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We report ALMA observations of polarized dust emission from the protostellarsource Ser-emb 8 at a linear resolution of 140 au. Assuming models ofdust-grain alignment hold, the observed polarization pattern gives a projectedview of the magnetic field structure in this source. Contrary to expectationsbased on models of strongly magnetized star formation, the magnetic field inSer-emb 8 does not exhibit an hourglass morphology. Combining the new ALMA datawith previous observational studies, we can connect magnetic field structurefrom protostellar core (~80,000 au) to disk (~100 au) scales. We compare ourobservations with four magnetohydrodynamic gravo-turbulence simulations madewith the AREPO code that have initial conditions ranging from super-Alfv\'enic(weakly magnetized) to sub-Alfv\'enic (strongly magnetized). These simulationsachieve the spatial dynamic range necessary to resolve the collapse ofprotostars from the parsec scale of star-forming clouds down to the ~100 auscale probed by ALMA. Only in the very strongly magnetized simulation do we seeboth the preservation of the field direction from cloud to disk scales and anhourglass-shaped field at < 1000 au scales. We conduct an analysis of therelative orientation of the magnetic field and the density structure in boththe Ser-emb 8 ALMA observations and the synthetic observations of the fourAREPO simulations. We conclude that the Ser-emb 8 data are most similar to theweakly magnetized simulations, which exhibit random alignment, in contrast tothe strongly magnetized simulation, where the magnetic field plays a role inshaping the density structure in the source. In the weak-field case, it isturbulence -- not the magnetic field -- that shapes the material that forms theprotostar, highlighting the dominant role that turbulence can play across manyorders of magnitude in spatial scale.

Unveiling the Role of the Magnetic Field at the Smallest Scales of Star Formation