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

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.

Accepted to MNRAS

References in corpus (16)

• Non-thermal desorption from interstellar dust grains via exothermic surface reactions
• The 2014 KIDA network for interstellar chemistry
• Observing the gas temperature drop in the high-density nucleus of L 1544
• Polycylcic Aromatic Hydrocarbons (PAH's) in dense cloud chemistry
• Formation of Complex Molecules in Prestellar Cores: a Multilayer Approach
• Isotopic fractionation of carbon, deuterium and nitrogen : a full chemical study
• Multilayer formation and evaporation of deuterated ices in prestellar and protostellar cores
• Benchmarking spin-state chemistry in starless core models
• Consistent dust and gas models for protoplanetary disks: II. Chemical networks and rates
• The abundances of nitrogen-containing molecules during pre-protostellar collapse
• The central 1000 AU of a pre-stellar core revealed with ALMA. I. 1.3 mm continuum observations
• Chemistry and Radiative Transfer of Water in Cold, Dense Clouds
• Dust opacity variations in the pre-stellar core L1544
• NH_3(1_0-0_0) in the pre-stellar core L1544
• UV photoprocessing of NH3 ice: photon-induced desorption mechanisms
• H${}_2$ ortho-to-para conversion on grains: A route to fast deuterium fractionation in dense cloud cores?

Cited by in corpus (28)

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• An Interferometric View of H-MM1. I. Direct Observation of NH3 Depletion
• LOC program for line radiative transfer
• Modelling Deuterated Isotopologues of Methanol toward the Pre-Stellar Core L1544
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• Effect of grain size distribution and size-dependent grain heating on molecular abundances in starless and pre-stellar cores
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• A large (~1 pc) contracting envelope around the prestellar core L1544
• Deuteration of c-C${}_3$H${}_2$ towards the pre-stellar core L1544
• Chemistry and dynamics of the prestellar core L1544
• First survey of HCNH${}^{+}$ in high-mass star-forming cloud cores
• Chemical post-processing of magneto-hydrodynamical simulations of star-forming regions: robustness and pitfalls
• Nitrogen fractionation in ammonia and its insights on nitrogen chemistry
• Similar levels of deuteration in the pre-stellar core L1544 and the protostellar core HH211
• Non-Equilibrium Abundances Treated Holistically (NEATH): the molecular composition of star-forming clouds
• Robust Binding Energy Distribution Sampling on Amorphous Solid Water Models. Method testing and validation with NH3, CO and CH4
• A multigrain-multilayer astrochemical model with variable desorption energy for surface species
• Tracing the contraction of the pre-stellar core L1544 with HC${}^{17}$O${}^{+}$ $J$ = 1-0 emission
• Hunting pre-stellar cores with APEX: Corona Australis 151, the densest pre-stellar core or the youngest protostar?
• Oscillations in Gas-grain Astrochemical Kinetics
• KAgoshima Galactic Object survey with Nobeyama 45-metre telescope by Mapping in Ammonia lines (KAGONMA): Discovery of parsec-scale CO depletion in the Canis Major star-forming region
• Impact of ice growth on the physical and chemical properties of dense cloud cores
• Effects of grain temperature distribution on organic protostellar envelope chemistry
• Modeling the UV-photon irradiation of CS${}_2$-bearing ices in the laboratory with the pyRate gas-grain astrochemical code. New insights into the missing sulfur problem