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

TL;DR

This work addresses how the heterogeneous sub-structures of hydrogenated amorphous carbon (a-C(:H)) nanoparticles govern their survival and evolution under UV irradiation in the ISM. It introduces a statistical, structure-based modelling approach within the THEMIS framework to characterize aromatic-domain sizes, network connectivity, and fragmentation pathways, incorporating a photo-thermo-dissociation (PTD) inspired mechanism and a Coulomb-fragmentation assessment. The findings show that sub-nanometer a-C(:H) grains typically host aromatic domains of $N_{ m R}\approx 2$–$3$, connected by aliphatic-olefinic bridges of about 4–6 carbons, with roughly half of edge sites linked to the network; photon-driven fragmentation lifetimes in the diffuse ISM are of order $t_{ m pf}\sim 10^6$–$10^7$ yr and are shorter in harsher environments, while Coulomb fragmentation is unlikely except under extreme charge states. These results refine our understanding of carbonaceous dust processing, support the THEMIS view of interstellar dust evolution, and inform interpretations of IR emission and extinction across diverse radiation fields.

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.

Figures (4)

• Figure 1: Aromatic clusters coordination numbers, $m_{\rm cluster}$ (red lines, upper and lower limits shown dashed), and number of network linking bonds, $n_{\rm link}$, in a-C(:H) nanoparticles as a function of $N_{\rm R}$ and radius (0.4, 0.6, 0.8, 1.0, 1.2 nm, thin to thick black and grey lines, respectively). For each particle radius (same line thickness) the blue cross ordinate indicates the size-dependent hydrogen to carbon atomic ratios, [H]/[C], multiplied by 10.
• Figure 2: Schematic structure of an idealised, simplified and unfolded, shell-like a-C nanoparticle, C$_{43}$H$_{40}$, which could also be a substructure within a larger a-C(:H) nanoparticle network. The dashed green arrows indicate the common linking bonds that close the shell structure. The red dots indicate the equally probable (i.e. lowest energy) bond cleavage sites. The unadorned, dangling bonds mark the positions of H atoms.
• Figure 3: THEMIS a-C(:H) nanoparticle rates constants for dissociation, $k_{\rm diss}$ (upper plot), and IR photon emission, $k_{\rm IR}$ (lower plot), as a function of the internal energy $E_{\rm int}$ and particle radius from 0.3 to 0.74 nm, thinner to thicker lines, respectively. The dashed lines indicate particles with $N_{\rm C} = 23$ and 61 (see text for explanation).
• Figure 4: THEMIS a-C(:H) nanoparticle initial temperatures (upper plot), and in the lower plot photon absorption timescales (blue), and lifetimes (black) for the ISRF conditions ($G_0$, $F_{\rm UV}$$E_{h\nu}$, and $A_{\rm V}$) given in Table \ref{['table_summary']}. The thin vertical dashed lines indicate the THEMIS minimum grain size in the diffuse ISM ($a \sim 0.4-0.5$ nm), and the derived limit for harsh PDR environments 2022AA...666A..49S and 2024AA...685A..76E. The horizontal dashed lines mark the million year lifetime (lower plot) and $T=10^3$ K (upper plot).