2017ApJ...834..152B

We present a new velocity-resolved survey of 2.9 µm spectra of hot H₂O 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 H₂O 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 H₂O 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 H₂O, 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.