SHAMPOO: A stochastic model for tracking dust particles under the influence of non-local disk processes

Abstract

Context. The abundances of carbon, hydrogen, nitrogen, oxygen, and sulfur (CHNOS) are crucial for understanding the initial composition of planetesimals and, by extension, planets. At the onset of planet formation, large amounts of these elements are stored in ices on dust grains in planet-forming disks. The evolution of the ice in dust, however, is affected by disk processes, including dynamical transport, collisional growth and fragmentation, and the formation and sublimation of ice. These processes can be highly coupled and non-local. Aims. In this work, we aim to constrain the disk regions where dynamical, collisional, and ice processing are fully coupled. Subsequently, we aim to develop a flexible modelling approach that is able to predict the effects of these processes acting simultaneously on the CHNOS budgets of planetesimal-forming material in these regions. Methods. We compared the timescales associated with these disk processes to constrain the disk regions where such an approach is necessary, and subsequently developed the SHAMPOO (StocHAstic Monomer PrOcessOr) code, which tracks the CHNOS abundances in the ice mantle of a single ‘monomer’ dust particle of bare mass mm, embedded in a larger ‘home aggregate’. The monomer inside its home aggregate is affected by aerodynamic drag, turbulent stirring, collision processes, and ice adsorption and desorption simultaneously. The efficiency of adsorption onto and the photodesorption of the monomer here depends on the depth zm at which the monomer is embedded in the home aggregate. We used SHAMPOO to investigate the effect of thefragmentation velocity υfrag and home aggregate filling factor ϕ on the amount of CHNOS-bearing ices for monomers residing at r = 10 AU. Results. The timescale analysis shows that the locations where disk processes are fully coupled depend on both grain size and ice species. We find that monomers released at 10 AU embedded in smaller, more fragile, aggregates with fragmentation velocities of 1 m s−1 are able to undergo adsorption and photodesorption more often than monomers in aggregates with fragmentation velocities of 5 and 10 m s−1. Furthermore, we find that at 10 AU in the midplane, aggregates with a filling factor of ϕ = 10−3 are able to accumulate ice 22 times faster on average than aggregates with ϕ = 1 under the same conditions. Conclusions. Since different grain sizes are coupled through collisional processes and the grain ice mantle typically consists of multiple ice species, it is difficult to isolate the locations where disk processes are fully coupled, necessitating the development of the SHAMPOO code. Furthermore, the processing of ice may not be spatially limited to dust aggregate surfaces for either fragile or porous aggregates.