# Why does ammonia not freeze out in the center of pre-stellar cores?

## TL;DR

This study investigates why ammonia does not freeze out in the center of pre-stellar core L1544, finding that current models cannot reproduce the observed increase and suggesting chemical desorption efficiency constraints.

## Contribution

The paper provides a detailed parameter-space exploration of ammonia abundance in L1544, highlighting limitations of existing models and proposing constraints on chemical desorption efficiency.

## Key findings

- Models fail to reproduce observed ammonia increase in core center.
- Chemical desorption efficiency likely below 1%.
- Time-dependent effects are crucial for accurate modeling.

## Abstract

We carried out a parameter-space exploration of the ammonia abundance in the pre-stellar core L1544, where it has been observed to increase toward the center of the core with no signs of freeze-out onto grain surfaces. We considered static and dynamical physical models coupled with elaborate chemical and radiative transfer calculations, and explored the effects of varying model parameters on the (ortho+para) ammonia abundance profile. None of our models are able to reproduce the inward-increasing tendency in the observed profile; ammonia depletion always occurs in the center of the core. In particular, our study shows that including the chemical desorption process, where exothermic association reactions on the grain surface can result in the immediate desorption of the product molecule, leads to ammonia abundances that are over an order of magnitude above the observed level in the innermost 15000 au of the core - at least when one employs a constant efficiency for the chemical desorption process irrespective of the ice composition. Our results seemingly constrain the chemical desorption efficiency of ammonia on water ice to below 1%. It is increasingly evident that time-dependent effects must be considered so that the results of chemical models can be reconciled with observations.