# Systematic Variations of CO Gas Abundance with Radius in Gas-rich Protoplanetary Disks

## TL;DR

This study reveals that CO gas abundance varies significantly with radius in protoplanetary disks, influenced by chemical processes and dust evolution, impacting planet formation materials.

## Contribution

It provides detailed CO abundance profiles in four disks, highlighting the roles of chemical processing and dust dynamics in CO depletion patterns.

## Key findings

- CO abundance varies by an order of magnitude with radius.
- Chemical processing reduces CO mainly in the disk's deep regions.
- Dust evolution influences volatile transport but doesn't fully explain high depletion.

## Abstract

CO 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 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.