The effect of nonlocal disk processes on the volatile CHNOS budgets of planetesimal-forming material

Abstract

The abundances of volatile CHNOS have a profound effect on the interior structure and evolution of a planet. Therefore, it is key to investigate the behavior of the abundances of these elements in the solid phase in the earliest stages of planet formation. However, the processes in planet-forming disks that shape these abundances are highly coupled and nonlocal. We aim to quantify the effects of the interplay between dynamical, collision, ice processing on the abundances of CHNOS in local disk solids as a function of position throughout the planet-forming region. We used SHAMPOO (StocHAstic Monomer PrOcessOr), which tracks the ice budgets of CHNOS-bearing molecules of a dust monomer as it undergoes nonlocal disk processing in a Class I disk. We used a large set of individual monomer evolutionary trajectories to make inferences about the properties of the local dust populations. We find that spatially, monomers can travel larger distances farther out in the disk, leading to a larger spread in positions of origin for a dust population at, for example, r=50 AU compared to r=2 AU. However, chemically, the inner disk (r<10 AU) is more nonlocal due to the closer spacing of ice lines in this disk region. We find that the ice mass associated with individual chemical species can change significantly. The largest differences with local dust were found near ice lines where the collisional timescale is comparable to the adsorption and desorption timescales. Here, aggregates may become significantly depleted in ice as a consequence of microscopic collisional mixing, a previously unknown effect where monomers are stored away in aggregate interiors through rapid cycles of coagulation and fragmentation. This suggests that ice processing is highly coupled to collisional processing in this disk region, which implies that the interiors of dust aggregates must be considered and not just their surfaces.