Chemistry in a forming protoplanetary disk: main accretion phase

TL;DR

This study models the chemical evolution in a forming protoplanetary disk, revealing how molecular compositions change during collapse and disk formation, with implications for organic molecule formation.

Contribution

It provides a detailed post-processing chemical analysis of a star-forming core's evolution using a three-phase model, highlighting the impact of temperature history on ice composition.

Findings

Stable molecules are abundant at collapse onset and sublimated inward.

Complex organic molecules form in the disk and depend on photodissociation processes.

S-bearing species show distinct abundance patterns during collapse and in the disk.

Abstract

We investigate the chemistry in a radiation-hydrodynamics model of star-forming core which evolves from a cold ($\sim 10$ K) prestellar core to the main accretion phase in $\sim 10^5$ yr. A rotationally-supported gravitationally unstable disk is formed around a protostar. We extract the temporal variation of physical parameters in $\sim 1.5 \times 10^3$ SPH particles which end up in the disk, and perform post-processing calculations of the gas-grain chemistry adopting a three-phase model. Inside the disk, the SPH particles migrate both inward and outward. Since a significant fraction of volatiles such as CO can be trapped in the water-dominant ice in the three-phase model, the ice mantle composition depends not only on the current position in the disk but also on whether the dust grain has ever experienced higher temperatures than the water sublimation temperature. Stable molecules such as H$_2$O, CH$_4$, NH$_3$ and CH$_3$OH are already abundant at the onset of gravitational collapse and simply sublimated as the fluid parcels migrate inside the water snow line. On the other hand, various molecules such as carbon chains and complex organic molecules (COMs) are formed in the disk. COMs abundance sensitively depends on the outcomes of photodissociation and diffusion rates of photofragments in bulk ice mantle. As for S-bearing species, H$_2$S ice is abundant in the collapse phase. In the warm regions in the disk, H$_2$S is sublimated to be destroyed, while SO, H$_2$CS, OCS and SO$_2$ become abundant.