2021ApJ...918L..10B

Water is a molecule that is tightly related to many facets of star and planet formation. Water's abundance and distribution, especially the location of its snowline has thus been the subject of much study. While water is seen to be abundant in the inner region of protoplanetary disks in infrared spectroscopy, detections of water in the disk in the submillimeter are rare, with only one detection toward AS 205. Here we put the multitude of nondetections and the single detection into context of recent physicochemical models. We find that the 321.2257 GHz (10_2,9-9_3,6) line detection toward AS 205 is inconsistent with a normal inner disk temperature structure and that the observed line must be masing. Furthermore, the emitting area derived from the line width, together with published analyses on water in disks around T-Tauri stars implies that the water snowline in the disk surface is at the same location as the snowline in the midplane. We propose that this is caused by vertical mixing continuously sequestering water from the warm surface layers into the cold disk midplane.