Dust evolution, a global view: II. Top-down branching, nano-particle fragmentation and the mystery of the diffuse interstellar band carriers

The origin of the diffuse interstellar bands is one of the longest-standing mysteries of the interstellar medium is explored within the framework of The Heterogeneous dust Evolution Model at the IaS (THEMIS). The likely nature of the diffuse interstellar band 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 interstellar medium. Based on the natural aromatic-rich moieties (ashphaltenes) 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) nano-particles, is at the heart of the formation and evolution of the diffuse interstellar band carriers and their associations with small molecules and radicals such as C2, C3, CH and CN. It is most likely that the diffuse interstellar bands are carried by dehydrogenated, ionised, hetero- cyclic, aromatic rich moieties that form an integral part of the contiguous structure of hetero-atom doped hydrogenated amorphous carbon nano-particles and their daughter fragmentation products.