Variations on a theme – the evolution of hydrocarbon solids

Abstract

Context. The properties of hydrogenated amorphous carbon (a-C:H) dust evolve in response to the local radiation field in the interstellar medium (ISM) and the evolution of these properties is particularly dependent upon the particle size. Aims. A model for finite-sized, low-temperature amorphous hydrocarbon particles, based on the microphysical properties of random and defected networks of carbon and hydrogen atoms, with surfaces passivated by hydrogen atoms, has been developed. Methods. The eRCN/DG and the optEC (s) models have been combined, adapted and extended into a new optEC (s) (a) model that is used to calculate the optical properties of hydrocarbon grain materials down into the sub-nanometre size regime, where the particles contain only a few tens of carbon atoms. Results. The optEC (s) (a) model predicts a continuity in properties from large to small (sub-nm) carbonaceous grains. Tabulated data of the size-dependent optical constants (from EUV to cm wavelengths) for a-C:H (nano-)particles as a function of the bulk material band gap [E g (bulk)], or equivalently the hydrogen content, are provided. The effective band gap [E g (eff.)] is found to be significantly larger than E g (bulk) for hydrogen-poor a-C(:H) nano-particles and their predicted long-wavelength (λ > 30 μm) optical properties differ from those derived for interstellar polycyclic aromatic hydrocarbons (PAHs). Conclusions. The optEC (s) (a) model is used to investigate the size-dependent structural and spectral evolution of a-C(:H) materials under ISM conditions, including: the IR-FUV extinction, the 217 nm bump and the infrared emission bands. The model makes several predictions that can be tested against observations.

H2formation via the UV photo-processing of a-C:H nano-particles

“…1 schematic view), as predicted by Jones (2012c) and Jones et al (2013), comes from the observed 3−4 μm region emission band spectra in many sources, e.g., NGC 7023 (Pilleri et al 2015), the Orion Bar (Sloan et al 1997;Verstraete et al 2001), M17 SW, NGC 2023 (Verstraete et al 2001), M 82 (Yamagishi et al 2012) and the Seyfert 1 galaxy NGC 1097 (Kondo et al 2012). Even in these energetic regions the 3.3 μm aromatic CH band always appears to be accompanied by a CH n aliphatic plateau in the 3.4−3.6 μm region.…”

The cycling of carbon into and out of dust

“…[10][11][12] The band gap is therefore a proxy for the a-C(:H) material optical properties of a given R and X H . 10,11 It is therefore the evolution of the band gap that can be used to trace and characterise the inherent variability in the a-C(:H) optical properties and that is of prime importance in unravelling the evolution of hydrocarbon grains in the ISM.…”

“…10,11 It is therefore the evolution of the band gap that can be used to trace and characterise the inherent variability in the a-C(:H) optical properties and that is of prime importance in unravelling the evolution of hydrocarbon grains in the ISM. 15,[26][27][28] Recent modelling work [10][11][12] shows that the optical properties of a-C(:H) materials can be completely determined by two parameters, the band gap, E g , and the particle size, a, which turn out to be coupled for a < 30 nm, in that the minimum-possible band gap is given by 13…”

“…Recently, a completely new dust modelling approach was proposed, which does not include polycyclic aromatic hydrocarbons (PAHs), graphite grains or "astronomical silicate". 15 Instead, it uses the size-dependent optical and thermal properties for hydrogenated amorphous carbons, a-C(:H), [10][11][12][13][14] and the optical properties of an amorphous silicate with metallic iron inclusions. 15 The new aspects in this model are:…”

Dust heating

“…This size limitation can be applied to both PAHs and hydrogenated amorphous carbon, a-C(:H), grains since the structure of the small a-C(:H) grains is very close to that of the PAHs, i.e. they are both highly aromatic (Jones 2012). This number of carbon atoms corresponds to a PAH of radius a ≈ 0.77 nm and an a-C:H grain of radius a ≈ 0.82 nm with the density and the geometry (both PAHs and a-C:H grains considered as spherical) used in the Compiègne et al (2011) dust model.…”