Astronomy and Astrophysics – Astrophysics
Scientific paper
Apr 2012
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2012a%26a...540a...2j&link_type=abstract
Astronomy & Astrophysics, Volume 540, id.A2
Astronomy and Astrophysics
Astrophysics
1
Dust, Extinction, Ism: General
Scientific paper
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, ɛ, 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.
Appendices A and B are available in electronic form at http://www.aanda.orgData files are available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/540/A2
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