Evolving interstellar extinction

Interstellar dust containing carbon will necessarily have optical constants that respond to the local environment of the dust. This response may be sufficiently slow that the extinction produced by such dust may change on time-scales that are comparable to the likely cloud age. Therefore, interstellar extinction caused by carbon-containing dust should evolve in time. We explore this concept through a model of dust in which H-rich amorphous carbon is deposited on silicate substrates in the interstellar medium, and is annealed to form H-poor amorphous carbon. We find that, in this model, the interstellar extinction is sensitive to the gas density (which controls the rate of carbon depletion) and to the UV radiation field (which controls the annealing rate). In clouds typical of the diffuse interstellar medium, the initial extinction curve is fairly flat in the far UV, becomes similar to the 'standard' interstellar curve after about 1-2 Myr, and remains similar to the standard curve for about a further 1 Myr. The curve then steepens in the far UV. The wide variations in the observed extinction may therefore simply reflect time-evolution of the dust properties.