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

Context. 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. The stability of interstellar a-C(:H) nanoparticles against photo-thermo-dissociation and Coulomb fragmentation needs to take their intrinsic heterogeneity into account. Aims. 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. Methods. Using a statistical approach, we determined 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. Results. The derived UV photon-induced fragmentation lifetimes for nanometre-sized a-C(:H) nanoparticles, with radii ∼0.4-0.5 nm radius and containing ∼40-60 carbon atoms, are of the order of 106-107 yr in the diffuse interstellar medium and likely 102-104 times shorter in photodissociation regions, depending on the local radiation field intensity. Grains larger than this are stable against photodissociation. In HII regions only a-C(:H) nanoparticles with radii greater than 0.7 nm (≳150 carbon atoms) are likely to survive. Conclusions. 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 H <ns0:sc>II</ns0:sc> regions. However, in these same regions Coulomb fragmentation is unlikely to be an important dust destruction process.