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

Abstract

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.

The Effects of Cosmic Rays on the Chemistry of Dense Cores

“…• Chemical models with and without the ionization dependency were compared to molecular abundances in the TMC-1 cyanopolyyne peak from Fuente et al (2019) Millar (2020).…”

“…All models had more difficulty reproducing N 2 H + abundances (the L model still performed better). The H model predicted abundances outside theFuente et al (2019) observational limits for both CS and HCO + and overpredicted the abundance of H 2 O compared to that in vonProcházka & Millar (2020).…”

“…Figure 7 shows data reported in von Procházka & Millar (2020), also compared to the UCLCHEM models at 10 K and a final density of 10 4 cm −3 with and without the inclusion of the CR ionization rate dependency during the post-collapse phase. In the cases of HCO + , N 2 H + and NH 3 , for all models the post-collapse abundances are within in the upper and lower limits of the observations.…”

“…Fuente et al (2019) and vonProcházka & Millar (2020). The comparisons show that the L model of the dependency tends to reproduce abundances more reliably than the standard handling or the H model.…”

The KOSMA-τPDR model

“…Depending on the value of ϵ this process can dominate other desorption processes and we found it to significantly contribute to the overall desorption. For a more detailed analysis see Roberts et al (2007) and more recently von Procházka & Millar (2021).…”