Variations on a theme – the evolution of hydrocarbon solids

II. Optical property modelling – the optEC_(s) model^⋆,⋆⋆

Institut d’Astrophysique Spatiale, CNRS, IAS UMR8617, 91405 Orsay, France Université Paris Sud, IAS UMR8617, Orsay 91405, France
e-mail: Anthony.Jones@ias.u-psud.fr

Received: 4 July 2011
Accepted: 30 October 2011

Abstract

Context. The properties of hydrogenated amorphous carbon (a-C:H) dust are known to evolve in response to the local conditions.

Aims. We present an adaptable model for the determination of the optical properties of low-temperature, interstellar a-C:H grains that is based on the fundamental physics of their composition.

Methods. The imaginary part of the refractive index, k, for a-C:H materials, from 50 eV to cm wavelengths, is derived and the real part, n, of the refractive index is then calculated using the Kramers-Kronig relations.

Results. The formulated optEC_(s) model allows a determination of the complex dielectric function, ϵ, and refractive index, m(n,k), for a-C:H materials as a continuous function the band gap, E_g, which is shown to lie in the range  ≃−0.1 to 2.7 eV. We provide expressions that enable a determination of their optical constants and tabulate m(n,k,E_g) for 14 different values of E_g. We explore the evolution of the likely extinction and emission behaviours of a-C:H grains and estimate the relevant transformation time-scales.

Conclusions. With the optEC_(s) model we are able to predict how the optical properties of an a-C:H dust component in the interstellar medium will evolve in response to, principally, the local interstellar radiation field. The evolution of a-C:H materials appears to be consistent with many dust extinction, absorption, scattering and emission properties, and also with H₂ molecule, daughter “PAH” and hydrocarbon molecule formation resulting from its photo-driven decomposition.