A Multiscale Picture of the Magnetic Field and Gravity from a Large-scale Filamentary Envelope to Core-accreting Dust Lanes in the High-mass Star-forming Region W51

We present 230 GHz continuum polarization observations with the Atacama Large Millimeter/Submillimeter Array at a resolution of 0.'' 1 (similar to 540 au) in the high-mass star-forming regions W51 e2 and e8. These observations resolve a network of core-connecting dust lanes, marking a departure from earlier coarser, more spherical continuum structures. At the same time, the cores do not appear to fragment further. Polarized dust emission is clearly detected. The inferred magnetic field orientations are prevailingly parallel to dust lanes. This key structural feature is analyzed together with the local gravitational vector field. The direction of local gravity is found to typically align with dust lanes. With these findings, we derive a stability criterion that defines a maximum magnetic field strength that can be overcome by an observed magnetic field-gravity configuration. Equivalently, this defines a minimum field strength that can stabilize dust lanes against a radial collapse. We find that the detected dust lanes in W51 e2 and e8 are stable, hence possibly making them a fundamental component in the accretion onto central sources, providing support for massive star formation models without the need of large accretion disks. When comparing to coarser resolutions, covering the scales of envelope, global, and local collapse, we find recurring similarities in the magnetic field structures and their corresponding gravitational vector fields. These self-similar structures point at a multiscale collapse-within-collapse scenario until finally the scale of core-accreting dust lanes is reached where gravity is entraining the magnetic field and aligning it with the dust lanes.