Abstract
The origin of the diffuse interstellar bands (DIBs), one of the longest-standing mysteries of the interstellar medium (ISM), is explored within the framework of The Heterogeneous dust Evolution Model for Interstellar Solids (THEMIS). The likely nature of the DIB carriers and their evolution is here explored within the framework of the structures and sub-structures inherent to doped hydrogenated amorphous carbon grains in the ISM. Based on the natural aromatic-rich moieties (asphaltenes) recovered from coal and oil, the likely structure of their interstellar analogues is investigated within the context of the diffuse band problem. It is here proposed that the top-down evolution of interstellar carbonaceous grains, and, in particular, a-C(:H) nanoparticles, is at the heart of the formation and evolution of the DIB carriers and their associations with small molecules and radicals, such as C2, C3, CH and CN. It is most probable that the DIBs are carried by dehydrogenated, ionized, hetero-cyclic, olefinic and aromatic-rich moieties that form an integral part of the contiguous structure of hetero-atom-doped hydrogenated amorphous carbon nanoparticles and their daughter fragmentation products. Within this framework, it is proposed that polyene structures in all their variants could be viable DIB carrier candidates.
Highlights
The search for the origin of the diffuse interstellar bands (DIBs) has been a long journey (e.g. [1,2,3]); a journey that is far from over despite the recent assignment of four DIBs to the fullerene cation, C+60 [4,5].2016 The Authors
This paper is structured as follows: §2 considers the challenge presented by the overwhelming variety of possible chemical structures; §3 proposes a new nomenclature scheme for characterizing hetero-aromatic moieties; §4 summarizes some of the known effects of dehydrogenation and ionization on aromatic species; §5 suggests a new form of hydrocarbon ring molecule; §6 investigates the likely nature of the DIB carriers in the interstellar medium (ISM); §7 proposes a new top-down branching route for nanoparticle evolution and the formation of the DIB carriers in the ISM; §8 suggests possible follow-on investigations and §9 concludes
It is likely that the DIB carriers are related to the extended red emission (ERE) carriers and that both arise in species that are marginally stable in a wide variety of environments, in the diffuse ISM [96]
Summary
The search for the origin of the diffuse interstellar bands (DIBs) has been a long journey (e.g. [1,2,3]); a journey that is far from over despite the recent assignment of four DIBs to the fullerene cation, C+60 [4,5]. The approach adopted here is generic and will, certainly not furnish the specific identification of a single DIB carrier If this exploration can bring something to bear on the likely nature and evolution of solid carbonaceous matter and its relationship to the DIBs, especially at nanoparticle and smaller size-scales, it could lead to the elucidation of whole families of related but structurally distinct species or grain sub-structures that may be central to an explanation for the DIBs, their origin, evolution and associations with particular molecules, ions and radicals. This paper is structured as follows: §2 considers the challenge presented by the overwhelming variety of possible chemical structures; §3 proposes a new nomenclature scheme for characterizing hetero-aromatic moieties; §4 summarizes some of the known effects of dehydrogenation and ionization on aromatic species; §5 suggests a new form of hydrocarbon ring molecule; §6 investigates the likely nature of the DIB carriers in the interstellar medium (ISM); §7 proposes a new top-down branching route for nanoparticle evolution and the formation of the DIB carriers in the ISM; §8 suggests possible follow-on investigations and §9 concludes
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