Magnetic Field Strength in Molecular Clouds: New Evidence of Super–Alfvénic 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 350 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, the super–Alfvénic simulation (model B) P (k) ∝ k. 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 , 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–Alfvénic turbulence in molecular clouds (model B) while model A is inconsistent (more than 99% confidence) with the observations.