Role of Diffusive and Non-Diffusive Grain-Surface Processes in Cold Cores: Insights from the PEGASIS Three-Phase Astrochemical Model

Abstract

Cold dense cores are unique among the structures found in the interstellar medium (ISM), as they harbor a rich chemical inventory, including complex organic molecules (COMs), which will be inherited by future evolutionary stages. These molecules exist both in the gas phase and as ices accreted onto grain surfaces. To model these environments, we present Pegasis, a new, fast, and extensible three-phase astrochemical code to explore the chemistry of cold cores, with an emphasis on the role of diffusive and non-diffusive chemistry in shaping their gas and grain chemical compositions. We incorporated the 2024 KIDA chemical network and compared our results with current astrochemical models. Using a traditional rate-equation-based approach, we implemented both diffusive and non-diffusive chemistry, coupled with either an inert or chemically active ice mantle. We identify crucial reactions that enhance the production of COMs through non-diffusive mechanisms on the grain surface as well as the mechanisms through which they can accumulate in the gas phase. Across all models with non-diffusive chemistry, we observe a definite enhancement in the concentration of COMs on both the grain surface as well as in the grain mantle. Finally, our model broadly reproduces the observed abundances of multiple gas-phase species in the cold dense core TMC-1 (CP) and provides insights into its chemical age. Our work demonstrates the capabilities of Pegasis in exploring a wide range of grain-surface chemical processes and modeling approaches for three-phase chemistry in the ISM, providing robust explanations for observed abundances in TMC-1 (CP). In particular, it highlights the role of non-diffusive chemistry in the production of gas-phase COMs on grain surfaces, which are subsequently chemically desorbed, especially when the precursors involved in their formation on the surfaces are heavier than atomic hydrogen.