Unveiling the Role of the Magnetic Field at the Smallest Scales of Star
  Formation
  We report ALMA observations of polarized dust emission from the protostellar
source Ser-emb 8 at a linear resolution of 140 au. Assuming models of
dust-grain alignment hold, the observed polarization pattern gives a projected
view of the magnetic field structure in this source. Contrary to expectations
based on models of strongly magnetized star formation, the magnetic field in
Ser-emb 8 does not exhibit an hourglass morphology. Combining the new ALMA data
with previous observational studies, we can connect magnetic field structure
from protostellar core (~80,000 au) to disk (~100 au) scales. We compare our
observations with four magnetohydrodynamic gravo-turbulence simulations made
with the AREPO code that have initial conditions ranging from super-Alfv\'enic
(weakly magnetized) to sub-Alfv\'enic (strongly magnetized). These simulations
achieve the spatial dynamic range necessary to resolve the collapse of
protostars from the parsec scale of star-forming clouds down to the ~100 au
scale probed by ALMA. Only in the very strongly magnetized simulation do we see
both the preservation of the field direction from cloud to disk scales and an
hourglass-shaped field at < 1000 au scales. We conduct an analysis of the
relative orientation of the magnetic field and the density structure in both
the Ser-emb 8 ALMA observations and the synthetic observations of the four
AREPO simulations. We conclude that the Ser-emb 8 data are most similar to the
weakly magnetized simulations, which exhibit random alignment, in contrast to
the strongly magnetized simulation, where the magnetic field plays a role in
shaping the density structure in the source. In the weak-field case, it is
turbulence -- not the magnetic field -- that shapes the material that forms the
protostar, highlighting the dominant role that turbulence can play across many
orders of magnitude in spatial scale.
