2016A&A...595A..83E

Context. The atmospheres of extrasolar planets are thought to be built largely through accretion of pebbles and planetesimals. Such pebbles are also the building blocks of comets. The chemical composition of their volatiles are usually taken to be inherited from the ices in the collapsing cloud. However, chemistry in the protoplanetary disk midplane can modify the composition of ices and gases.
Aims. To investigate if and how chemical evolution affects the abundances and distributions of key volatile species in the midplane of a protoplanetary disk in the 0.2-30AU range.
Methods. A disk model used in planet population synthesis models is adopted, providing temperature, density and ionisation rate at different radial distances in the disk midplane. A full chemical network including gas-phase, gas-grain interactions and grain-surface chemistry is used to evolve chemistry in time, for 1Myr. Both molecular (inheritance from the parent cloud) and atomic (chemical reset) initial conditions are investigated.
Results. Great diversity is observed in the relative abundance ratios of the main considered species: H₂O, CO, CO₂, CH₄, O₂, NH₃ and N₂. The choice of ionisation level, the choice of initial abundances, as well as the extent of chemical reaction types included are all factors that affect the chemical evolution. The only exception is the inheritance scenario with a low ionisation level, which results in negligible changes compared with the initial abundances, regardless of whether or not grain-surface chemistry is included. The grain temperature plays an important role, especially in the critical 20-28K region where atomic H no longer sticks long enough to the surface to react, but atomic O does. Above 28K, efficient grain-surface production of CO₂ ice is seen, as well as O₂ gas and ice under certain conditions, at the expense of H₂O and CO. H₂O ice is produced on grain surfaces only below 28K. For high ionisation levels at intermediate disk radii, CH₄ gas is destroyed and converted into CO and CO₂ (in contrast with previous models), and similarly NH₃ gas is converted into N₂. At large radii around 30AU, CH₄ ice is enhanced leading to a low gaseous CO abundance. As a result, the overall C/O ratios for gas and ice change significantly with radius and with model assumptions. For high ionisation levels, chemical processing becomes significant after a few times 10⁵yr.
Conclusions. Chemistry in the disk midplane needs to be considered in the determination of the volatile composition of planetesimals. In the inner <30AU disk, interstellar ice abundances are preserved only if the ionisation level is low, or if these species are included in larger bodies within 10⁵yr.