Molecular abundances and C/O ratios in chemically evolving planet-forming disk midplanes

TL;DR

This study models the chemical evolution in planet-forming disk midplanes, revealing that C/O ratios and molecular abundances change over time due to ionisation-driven chemistry, impacting planet atmosphere predictions.

Contribution

It introduces a comprehensive kinetic chemistry model showing that disk chemistry evolves significantly over millions of years, affecting traditional assumptions about C/O ratios at icelines.

Findings

Chemical abundances evolve over time, especially after 10^5 years.

C/O ratios in gas and ice are not fixed but change dynamically.

Gaseous CO can convert into CO2 at low temperatures, affecting observed icelines.

Abstract

(Abridged) Exoplanet atmospheres are thought be built up from accretion of gas as well as pebbles and planetesimals in the midplanes of planet-forming disks. The chemical composition of this material is usually assumed to be unchanged during the disk lifetime. However, chemistry can alter the relative abundances of molecules in this planet-building material. To assess the impact of disk chemistry during the era of planet formation, an extensive kinetic chemistry gas-grain reaction network is utilised to evolve the abundances of chemical species over time. Given a high level of ionisation, chemical evolution in protoplanetary disk midplanes becomes significant after a few times $10^{5}$ yrs, and is still ongoing by 7 Myr between the H$_{2}$O and the O$_{2}$ icelines. Importantly, the changes in the abundances of the major elemental carbon and oxygen-bearing molecules imply that the traditional "stepfunction" for the C/O ratios in gas and ice in the disk midplane (as defined by sharp changes at icelines of H$_{2}$O, CO$_{2}$ and CO) evolves over time, and cannot be assumed fixed. In addition, at lower temperatures (< 29 K), gaseous CO colliding with the grains gets converted into CO$_{2}$ and other more complex ices, lowering the CO gas abundance between the O$_{2}$ and CO thermal icelines. This effect can mimic a CO iceline at a higher temperature than suggested by its binding energy. Chemistry in the disk midplane is ionisation-driven, and evolves over time. In order to reliably predict the atmospheric compositions of forming planets, as well as to relate observed atmospheric C/O ratios of exoplanets to where and how the atmospheres have formed in a disk midplane, chemical evolution needs to be considered and implemented into planet formation models.