Chemical evolution in planet-forming regions with growing grains

Abstract

Context. Planets and their atmospheres are built from gas and solid material in protoplanetary disks. This solid material grows from smaller micron-sized grains to larger sizes in the disks during the process of planet formation. This solid growth may influence the efficiency of chemical reactions that take place on the surfaces of the grains and in turn affect the chemical evolution that the gas and solid material in the disk undergoes, with implications for the chemical composition of the planets. Aims. Our goal is to model the compositional evolution of volatile ices on grains of different sizes, assuming both time-dependent grain growth and several constant grain sizes. We also examine the dependence on the initial chemical composition. Methods. The custom Walsh chemical kinetics code was used to model the chemical evolution. This code was upgraded to account for the time-evolving sizes of solids. Chemical evolution was modelled locally at four different radii in a protoplanetary disk midplane (with associated midplane temperatures of 120, 57, 25, and 19.5 K) for up to 10 Myr. The evolution was modelled for five different constant grain sizes, and in one model, the grain size changed with time according to a grain-growth model appropriate for the disk midplane. Results. Local grain growth, with conservation of the total grain mass, and assuming spherical grains, acts to reduced the total grain-surface area that is available for ice-phase reactions. This reduces the efficiency of these reactions compared to a chemical scenario with a conventional grain-size choice of 0.1 μm. The chemical evolution modelled with grain growth leads to increased abundances of H2O ice. For carbon in the inner disk, grain growth causes CO gas to overtake CO2 ice as the dominant carrier, and in the outer disk, CH4 ice becomes the dominant carrier. Larger grain sizes cause less change the C/O ratio in the gas phase over time than when 0.1 μm sized grains are considered. Overall, a constant grain size adopted from a grain evolution model leads to an almost identical chemical evolution as a chemical evolution with evolving grain sizes. A constant grain size choice, albeit larger than 0.1 μm, may therefore be an appropriate simplification when modelling the impact of grain growth on chemical evolution.