Magnetic Fields In Massive Star-Forming Regions (Magmar). I. Linear Polarized Imaging Of The Ultracompact H Ii Region G5.89-0.39

We report 1.2 mm polarized continuum emission observations carried out with the Atacama Large Millimeter/submillimeter Array toward the high-mass star formation region G5.89-0.39. The observations show a prominent 0.2 pc north-south filamentary structure. The ultracompact H ii region in G5.89-0.39 breaks the filament into two pieces. Its millimeter emission shows a dusty belt with a mass of 55-115 M (circle dot) and 4500 au in radius, surrounding an inner part comprising mostly ionized gas, with dust emission only accounting for about 30% of the total millimeter emission. We also found a lattice of convex arches that 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 and 600 M (circle dot) and harbors a cluster of about 20 mm envelopes with a median size and mass of 1700 au and 1.5 M (circle dot), 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 similar to 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 4500 and 7500 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.