2026ApJ...996L..17R

We present spatially resolved Atacama Large Millimeter/submillimeter Array (ALMA) observations of the water line at 183 GHz in the disk around the Herbig star HD 100546. The water vapor emission peaks at the inner edge of the warm dust cavity, located ∼15 au from the central star. We attribute this to thermal desorption at the water snow line, shifted outward at the dust cavity wall directly heated by the intense radiation. This represents the first spatially resolved image of the water snow line using ALMA observations of the main water isotopologue in a protoplanetary disk. The water emission morphology peaking inside the first dust ring is consistent with previous ALMA detections of oxygen-bearing complex organic molecules in the disk, including thermally desorbed methanol. These findings signal that warm cavities of transition disks provide ideal targets to directly reconstruct the spatial distribution of water vapor and the snow line location with ALMA and directly connect water vapor emission to ice desorption of complex organic species.