Reimagining the water snowline

TL;DR

This paper investigates the water snowline in protoplanetary disks, revealing that water emission lines are likely masing and that the snowline in the disk surface coincides with the mid-plane due to vertical mixing.

Contribution

It challenges the traditional view of the water snowline location by integrating observational data with physico-chemical models, highlighting the role of vertical mixing.

Findings

Water line detected towards AS 205 is masing.

Water snowline in surface and mid-plane are co-located.

Vertical mixing influences water distribution in disks.

Abstract

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 it's snowline has thus been the subject of much study. While water is seen to be abundant in the inner region of proto-planetary disks in infrared spectroscopy, detections of water in the disk in the sub-millimeter are rare, with only one detection towards AS 205. Here we put the multitude of non-detections and the single detection into context of recent physico-chemical models. We find that the 321.2257 GHz (10(2,9) - 9(3,6)) line detection towards 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 mid-plane. We propose that this is caused by vertical mixing continuously sequestering water from the warm surface layers into the cold disk midplane.