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

TL;DR

This paper models the combined effects of chemical processing and physical sequestration on CO depletion in protoplanetary disks, showing that a comprehensive model can reproduce observed depletion levels and influence disk composition and planetesimal formation.

Contribution

It introduces a unified model including chemistry, pebble dynamics, and turbulence to explain CO depletion in disks, aligning with observations and linking to planetesimal compositions.

Findings

Only the complete model reaches observed CO depletion levels.

CO abundance varies with time and location in the disk.

Physical and chemical processes affect elemental ratios and disk mass estimates.

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.