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 issuein several astrophysical contexts. We use the time-dependent photoionizationcode in a dusty medium developed by Perna & Lazzati (2002), to study themodifications in the dust distribution and the relative optical opacities whena strong X-ray UV radiation flux propagates into a medium. We find thatsilicates are preferentially destroyed with respect to graphite, and theextinction curve becomes significantly flatter (hence implying less reddening),with the characteristic bump at lambda 2175 A highly suppressed, due to thedestruction of the small graphite grains. This could explain the observationallack of such a feature in GRB afterglow and AGN spectra. For a very intense andhighly variable source irradiating a compact and dense region, time variabilityin the optical opacity resulting from dust destruction can be observed on arelatively short timescale. We show that, under these circumstances, monitoringthe time variability of the opacity can yield powerful clues on the propertiesof dust in the environment of the source. In particular, it allows to break theobservational degeneracy as to whether a grey extinction is the result of a lowdust-to-gas ratio or of a dust grain distribution that is skewed towards largegrains.Comment: ApJ in press; corrected typo in Fig.