Time‐dependent Photoionization in a Dusty Medium. I. Code Description and General Results

We present a time-dependent photoionization code that combinesself-consistently metal evolution and dust destruction under an intense X-rayUV radiation field. Firstly, we extend the mathematical formulation of thetime-dependent evolution of dust grains under an intense radiation flux withthe inclusion of the process of ion field emission (IFE). We determine therelative importance of IFE with respect to X-ray and UV sublimation as afunction of grain size, intensity and hardness of the incident spectrum. Wethen combine the processes of dust destruction with a photoionization code thatfollows the evolution of the ionization states of the metals and the relativeradiative transitions. Our code treats, self-consistently, the gradualrecycling of metals into gas as dust is sublimated away; it allows for anyinitial dust grain distribution and follows its evolution in space and time. Inthis first paper, we use our code to study the time-dependent behaviour of theX-ray and optical opacities in the nearby environment of a Gamma-ray Burst, andshow how the time variability of the low-energy and high-energy opacities canyield powerful clues on the characteristics of the medium in which the burstsoccur.Comment: 37 pages, 12 figures, final version accepted to Ap