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 350 3 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-Alfvénic 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, P 0 ( 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-Alfvénic turbulence in molecular clouds (model B), while model A is inconsistent (more than 99% confidence) with the observations.