Role of NH₃ binding energy in the early evolution of protostellar cores

¹ Centre for Astrochemical Studies, Max Planck Institute for Extraterrestrial Physics, Giessenbachstraße 1, 85748 Garching, Germany
² Laboratoire d’Instrumentation et de Recherche en Astrophysique, CY Cergy Paris Université, Observatoire de Paris, PSL University, Sorbonne Université, Université Paris Cité, CNRS, 95000 Cergy, France

^★ Corresponding author; shreyaks@mpe.mpg.de

Received: 26 July 2024
Accepted: 17 February 2025

Abstract

Context. NH₃ (ammonia) plays a critical role in the chemistry of star and planet formation, yet uncertainties in its binding energy (BE) values complicate accurate estimates of its abundance. Recent research suggests a multi-binding energy approach, challenging the previous single-value notion.

Aims. In this work, we use different values of NH₃ binding energy to examine its effects on the NH₃ abundances and the chemistry of Class 0 protostellar cores.

Methods. Using a gas-grain chemical network, we systematically vary the values of NH₃ binding energies in a model of a Class 0 protostellar core (using the model of IRAS 16293-2422 as a template) and study the effects of these binding energies on the NH₃ abundances.

Results. Simulations indicate that, in our model, the abundance profiles of NH₃ are highly sensitive to the binding energy used, particularly in the warmer inner regions of the core. Higher binding energies lead to lower gas-phase NH₃ abundances, while lower values of binding energy have the opposite effect. Furthermore, this BE-dependent abundance variation of NH₃ significantly affects the formation pathways and abundances of key species such as HNC, HCN, and CN. Our tests also reveal that the size variation of the emitting region due to binding energy becomes discernible only with beam sizes of 10 arcsec or less.

Conclusions. These findings underscore the importance of considering a range of binding energies in astrochemical models and highlight the need for higher resolution observations to better understand the subtleties of molecular cloud chemistry and star formation processes.