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

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.