Chemical Feedbacks of Pebble Growth: Impacts on CO depletion and C/O ratios

TL;DR

This study introduces a new 1D model for protoplanetary disk chemistry and pebble growth, revealing how these processes influence CO depletion and C/O ratios, aligning with some observations but not all.

Contribution

The paper presents a novel 1D model that simultaneously simulates pebble growth and chemistry, incorporating vertical diffusion of gas and small dust particles.

Findings

CO is depleted by 1-2 orders of magnitude in surface layers for turbulent disks.

Pebble growth and sequestration can increase gas-phase C/O ratios to about unity.

The model cannot reproduce high C/O ratios of 1.5-2.0 observed in some disks.

Abstract

Observations of protoplanetary disks have revealed them to be complex and dynamic, with vertical and radial transport of gas and dust occurring simultaneously with chemistry and planet formation. Previous models of protoplanetary disks focused primarily on chemical evolution of gas and dust in a static disk, or dynamical evolution of solids in a chemically passive disk. In this paper, we present a new 1D method for modelling pebble growth and chemistry simultaneously. Gas and small dust particles are allowed to diffuse vertically, connecting chemistry at all elevations of the disk. Pebbles are assumed to form from the dust present around the midplane, inheriting the composition of ices at this location. We present the results of this model after 1 Myr of disk evolution around a 1$M_\odot$ star at various locations both inside and outside of the CO snowline. We find that for a turbulent disk ($\alpha = 10^{-3}$), CO is depleted from the surface layers of the disk by roughly 1-2 orders of magnitude, consistent with observations of protoplanetary disks. This is achieved by a combination of ice sequestration and decreasing UV opacity, both driven by pebble growth. Further, we find the selective removal of ice species via pebble growth and sequestration can increase gas phase C/O ratios to values of approximately unity. However, our model is unable to produce C/O values of $\sim$1.5-2.0 inferred from protoplanetary disk observations, implying selective sequestration of ice is not sufficient to explain C/O ratios $>1$.