First Detection of Hourglass-shaped Magnetic Fields in the L1544 Prestellar Core

Stars form in dense, cold regions of interstellar clouds where gravity, turbulence, and magnetic fields interact. Magnetic fields are expected to regulate how gas contracts, but their role at the earliest, prestellar stage is poorly constrained, because polarized dust emission from the coldest cores is faint. Here we report submillimeter dust polarization observations of the well-studied prestellar core L1544, obtained with the James Clerk Maxwell Telescope SCUBA-2/POL-2 instrument as part of the BISTRO (B-fields in Star-forming Region Observations) survey. The data resolve a clearly pinched, hourglass-shaped magnetic field at the core center, representing the first detection in a prestellar core. The plane-of-sky field morphology matches classical predictions for magnetically regulated contraction and is consistent with previously reported slow infall and filament-fed accretion flows that deliver material into the core. From the polarization angle dispersion, we infer a magnetic field strength that implies an Alfvén speed above the measured infall speed and well below the free-fall velocity, indicating sub-Alfvénic, magnetically moderated contraction. These results show that the hourglass-shaped field geometry reported in some protostellar envelopes can already be present before a protostar forms, thereby bridging a key observational gap in the timeline of magnetically regulated star formation. As a nearby, representative core on the verge of star formation, L1544 now serves as a benchmark for testing theoretical models that couple gravity and magnetic fields in dense cores.