The essential elements of dust evolution: a-C(:H) nanoparticle sub-structures and photo-fragmentation

Abstract

Hydrogenated amorphous carbon materials, a-C(:H), are heterogeneous structures consisting of carbon atoms in different hybridisation states and bonding configurations and are thought to constitute a significant and observationally important fraction of the interstellar dust material. This work aims to characterise semi-conducting a-C(:H) nanoparticle structures and, in particular, their property-characterising aromatic domain size distribution and so predict how they will behave in intense UV radiation fields that can fragment them through dissociative and charge effects as a result of carbon-carbon bond-breaking. Using a statistical approach we determine the typical sizes of the aromatic domains, their size distribution, how they are network-bonded, and where they are to be found within the structure. We consider the effects of thermal excitation, photo-dissociation and charging of a-C(:H) nanoparticles, and the products of their fragmentation. The derived UV photon-induced fragmentation lifetimes for nanometre-sized a-C(:H) nanoparticles, with radii ~0.4-0.5nm radius and containing ~40-60 carbon atoms, are of the order of 10^6-10^7yr in the diffuse interstellar medium and likely 10^2-10^4 times shorter in photodissociation regions, depending on the local radiation field intensity. Grains larger than this are stable against photodissociation. In H{\footnotesize II} regions only a-C(:H) nanoparticles with radii greater than 0.7nm (> 150 carbon atoms) are likely to survive. The photon-driven fragmentation of sub-nanometre a-C(:H) particles was determined to be important in the diffuse interstellar medium and also in high excitation regions, such as photodissociation and HII regions. However, in these same regions Coulomb fragmentation is unlikely to be an important dust destruction process.