Reprocessing of Ices in Turbulent Protoplanetary Disks: Carbon and Nitrogen Chemistry

TL;DR

This study investigates how turbulent mixing in protoplanetary disks affects ice chemistry, especially for carbon and nitrogen molecules, influencing molecule abundances and complex organic molecule formation.

Contribution

It introduces a model incorporating turbulent transport into ice chemistry, revealing its impact on molecule reprocessing and organic synthesis in disks.

Findings

Turbulence decreases methanol and ammonia ices within 30 AU.

Mixing enhances complex organic molecule formation in the disk surface.

Vertical mixing suppresses the CO and N2 sink mechanism in inner disks.

Abstract

We study the influence of the turbulent transport on ice chemistry in protoplanetary disks, focusing on carbon and nitrogen bearing molecules. Chemical rate equations are solved with the diffusion term, mimicking the turbulent mixing in the vertical direction. Turbulence can bring ice-coated dust grains from the midplane to the warm irradiated disk surface, and the ice mantles are reprocessed by photoreactions, thermal desorption, and surface reactions. The upward transport decreases the abundance of methanol and ammonia ices at r < 30 AU, because warm dust temperature prohibits their reformation on grain surfaces. This reprocessing could explain the smaller abundances of carbon and nitrogen bearing molecules in cometary coma than those in low-mass protostellar envelopes. We also show the effect of mixing on the synthesis of complex organic molecules (COMs) are two ways: (1) transport of ices from the midplane to the disk surface and (2) transport of atomic hydrogen from the surface to the midplane. The former enhances the COMs formation in the disk surface, while the latter suppresses it in the midplane. Then, when mixing is strong, COMs are predominantly formed in the disk surface, while their parent molecules are (re)formed in the midplane. This cycle expands the COMs distribution both vertically and radially outward compared with that in the non-turbulent model. We derive the timescale of the sink mechanism by which CO and N2 are converted to less volatile molecules to be depleted from the gas phase, and find that the vertical mixing suppresses this mechanism in the inner disks.