The Depletion of Water During Dispersal of Planet-forming Disk Regions

We present a new velocity-resolved survey of 2.9 μm spectra of hot H_2O and OH gas emission from protoplanetary disks, obtained with the Cryogenic Infrared Echelle Spectrometer at the VLT (R ~ 96,000). With the addition of archival Spitzer-IRS spectra, this is the most comprehensive spectral data set of water vapor emission from disks ever assembled. We provide line fluxes at 2.9–33 μm that probe from the dust sublimation radius at ~0.05 au out to the region of the water snow line. With a combined data set for 55 disks, we find a new correlation between H_2O line fluxes and the radius of CO gas emission, as measured in velocity-resolved 4.7 μm spectra (R_(co)), which probes molecular gaps in inner disks. We find that H_2O emission disappears from 2.9 μm (hotter water) to 33 μm (colder water) as R_(co) increases and expands out to the snow line radius. These results suggest that the infrared water spectrum is a tracer of inside-out water depletion within the snow line. It also helps clarify an unsolved discrepancy between water observations and models by finding that disks around stars of M⋆ > 1.5 M⊙ generally have inner gaps with depleted molecular gas content. We measure radial trends in H_2O, OH, and CO line fluxes that can be used as benchmarks for models to study the chemical composition and evolution of planet-forming disk regions at 0.05–20 au. We propose that JWST spectroscopy of molecular gas may be used as a probe of inner disk gas depletion, complementary to the larger gaps and holes detected by direct imaging and by ALMA