Anchoring Magnetic Fields in Turbulent Molecular Clouds II - from 0.1 to
  0.01 parsec
  We (Li et al. 2009; Paper-I) compared the magnetic field directions inferred
from polarimetry data obtained from 100-pc scale inter-cloud media (ICM) and
from sub-pc scale molecular cloud cores. The highly correlated result led us to
conclude that cloud turbulence must be sub-Alfvenic. Here we extend the study
with 0.01-pc cores observed by interferometers. The inferred field directions
at this scale significantly deviate from that of the surrounding ICM. An
obvious question to ask is whether this high-resolution result contradicts the
sub-Alfvenic picture concluded earlier. We performed MHD simulations of a
slightly super-critical (magnetic criticality = 2) clouds with Alfvenic Mach
number $M_A = 0.63$, which can reproduce the Paper-I results, and observed the
development towards smaller scales. Interestingly, all subregions hosting cores
with $n_H$$_2$ > $10^{5}$/cc (the typical density observed by interferometers)
possess $M_A = 2-3$. Not too surprisingly, these slightly super-Alfvenic cores
result in B-field orientation offsets comparable to the interferometer
observations. The result suggests that gravity can concentrate (and maybe also
contribute to, which takes more study to confirm) turbulent energy and create
slightly super-Alfvenic cores out from sub-Alfvenic clouds. The results of our
simulations also agree with the observed velocity-scale (Kauffmann et al.
2013), mass-scale (Lombardi et al. 2010) and field strength-density (Li et al.
2015; Crutcher et al. 2010) relations.
