Fitting an Analytic Magnetic Field to a Prestellar Core
  We deploy and demonstrate the capabilities of the magnetic field model
developed by Ewertowski & Basu (2013) by fitting observed polarimetry data of
the prestellar core FeSt 1-457. The analytic hourglass magnetic field function
derived directly from Maxwell's equations yields a central-to-surface magnetic
field strength ratio in the equatorial plane, as well as magnetic field
directions with relative magnitudes throughout the core. This fit emerges from
a comparison of a single plane of the model with the polarization map that
results from the integrated properties of the magnetic field and dust
throughout the core. Importantly, our fit is independent of any assumed density
profile of the core. We check the robustness of the fit by using the POLARIS
code to create synthetic polarization maps that result from the integrated
scattering and emission properties of the dust grains and their radiative
transfer, employing an observationally-motivated density profile. We find that
the synthetic polarization maps obtained from the model also provides a good
fit to the observed polarimetry. Our model fits the striking feature of
significant curvature of magnetic field lines in the outer part of FeSt 1-457.
Combined with independent column density estimates, we infer that the core of
size $R_{\rm gas}$ has a mildly supercritical mass-to-flux ratio and may have
formed through dynamical motions starting from a significantly larger radius
$R$. A breakdown of flux-freezing through neutral-ion slip (ambipolar
diffusion) could be responsible for effecting such a transition from a
large-scale magnetic field structure to a more compact gas structure.
