Time-dependent photoionization in a dusty medium. II. Evolution of dust distributions and optical opacities

The interaction of a radiation field with a dusty medium is a relevant issue in several astrophysical contexts. We use the time-dependent photoionization code in a dusty medium developed by Perna & Lazzati to study the modi cations in the dust distribution and the relative optical opacities when a strong X-ray/UV radiation flux propagates into a medium. We find that silicates are preferentially destroyed with respect to graphite, and the extinction curve becomes significantly flatter (hence implying less reddening), with the characteristic bump at 2175 Angstrom being highly suppressed because of the destruction of the small graphite grains. This could explain the observational lack of such a feature in gamma-ray burst afterglow and active galactic nuclei spectra. For a very intense and highly variable source irradiating a compact and dense region, time variability in the optical opacity resulting from dust destruction can be observed on a relatively short timescale. We show that, under these circumstances, monitoring the time variability of the opacity can yield powerful clues to the properties of dust in the environment of the source. In particular, it allows one to break the observational degeneracy as to whether a gray extinction is the result of a low dust-to-gas ratio or of a dust grain distribution that is skewed toward large grains.