Grain Size Distributions and Photo-Electric Heating in Ionized Regions

In this paper we present results obtained with the new grain code in Cloudy which underline the strong effect of photo-electric heating by grains in photo-ionized regions. We study the effect that the distribution of grain sizes has on the magnitude of the effect, and show that this effect is nothing short of dramatic. This makes the grain size distribution an important parameter in modeling of photo-ionized regions such as H II regions and planetary nebulae.