Magnetic Criticality across Scales: From Support to Collapse in the L1287 Molecular Cloud

The relative importance of magnetic fields, turbulence, and gravity during the early stages of star formation remains uncertain. We present a multiscale polarimetric investigation of L1287, a clustered low-mass star-forming region hosting a bipolar molecular outflow and a protostar. Our study combines new R -band optical polarimetric observations obtained with AIMPOL, tracing the magnetic field on envelope scales, with archival 850 μ m SCUPOL polarization data probing the dense molecular clump. The clump-scale magnetic field morphology is offset by ∼59° from the envelope field, and the polarization fraction decreases systematically toward higher-density regions, indicating depolarization. This trend is primarily attributed to reduced grain-alignment efficiency, with possible contributions from radiative torque disruption, while magnetic field tangling is unlikely to be the dominant factor. The density structure is derived using extinction map and Herschel SPIRE data. Combining Two Micron All Sky Survey (2MASS) photometry with Gaia parallaxes yields a distance of 920 ± 76 pc. Using the Davis–Chandrasekhar–Fermi method, we estimate magnetic field strengths of ∼111 ± 27 μ G in the envelope and ∼195 ± 70 μ G in the dense clump. The envelope is magnetically subcritical, whereas the clump is supercritical. The Alfvén Mach number indicates sub-Alfvénic conditions in the envelope and a transition toward trans-Alfvénic conditions in the dense gas. Virial analysis further shows that magnetic support dominates the envelope, while gravity dominates the clump. The alignment of the clump magnetic field with both the minor axis and the molecular outflow supports a magnetically regulated evolutionary scenario that transitions toward gravitational collapse at higher densities. These results provide observational evidence that magnetic criticality evolves significantly across spatial scales in star-forming clouds.