Formation of Water in the Warm Atmospheres of Protoplanetary Disks
  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.
