Chemical modeling of water deuteration in IRAS16293-2422

TL;DR

This study models water deuteration in IRAS16293-2422 using gas-grain chemistry, revealing discrepancies between observed and predicted abundances and suggesting surface reactions as a key formation mechanism.

Contribution

It introduces a detailed chemical model incorporating an extended deuterium network and analyzes the physical conditions affecting water deuteration in a protostellar environment.

Findings

Gas-phase water and deuterated water abundances are lower than model predictions.

Outer envelope shows chaotic abundance profiles due to adsorption and evaporation.

High abundances in the absorption layer are likely due to surface reactions, not photodesorption.

Abstract

IRAS 16293-2422 is a well studied low-mass protostar characterized by a strong level of deuterium fractionation. In the line of sight of the protostellar envelope, an additional absorption layer, rich in singly and doubly deuterated water has been discovered by a detailed multiline analysis of HDO. To model the chemistry in this source, the gas-grain chemical code Nautilus has been used with an extended deuterium network. For the protostellar envelope, we solve the chemical reaction network in infalling fluid parcels in a protostellar core model. For the foreground cloud, we explored several physical conditions (density, cosmic ionization rate, C/O ratio). The main results of the paper are that gas-phase abundances of H2O, HDO and D2O observed in the inner regions of IRAS16293-2422 are lower than those predicted by a 1D dynamical/chemical (hot corino) model in which the ices are fully evaporated. The abundance in the outer part of the envelope present chaotic profiles due to adsorption/evaporation competition, very different from the constant abundance assumed for the analysis of the observations. We also found that the large abundances of gas-phase H2O, HDO and D2O observed in the absorption layer are more likely explained by exothermic surface reactions rather than photodesorption processes.