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

Abstract

Context. 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. Aims. To assess the impact of disk chemistry during the era of planet formation. This is done by investigating the chemical changes to volatile gases and ices in a protoplanetary disk midplane out to 30 AU for up to 7 Myr, considering a variety of different conditions, including a physical midplane structure that is evolving in time, and also considering two disks with different masses.Methods. An extensive kinetic chemistry gas-grain reaction network is utilised to evolve the abundances of chemical species over time. Two disk midplane ionisation levels (low and high) are explored, as well as two different makeups of the initial abundances ("inheritance" or "reset"). Results. 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. Inside the H 2 O iceline, and in the outer, colder regions of the disk midplane outside the O 2 iceline, the relative abundances of the species reach (close to) steady state by 7 Myr. 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, with the C/O ratio in the gas even becoming smaller than the C/O ratio in the ice. 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. Conclusions. Chemistry in the disk midplane is ionisation-driven, and evolves over time. This affects which molecules go into forming planets and their atmospheres. 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.

A Three-phase Approach to Grain Surface Chemistry in Protoplanetary Disks: Gas, Ice Surfaces, and Ice Mantles of Dust Grains

“…This efficient conversion of sCO to sCO 2 has also been reported in several recent studies, e.g. Furuya & Aikawa (2014), Aikawa et al (2015), Reboussin et al (2015), Molyarova et al (2017), Eistrup et al (2016), Bosman et al (2018).…”

Influence of sub- and super-solar metallicities on the composition of solid planetary building blocks

“…However, some models (e.g. Eistrup et al 2018) seem to indicate that carbon is most abundant in the form of CO or CO 2 rather than organic material without any oxygen, indicating a trend more consistent with our nominal chemical model (Fig. 10).…”

“…On the other hand, at high [Fe/H] also the rock forming species that bind oxygen like Fe, Si and Mg are more abundant than oxygen implying that the water ice ratio will decrease with increasing [Fe/H] independently of how much oxygen is bound in carbon to form CO or CO 2 . The exact water content though can be estimated more accurately with more detailed chemical models, however, detailed models seem to indicate that the ration between CO to CO 2 to non-oxygen bearing carbon molecules is maximal equal, where in most cases the non-oxygen bearing carbon molecules are less abundant than CO and CO 2 (Eistrup et al 2018) which points to a water ice ratio as predicted by our nominal model (Fig. 10).…”

“…Our used model also does not account for the formation of molecules during the infall stage, however simulations using solar-like composition (Furuya et al 2015) seem to be in agreement with our model at solar composition. However, the impact of a more advanced model seems to be much more profound on the gaseous component (Eistrup et al 2018) and thus the planetary atmosphere composition (Cridland et al 2019) than on the solid composition. However, to calculate the solid composition of the material, also the gaseous component has to be taken into account.…”

Dust in brown dwarfs and extrasolar planets

“…Disk chemistry simulations suggest that the local disk gas can become carbon-rich but only under special circumstances and at certain times Ali-Dib et al 2014;Eistrup et al 2016). This suggests that carbon-rich planets could be a tracer of these rare episodes in an originally oxygen-rich disk.…”