Grains in Photo-Ionized Environments

University of Calgary, 2500 University Dr. NW, Calgary, ABT2N 1N4, CanadaAbstract. Ever since the pioneering study of Spitzer (1948), it has beenwidely recognized that grains play an important role in the heating andcooling of photo-ionized environments. This includes the diffuse ISM, aswell as H II regions, planetary nebulae, and photo-dissociation regions.A detailed code is necessary to model grains in a photo-ionized mediumsince the interactions of grains with their environment include a hostof microphysical processes, and their importance can only be judged byperforming a complete simulation. In this paper we will use the spectralsynthesis code Cloudy for this purpose. A comprehensive upgrade of thegrain model has been recently incorporated in Cloudy, and certain aspectsof this upgrade will be discussed. Special emphasis will be on the newgrain charge model. We will consider in detail the physics of grains inboth ionized and neutral environments, and will present a calculation ofphoto-electric heating and collisional cooling rates for a range of physicalconditions and grain materials and for a range of grain sizes (includinga realistic size distribution). We conclude with a brief discussion of theproblems currently hampering progress in this field. The new grain modelwill be used to model the silicate emission in the Ney-Allen nebula, andwill help us better understand the nature of the grains in that part of theOrion complex.1. IntroductionGrains are ubiquitous in the interstellar medium (ISM), and they can be de-tected either directly through their far-infrared emission or indirectly throughextinction or polarization studies. Despite the vast number of observations,many questions regarding grain composition and grain physics remain unan-swered. Further study is therefore required, and detailed models are needed tointerpret the results. Ever since the pioneering study of Spitzer (1948), it hasbeen widely recognized that grains play an important role in the heating andcooling of the diffuse ISM (see also the more recent studies by Bakes & Tielens1994, and Weingartner & Draine 2001a, hereafter WD). Grains also play animportant role in the physics of Hii regions and planetary nebulae (e.g., Maciel& Pottasch 1982, Baldwin et al. 1991, hereafter BFM, Borkowski & Harrington1