Turbulent-diffusion Mediated CO Depletion in Weakly Turbulent Protoplanetary disks

TL;DR

This paper presents a new mechanism called 'runaway freeze-out' that explains how weak turbulence in protoplanetary disks leads to significant CO depletion in the gas phase through turbulent mixing and freeze-out processes.

Contribution

It introduces a simple 1D model demonstrating how layered turbulence facilitates volatile depletion, highlighting the role of weak midplane turbulence in CO depletion.

Findings

CO can be depleted by a factor of a few over Myr timescales

Depletion level depends on turbulence strength and disk layering

Weak midplane turbulence enables efficient volatile freeze-out

Abstract

Volatiles, especially CO, are important gas tracers of protoplanetary disks (PPDs). Freeze-out and sublimation processes determine their division between gas and solid phases, which affects both which disk regions can be traced by which volatiles, and the formation and composition of planets. Recently, multiple lines of evidence suggest that CO is substantially depleted from the gas in the outer regions of PPDs. In this letter, we show that the gas dynamics in the outer PPDs facilitates volatile depletion through a mechanism which we term "runaway freeze-out". Using a simple 1D model that incorporates dust settling, turbulent diffusion of dust and volatiles, as well as volatile freeze-out/sublimation processes, we show that as long as turbulence in the cold midplane is sufficiently weak to allow majority of the small grains to settle, CO in the warm surface layer can be turbulently mixed into the midplane region and depleted by freeze-out. The level of depletion sensitively depends on the level of disk turbulence. Based on recent disk simulations that suggest a layered turbulence profile with very weak midplane turbulence and strong turbulence at disk surface, CO and other volatiles can be efficiently depleted by a factor of a few over Myr timescales.