Species cycling and the enhancement of ammonia in prestellar cores
  The quantity of NH3 produced on grain surfaces in the prestellar 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.
