Dust evolution, a global view: III. Core/mantle grains, organic nano-globules, comets and surface chemistry

TL;DR

This paper explores how detailed surface chemistry of core/mantle interstellar and cometary grains can explain various astrophysical phenomena and suggests that dust chemistry is closely linked to interstellar gas chemistry.

Contribution

It introduces a comprehensive model of grain surface processes that accounts for multiple observed features in interstellar and cometary environments, proposing new mechanisms for organic and ice formation.

Findings

Explanation of oxygen depletion and CO dark gas

Formation of polar ice mantles and organic globules

Link between grain surface chemistry and observed spectral features

Abstract

Within the framework of The Heterogeneous dust Evolution Model at the IaS (THEMIS) this work investigates in detail the surface processes and chemistry relating to core/mantle interstellar and cometary grain structures and its influence on the nature of these fascinating particles. It appears that a realistic consideration of the nature and chemical reactivity of interstellar grain surfaces could self-consistently and within a coherent framework explain: the anomalous oxygen depletion, the nature of the CO dark gas, the formation of 'polar ice' mantles, the red wing on the 3 um water ice band, the basis for the O-rich chemistry observed in hot cores, the origin of organic nano-globules and the \sim 3.2 um 'carbonyl' absorption band observed in comet reflectance spectra. It is proposed that the reaction of gas phase species with carbonaceous a- C(:H) grain surfaces in the interstellar medium, in particular the incorporation of atomic oxygen into grain surfaces in epoxide functional groups, is the key to explaining these observations. Thus, the chemistry of cosmic dust is much more intimately related with that of the interstellar gas than has previously been considered. The current models for interstellar gas and dust chemistry will therefore most likely need to be fundamentally modified to include these new grain surface processes.