The depletion of water during dispersal of planet-forming disk regions

Abstract

We present a new velocity-resolved survey of 2.9 $\mu$m spectra of hot H${}_2$O and OH gas emission from protoplanetary disks, obtained with CRIRES at the VLT ($\Delta v\sim$ 3 km s${}^{-1}$). With the addition of archival Spitzer-IRS spectra, this is the most comprehensive spectral dataset of water vapor emission from disks ever assembled. We provide line fluxes at 2.9-33 $\mu$m that probe from disk radii of $\sim 0.05$ au out to the region across the water snow line. With a combined dataset for 55 disks, we find a new correlation between H${}_2$O line fluxes and the radius of CO gas emission as measured in velocity-resolved 4.7 $\mu$m spectra (R${}_{\mathrm{co}}$), which probes molecular gaps in inner disks. We find that H${}_2$O emission disappears from 2.9 $\mu$m (hotter water) to 33 $\mu$m (colder water) as R${}_{\mathrm{co}}$ 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 clarifying an unsolved discrepancy between water observations and models, by finding that disks around stars of M${}_{\star }>1.5$ M${}_{\odot }$ generally have inner gaps with depleted molecular gas content. We measure radial trends in H${}_2$O, 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.

Accepted for publication on ApJ

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