Formation of Water in the Warm Atmospheres of Protoplanetary Disks

TL;DR

This paper models the formation of water in protoplanetary disk atmospheres, highlighting the roles of X-ray ionization, grain chemistry, and heating in producing observable warm water, challenging previous transport-based explanations.

Contribution

It introduces a comprehensive model of water formation in disk atmospheres emphasizing the importance of ionization, grain chemistry, and heating processes, without relying on water transport from cooler regions.

Findings

High water abundances can be produced in the inner disk atmosphere.

Grain formation of H₂ and atmospheric heating significantly influence water levels.

The model explains water emission observations without water transport from outer regions.

Abstract

The gas-phase chemistry of water in protoplanetary disks is analyzed with a model based on X-ray heating and ionization of the disk atmosphere. Several uncertain processes appear to play critical roles in generating the column densities of warm water that are detected from disks at infrared wavelengths. The dominant factors are the reactions that form molecular hydrogen, including formation on warm grains, and the ionization and heating of the atmosphere. All of these can work together to produce a region of high water abundances in the molecular transition layer of the inner disk atmosphere, where atoms are transformed into molecules, the temperature drops from thousands to hundreds of Kelvins, and the ionization begins to be dominated by the heavy elements. Grain formation of molecular hydrogen and mechanical heating of the atmosphere can play important roles in this region and directly affect the amount of warm water in protoplanetary disk atmospheres. Thus it may be possible to account for the existing measurements of water emission from Tauri disks without invoking transport of water from cooler to warmer regions. The hydroxyl radical OH is under-abundant in this model of disk atmospheres and requires consideration of additional production and excitation processes.