Interstellar Hydrocarbon Grains: Nature, Origin and Evolution
Anthony JONES
Institut d'Astrophysique Spatiale, Orsay
Résumé :
Carbon is the most abundant dust-forming element in the ISM. In this
talk I will restrict my definition of carbon grains to those comprised
principally of hydrocarbon materials, including those where the
hydrogen content is minimal. Following this definition the
interstellar hydrocarbon grains that I will discuss include: graphite,
hydrogenated amorphous aliphatic and/or aromatic hydrocarbons (a-C,
a-C:H) and (nano)diamond. These hydrocarbon dusts indeed play a
pivotal role in determining, amongst other things, the interstellar
extinction, the dust thermal emission and the photo-electric heating
of the gas in the ISM. Hydrocarbon grains are formed around C-rich
evolved stars, in SN ejecta and also in the ISM itself via accretion
and solid-state chemistry. The physico-chemical properties of
hydrocarbon grains are indeed complex and can vary in response to the
ambient conditions (density, temperature, radiation field, ...). For
example they can undergo chemical processing (growth and changes in
chemical composition through accretion and reaction, and erosion via
inertial or chemi-sputtering) and physical processing
(photo-darkening, bleaching, fragmentation in shocks and
photo-disruption in intense radiation fields). The physics and
chemistry of hydrocarbon grains is incredibly complex and it is
therefore unlikely that we will ever completely understand their exact
composition in the ISM based solely on the observational evidence
available. They are, in any event, likely to be a complex mixture of
many different forms arising from many different
sources. Nevertheless, we should clearly appreciate that this complex
material evolves chemically, structurally and physically as a function
of the ambient conditions and that the properties vary in a systematic
way. I will give a general overview of the nature, origin and
evolution of hydrocarbon materials, as constrained by observational,
laboratory experiments and detailed modelling, and apply this to
hydrocarbon grains in the ISM.
