Invited Talk

Amorphous Hydrocarbon Dust Evolution in the ISM: Theory, Modelling and Observation

Anthony Jones (Institut d'Astrophysique Spatiale d'Orsay)

Various forms of carbonaceous matter have been proposed as models for carbon-rich dust in the ISM, including, but not exclusively: graphite, (nano)diamond, soot, coal, kerogen, organic refractory ("yellow stuff"), quenched carbonaceous composite (QCC) and (hydrogenated) amorphous carbons (a-C, HAC, a-C:H). Observational signatures that have been tied to carbon-rich dust in the ISM and circumstellar media include: the FUV extinction, the UV bump at 217 nm, the visible-NIR extinction, the extended red emission (ERE) and NIR emission, the MIR emission bands and a contribution to the MIR-FIR-mm dust emission continuum. Perhaps the most promising candidate material for cosmic carbonaceous dust is the suite of hydrogenated amorphous carbon, a-C(:H), materials whose optical properties were recently explored in detail within the astrophysical context (Jones 2012a,b,c). The properties of a-C(:H) grains in the ISM evolve, principally, in response to the local UV radiation field. The exact nature of this evolution is particularly dependent upon the particle size. In this talk I will introduce the optEC(s)(a) model for determining the optical properties of finite-sized, low-temperature-deposited, a:C(:H) particles. This optEC(s)(a) model is based on the microphysical properties of surface-hydrogenated hydrocarbon particles determined using extended random covalent network (eRCN) and defected graphite (DG) approaches. The eRCN and DG models, combined with the optEC(s) model for bulk materials, are used to derive the size-, band gap- and hydrogen atom fraction-dependent optical properties of hydrocarbon grains down into the sub-nm size regime. The effective band gap of nm and sub-nm a-C(:H) particles, Eg(eff.), is found to be significantly larger than that for the equivalent bulk material, Eg(bulk), for hydrogen-poor a-C(:H). Further, their predicted long-wavelength (> 30 micron) optical properties differ from those derived for interstellar PAHs. The optEC(s)(a)-modelled a:C(:H) particle optical properties have been used to investigate the size-dependent evolution of a-C(:H) materials under ISM conditions, including their: IR-FUV extinction, UV bump and IR absorption and emission bands.

References

Jones, A.P. 2012a, A&A, 540, A1
Jones, A.P. 2012b, A&A, 540, A2
Jones, A.P. 2012c, A&A, in press

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