Variations on a theme - the evolution of hydrocarbon solids: II. Optical property modelling - the optEC(s) model

TL;DR

The paper introduces the optEC(s) model, a physics-based tool to predict the optical properties and evolution of hydrogenated amorphous carbon dust in the interstellar medium, accounting for local environmental effects.

Contribution

It provides a continuous function for optical constants of a-C:H materials based on band gap, enabling better modeling of dust evolution in space.

Findings

The model covers a wide wavelength range from 50 eV to cm.

Optical properties depend on the band gap, which varies from -0.1 to 2.7 eV.

Predicted dust evolution aligns with observed extinction and emission properties.

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, epsilon, and refractive index, m(n, k), for a-C:H materials as a continuous function the band gap, Eg , 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, Eg ) for 14 different values of Eg . 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.