Dust evolution in the ISM: theory, modelling and observation
Recent research implies that silicate and carbonaceous dust may follow different evolutionary pathways in their sojourn through the interstellar medium (ISM) from their birth-sites. It now appears that the carbonaceous dust component may not be as resilient as silicate dust (e.g., Serra Diaz-Cano & Jones 2008; Jones & Nuth 2011; Micelotta, Jones & Tielens 2011a,b).
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), (hydrogenated) amorphous carbons (a-C, a-C:H, HAC) and polycyclic aromatic hydrocarbons (PAHs). Observational signatures that have been tied to carbon-rich dust in the ISM and circumstellar media include: the FUV extinction, the UV bump at 217nm, 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.
A major and unavoidable conclusion of the current dust ‘lifetime’ calculations is that ‘graphite-like’ materials are probably not going to be able to re-form in the ISM and that they are therefore not a tenable model for the bulk of the interstellar carbon-rich dust. Nevertheless, some form of carbonaceous dust clearly needs to be (re)formed in the ISM.
Perhaps the most promising candidate material for cosmic carbonaceous dust is the suite of hydrogenated amorphous carbon, a-C(:H), materials (Jones 2012a,b,c). In this talk I will introduce a new model for the derivation of the size- and composition-dependent optical properties of low-temperature-deposited, a:C(:H) particles. The model is calibrated against the available laboratory data, rather than being tuned to fit astrophysical observations.
In this talk I will consider the survivability of carbonaceous and silicate dust in the ISM and how they evolve in response to the local conditions. The primary agents that drive dust evolution in the ISM are irradiation by UV photons, and irradiation by ions and electrons in interstellar shock waves and cosmic rays. In an attempt to ‘put it all together’ I will also re-visit the question of the ‘dust lifetime’ in the ISM and whether such a globally-applied concept really has any useful meaning in the sense that it is usually used.