Systematic Variations of CO Gas Abundance with Radius in Gas-rich Protoplanetary Disks
AffiliationUniv Arizona, Lunar & Planetary Lab
Univ Arizona, Steward Observ, Dept Astron
MetadataShow full item record
PublisherIOP PUBLISHING LTD
CitationKe Zhang et al 2019 ApJ 883 98
RightsCopyright © 2019. The American Astronomical Society. All rights reserved.
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AbstractCO is the most widely used gas tracer of protoplanetary disks. Its abundance is usually assumed to be an interstellar ratio throughout the warm molecular layer of the disk. But recent observations of low CO gas abundance in many protoplanetary disks challenge our understanding of physical and chemical evolutions in disks. Here we investigate the CO abundance structures in four well-studied disks and compare their structures with predictions of chemical processing of CO and transport of CO ice-coated dust grains in disks. We use spatially resolved CO isotopologue line observations and detailed thermo-chemical models to derive CO abundance structures. We find that the CO abundance varies with radius by an order of magnitude in these disks. We show that although chemical processes can efficiently reduce the total column of CO gas within 1 Myr under an ISM level of cosmic-ray ionization rate, the depletion mostly occurs at the deep region of a disk. Without sufficient vertical mixing, the surface layer is not depleted enough to reproduce the weak CO emissions observed. The radial profiles of CO depletion in three disks are qualitatively consistent with predictions of pebble formation, settling, and drifting in disks. But the dust evolution alone cannot fully explain the high depletion observed in some disks. These results suggest that dust evolution may play a significant role in transporting volatile materials and a coupled chemical–dynamical study is necessary to understand what raw materials are available for planet formation at different distances from the central star.
VersionFinal published version
SponsorsNASA - Space Telescope Science InstituteSpace Telescope Science InstituteNational Aeronautics & Space Administration (NASA) [HST-HF2-51401.001, HST-HF2-51394.001, HST-HF2-51419.001]; NASANational Aeronautics & Space Administration (NASA) [NAS5-26555]