Mapping the magnetic field in the Taurus/B211 filamentary cloud with
  SOFIA HAWC+ and comparing with simulation
  Optical and infrared polarization mapping and recent Planck observations of
the filamentary cloud L1495 in Taurus show that the large-scale magnetic field
is approximately perpendicular to the long axis of the cloud. We use the HAWC+
polarimeter on SOFIA to probe the complex magnetic field in the B211 part of
the cloud. Our results reveal a dispersion of polarization angles of
$36^\circ$, about five times that measured on a larger scale by Planck.
Applying the Davis-Chandrasekhar-Fermi (DCF) method with velocity information
obtained from IRAM 30m C$^{18}$O(1-0) observations, we find two distinct
sub-regions with magnetic field strengths differing by more than a factor 3.
The quieter sub-region is magnetically critical and sub-Alfv\'enic; the field
is comparable to the average field measured in molecular clumps based on Zeeman
observations. The more chaotic, super-Alfv\'enic sub-region shows at least
three velocity components, indicating interaction among multiple substructures.
Its field is much less than the average Zeeman field in molecular clumps,
suggesting that the DCF value of the field there may be an underestimate.
Numerical simulation of filamentary cloud formation shows that filamentary
substructures can strongly perturb the magnetic field. DCF and true field
values in the simulation are compared. Pre-stellar cores are observed in B211
and are seen in our simulation. The appendices give a derivation of the
standard DCF method that allows for a dispersion in polarization angles that is
not small, present an alternate derivation of the structure function version of
the DCF method, and treat fragmentation of filaments.
