UV-processing of icy pebbles in the outer parts of turbulent disks
Abstract
Icy dust particles emerge in star-forming clouds and are subsequently incorporated in protoplanetary disks, where they coagulate into larger pebbles up to mm in size. In the disk, moderate levels of disk turbulence can lift small particles to the disk surface, where they can be destroyed. Nevertheless, studies of comets and meteorites generally find that ices at least partly retained their ISM composition before being accreted onto planetesimals. Here we model this process using hydrodynamical simulations with turbulence in the outer protoplanetary disk. We use the PLUTO code in a 2.5 D global accretion setup and include Lagrangian dust particles of 0.1 and 1 mm sizes. In a post-processing step, we use the RADMC3D code to generate the local UV radiation field to assess the level of ice processing of pebbles. We find that a small fraction (~17%) of 100 microns size particles are frequently lifted up to Z/R=0.2 which can result in the loss of their pristine composition as their residence time in this layer allows for effective CO and water photodissociation. The larger 1 mm size particles remain UV-shielded in the disk midplane throughout the dynamical evolution of the disk. Our results indicate that the assembly of icy bodies via the accretion of drifting mm-size icy pebbles can explain the pristine ice from the ISM. Nevertheless, particles smaller than 100 microns experience UV processing and may mix with unaltered icy pebbles, resulting in a less ISM-like composition.