The Average magnetic field strength in molecular clouds: New evidence of superAlfvenic turbulence

Nov, 2003
Published in:
  • Astrophys.J.Lett. 604 (2004) L49-L52
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Abstract: (arXiv)
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 3503350^3 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)k2.25±0.01P(k)\propto k^{-2.25\pm 0.01}, the super--Alfv\'{e}nic simulation (model B) P(k)k2.71±0.01P(k)\propto k^{-2.71\pm 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)kaP_{\rm o}(k)\propto k^{-a}, with a=2.74±0.07a=2.74\pm 0.07, 2.74±0.082.74\pm 0.08 and 2.76±0.082.76\pm 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.