Species cycling and the enhancement of ammonia in prestellar cores

TL;DR

This study investigates how gas-grain cycling influences ammonia production in prestellar cores, revealing its impact on chemical complexity and observable ammonia abundances through advanced chemical modeling.

Contribution

It introduces a modified chemical model incorporating updated reaction rates and grain processes to analyze ammonia formation and its dependence on gas-grain cycling.

Findings

NH3 can be observed in early stages due to hydrocarbon chemistry

A gradient in species agreement suggests physical substructures in the cloud

NH3 abundances are sensitive to gas-grain cycling processes

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.