UV-driven Chemistry as a Signpost for Late-stage Planet Formation

TL;DR

This paper explores how ultraviolet-driven chemistry in protoplanetary disks influences the chemical composition of forming planets, highlighting a transition from cosmic-ray to UV dominance that affects organic molecule formation.

Contribution

It introduces a model explaining the evolution of disk chemistry from cosmic-ray/X-ray to UV dominance, impacting planetary composition and organic molecule availability.

Findings

UV photons penetrate deeper in evolved disks with high gas-to-dust ratios.

Carbon-rich gas (C/O > 1) promotes formation of organic radicals and ions.

Photochemical processes lead to organic molecule formation in planet-forming regions.

Abstract

The chemical reservoir within protoplanetary disks has a direct impact on planetary compositions and the potential for life. A long-lived carbon-and nitrogen-rich chemistry at cold temperatures (<=50K) is observed within cold and evolved planet-forming disks. This is evidenced by bright emission from small organic radicals in 1-10 Myr aged systems that would otherwise have frozen out onto grains within 1 Myr. We explain how the chemistry of a planet-forming disk evolves from a cosmic-ray/X-ray-dominated regime to an ultraviolet-dominated chemical equilibrium. This, in turn, will bring about a temporal transition in the chemical reservoir from which planets will accrete. This photochemical dominated gas phase chemistry develops as dust evolves via growth, settling and drift, and the small grain population is depleted from the disk atmosphere. A higher gas-to-dust mass ratio allows for deeper penetration of ultraviolet photons is coupled with a carbon-rich gas (C/O > 1) to form carbon-bearing radicals and ions. This further results in gas phase formation of organic molecules, which then would be accreted by any actively forming planets present in the evolved disk.