Sense and Sensitivity - I. Uncertainty analysis of the gas-phase chemistry in AGB outflows

TL;DR

This study performs the first uncertainty analysis of chemical models in AGB star outflows, revealing how reaction rate uncertainties affect predicted molecular abundances and the interpretation of observational data.

Contribution

It introduces a Monte Carlo-based uncertainty quantification method for chemical models of AGB outflows, highlighting the impact of reaction rate uncertainties on model predictions.

Findings

Errors in parent species envelope sizes are influenced by chemistry, not photodissociation rates.

Uncertainty in CO envelope size can lead to a twofold error in mass-loss rate estimates.

Peak fractional abundance errors for daughter species can reach three orders of magnitude.

Abstract

Chemical reaction networks are central to all chemical models. Each rate coefficient has an associated uncertainty, which is generally not taken into account when calculating the chemistry. We performed the first uncertainty analysis of a chemical model of C-rich and O-rich AGB outflows using the Rate22 reaction network. Quantifying the error on the model predictions enables us to determine the need for adding complexity to the model. Using a Monte Carlo sampling method, we quantified the impact of the uncertainties on the chemical kinetic data on the predicted fractional abundances and column densities. The errors are caused by a complex interplay of reactions forming and destroying each species. Parent species show an error on their envelope sizes, which is not caused by the uncertainty on their photodissociation rate, but rather the chemistry reforming the parent after its photodissociation. Using photodissociation models to estimate the envelope size might be an oversimplification. The error on the CO envelope impacts retrieved mass-loss rates by up to a factor of two. For daughter species, the error on the peak fractional abundance ranges from a factor of a few to three orders of magnitude, and is on average about 10\% of its value. This error is positively correlated with the error on the column density. The standard model suffices for many species, e.g., the radial distribution of cyanopolyynes and hydrocarbon radicals around IRC +10216. However, including spherical asymmetries, dust-gas chemistry, and photochemistry induced by a close-by stellar companion are still necessary to explain certain observations.