CO Depletion in Protoplanetary Disks: A Unified Picture Combining Physical Sequestration and Chemical Processing

Abstract

The gas-phase CO abundance (relative to hydrogen) in protoplanetary disks decreases by up to 2 orders of magnitude from its ISM value $\sim 10^{-4}$, even after accounting for freeze-out and photo-dissociation. Previous studies have shown that while local chemical processing of CO and the sequestration of CO ice on solids in the midplane can both contribute, neither of these processes appears capable of consistently reaching the observed depletion factors on the relevant timescale of $1-3\mathrm{Myr}$. In this study, we model these processes simultaneously by including a compact chemical network (centered on carbon and oxygen) to 2D ($r+z$) simulations of the outer ($r>20\mathrm{au}$) disk regions that include turbulent diffusion, pebble formation, and pebble dynamics. In general, we find that the CO/H${}_2$ abundance is a complex function of time and location. Focusing on CO in the warm molecular layer, we find that only the most complete model (with chemistry and pebble evolution included) can reach depletion factors consistent with observations. In the absence of pressure traps, highly-efficient planetesimal formation, or high cosmic ray ionization rates, this model also predicts a resurgence of CO vapor interior to the CO snowline. We show the impact of physical and chemical processes on the elemental (C/O) and (C/H) ratios (in the gas and ice phases), discuss the use of CO as a disk mass tracer, and, finally, connect our predicted pebble ice compositions to those of pristine planetesimals as found in the Cold Classical Kuiper Belt and debris disks.

Accepted for publication in The Astrophysical Journal

References in corpus (39)

• Complex Chemistry in Star-Forming Regions: An Expanded Gas-Grain Warm-up Chemical Model
• A Steeper than Linear Disk Mass-Stellar Mass Scaling Relation
• Ring shaped dust accumulation in transition disks
• Formation of planetary systems by pebble accretion and migration: How the radial pebble flux determines a terrestrial-planet or super-Earth growth mode
• Chemical enrichment of giant planets and discs due to pebble drift
• Constraining the X-ray and Cosmic Ray Ionization Chemistry of the TW Hya Protoplanetary Disk: Evidence for a Sub-interstellar Cosmic Ray Rate
• Meridional flows in the disk around a young star
• Lupus disks with faint CO isotopologues: low gas/dust or large carbon depletion?
• Hydrocarbon emission rings in protoplanetary disks induced by dust evolution
• Mass measurements in protoplanetary disks from hydrogen deuteride
• Mass inventory of the giant-planet formation zone in a solar nebula analog
• Planet-forming material in a protoplanetary disc: the interplay between chemical evolution and pebble drift
• Detection of exocometary CO within the 440 Myr-old Fomalhaut belt: a similar CO+CO${}_2$ ice abundance in exocomets and Solar System comets
• Reprocessing of Ices in Turbulent Protoplanetary Disks: Carbon and Nitrogen Chemistry
• An ALMA Survey of CO isotopologue emission from Protoplanetary Disks in Chamaeleon I
• The carbon monoxide-rich interstellar comet 2I/Borisov
• Two-Dimensional Transport of Solids in Viscous Protoplanetary Disks
• Excess C/O and C/H in outer protoplanetary disk gas
• Unusually High CO Abundance of the First Active Interstellar Comet
• A survey of C2H, HCN, and C18O in protoplanetary disks
• Gas-phase CO in protoplanetary disks: a challenge for turbulent mixing
• The efficiency of dust trapping in ringed proto-planetary discs
• Planetesimal Population Synthesis: Pebble Flux Regulated Planetesimal Formation
• Color, Composition, and Thermal Environment of Kuiper Belt Object (486958) Arrokoth
• Exploring the Origins of Carbon in Terrestrial Worlds
• Chemistry in Protoplanetary Disks: the gas-phase CO/H2 ratio and the Carbon reservoir
• The Flying Saucer: Tomography of the thermal and density gas structure of an edge-on protoplanetary disk
• Midplane temperature and outer edge of the protoplanetary disk around HD 163296
• Connecting planet formation and astrochemistry: Refractory carbon depletion leading to super-stellar C/O in giant planetary atmospheres
• New Constraints From Dust Lines On The Surface Densities Of Protoplanetary Disks
• Turbulence driven diffusion in protoplanetary disks - chemical effects in the outer disk
• An evolutionary study of volatile chemistry in protoplanetary disks
• Mass constraints for 15 protoplanetary disks from HD 1-0
• Turbulent-diffusion Mediated CO Depletion in Weakly Turbulent Protoplanetary disks
• The role of C/O in nitrile astrochemistry in PDRs and planet-forming disks
• Unlocking CO Depletion in Protoplanetary Disks II. Primordial C/H Predictions Inside the CO Snowline
• The dry and carbon poor inner disk of TW Hya: evidence for a massive icy dust trap
• Tracking Dust Grains During Transport and Growth in Protoplanetary Disks
• Extrasolar Kuiper Belts