Water vapor emission at the warm cavity wall of the HD 100546 disk as revealed by ALMA

TL;DR

This study uses ALMA to spatially resolve water vapor emission in the HD 100546 disk, revealing the water snowline at the warm cavity wall and linking it to organic molecule desorption.

Contribution

First spatially resolved imaging of the water snowline in a protoplanetary disk using ALMA observations of the main water isotopologue.

Findings

Water vapor peaks at the inner edge of the dust cavity.

Water snowline is located at ~15 au from the star.

Water emission correlates with desorption of complex organic molecules.

Abstract

We present spatially resolved 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 snowline, shifted outward at the dust cavity wall directly heated by the intense radiation. This represents the first spatially resolved image of the water snowline 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 snowline location with ALMA, and directly connect water vapor emission to ice desorption of complex organic species.