UV-driven organic precursor formation and dust evolution outside the snow line

Abstract: In the outer regions of protoplanetary disks, dust grains are often mantled with a layer of ice containing simple molecules. The irradiation of the disk by far-ultraviolet (FUV) photons breaks such molecules, producing photoproducts (radicals, ions, and intermediary molecules) in the ice layer that can act as precursors to complex organics. As grains drift towards the warmer, inner region of the disk, their consequent sublimation of ices causes organic synthesis on their surfaces, potentially enhancing grain sticking and dust growth. In addition to interstellar medium radiation, FUV emission from the central star and nearby massive stars can contribute to photoproduct formation. We perform a vertical dust size evolution simulation including grain growth and fragmentation, and calculate photoproduct formation under irradiation from these three sources, treating the central star's FUV continuum and Lyman-$α$ emission separately. We find that enhanced UV irradiation, particularly from stellar Lyman-$α$ emission, significantly increases the total abundance of photoproducts formed. This results from both the strong stellar Lyman-$α$ flux and resonant scattering by the overlying hydrogen layer, which redistributes photons and directs a fraction toward the dust layer. Turbulent mixing and fragmentation cycles further redistribute grains vertically, allowing photoproducts to form throughout the disk. We find that stellar Lyman-$α$ irradiation can produce sufficient photoproducts to produce an organic mantle with an {organic-to-rock ratio of $\approx 10\%$}, which could enhance grain sticking. Our results suggest that FUV-driven organic synthesis may facilitate dust growth and contribute to overcoming barriers to planetesimal formation.