Chemistry in a gravitationally unstable protoplanetary disc

TL;DR

This study investigates how the dynamic, self-gravitating nature of massive protoplanetary discs influences their chemical composition and observable molecular emissions, highlighting the importance of shocks and temperature variations.

Contribution

It introduces a dynamical model of a self-gravitating protoplanetary disc to analyze chemical abundances, contrasting with traditional axisymmetric models.

Findings

Shock-induced temperature variations affect molecular desorption.

Gas-phase abundances of certain species are altered by shocks and reactions.

Emission maps can reveal shock locations and disc dynamics.

Abstract

Until now, axisymmetric, alpha-disc models have been adopted for calculations of the chemical composition of protoplanetary discs. While this approach is reasonable for many discs, it is not appropriate when self-gravity is important. In this case, spiral waves and shocks cause temperature and density variations that affect the chemistry. We have adopted a dynamical model of a solar-mass star surrounded by a massive (0.39 Msun), self-gravitating disc, similar to those that may be found around Class 0 and early Class I protostars, in a study of disc chemistry. We find that for each of a number of species, e.g. H2O, adsorption and desorption dominate the changes in the gas-phase fractional abundance; because the desorption rates are very sensitive to temperature, maps of the emissions from such species should reveal the locations of shocks of varying strengths. The gas-phase fractional abundances of some other species, e.g. CS, are also affected by gas-phase reactions, particularly in warm shocked regions. We conclude that the dynamics of massive discs have a strong impact on how they appear when imaged in the emission lines of various molecular species.