Grains in Photo-Ionized Environments

Ever since the pioneering study of Spitzer, it has been widely recognized that grains play an important role in the heating and cooling of photo-ionized environments. This includes the diffuse ISM, as well as H II regions, planetary nebulae, and photo-dissociation regions. A detailed code is necessary to model grains in a photo-ionized medium since the interactions of grains with their environment include a host of microphysical processes, and their importance can only be judged by performing a complete simulation. In this paper we will use the spectral synthesis code Cloudy for this purpose. A comprehensive upgrade of the grain model has been recently incorporated in Cloudy, and certain aspects of this upgrade will be discussed. Special emphasis will be on the new grain charge model. We will consider in detail the physics of grains in both ionized and neutral environments, and will present a calculation of photo-electric heating and collisional cooling rates for a range of physical conditions and grain materials and for a range of grain sizes (including a realistic size distribution). We conclude with a brief discussion of the problems currently hampering progress in this field. The new grain model will be used to model the silicate emission in the Ney-Allen nebula, and will help us better understand the nature of the grains in that part of the Orion complex.