Variations on a theme – the evolution of hydrocarbon solids
Institut d’Astrophysique Spatiale, CNRS, IAS UMR8617, 91405 Orsay, France
Université Paris Sud, IAS UMR8617, Orsay 91405, France
Received: 4 July 2011
Accepted: 30 October 2011
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, 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.
Key words: dust, extinction / ISM: general
Appendices A and B are available in electronic form at http://www.aanda.org
Data files are available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (184.108.40.206) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/540/A2
© ESO, 2012