Magnetic Fields in Massive Star-Forming Regions (MagMaR) I. Linear Polarized Imaging of the UCHII Region G5.89-0.39

We report 1.2 mm polarized continuum emission observations carried out with the Atacama Large Millimeter/submillimeter Array (ALMA) toward the high-mass star formation region G5.89-0.39. The observations show a prominent 0.2 pc north-south filamentary structure. The UCHII in G5.89-0.39 breaks the filament in two pieces. Its millimeter emission shows a dusty belt with a mass of 55-115 M$_{\odot}$ and 4,500 au in radius, surrounding an inner part comprising mostly ionized gas with a dust emission only accounting about 30% of the total millimeter emission. We also found a lattice of convex arches which may be produced by dragged dust and gas from the explosive dispersal event involving the O5 Feldt's star. The north-south filament has a mass between 300-600 M$_{\odot}$ and harbours a cluster of about 20 millimeter envelopes with a median size and mass of 1700 au and 1.5 M$_{\odot}$, respectively, some of which are already forming protostars. We interpret the polarized emission in the filament as mainly coming from magnetically aligned dust grains. The polarization fraction is ~4.4% in the filaments and 2.1% at the shell. The magnetic fields are along the North Filament and perpendicular to the South Filament. In the Central Shell, the magnetic fields are roughly radial in a ring surrounding the dusty belt between 4,500 and 7,500 au, similar to the pattern recently found in the surroundings of Orion BN/KL. This may be an independent observational signpost of explosive dispersal outflows and should be further investigated in other regions.