Species cycling and the enhancement of ammonia in pre-stellar cores

Abstract

ABSTRACT The quantity of NH3 produced on grain surfaces in the pre-stellar core is thought to be one of the determining factors regarding the chemical complexity achievable at later stages of stellar birth. In order to investigate how this quantity might be influenced by the gas–grain cycling of molecular material within the cloud, we employ a modified rates gas–grain chemical code and follow the time-dependent chemistry of NH3 as the system evolves. Our models incorporate an updated version of the most recent UDfA network of reaction rate coefficients, desorption from the grains through standard thermal and non-thermal processes, and physisorbed and chemisorbed binding of atomic and molecular hydrogen to a population of carbonaceous and siliceous grains. We find that (1) observable abundances of NH3 can exist in the gas phase of our models at early times when the N atom is derived from CN via an efficient early-time hydrocarbon chemistry, (2) a time-dependent gradient exists in the observational agreement between different species classes in our models, consistent with possible physical substructures within the TMC-1 Cyanopolyyne Peak, and (3) the gaseous and solid-state abundances of NH3 are sensitive to the presence of gas–grain cycling within the system. Our results suggest that the degree of chemical complexity achievable at later stages of the cloud’s chemical evolution is indeed influenced by the manner in which the gas–grain cycling occurs.