Numerical simulations of infrared emitting regions

This paper centers on the development of Cloudy, a large-scale code designed to compute the spectrum of gas in photo-ionization or collisional balance. Such plasma is far from equilibrium, and its conditions are set by the balance of a host of micro-physical processes. The development of Cloudy is a three-pronged effort requiring advances in the underlying atomic data base, the numerical and computational methods used in the simulation, culminating in the application to astronomical problems. These three steps are strongly interwoven. A complete simulation involves many hundreds of stages of ionization, many thousands of levels, with populations determined by a vast sea of atomic/molecular processes, many with accurate cross sections and rate coefficients only now becoming available. The scope of the calculations and the numerical techniques they use can be improved as computers grow ever faster, since previous calculations were naturally limited by the available hardware. In recent years, Cloudy has undergone a major upgrade in the atomic data. A large part of this effort was concentrated on improving the photo-ionization cross sections. This work is not complete however, and this paper will give a detailed discussion of the remaining problems and our future plans for improving the atomic database. First of all we will discuss our plans for further improvement of the photo-ionization cross section database. Next we will discuss the need for more accurate recombination rates. In particular we will discuss the problems surrounding the calculation of accurate dielectronic recombination rates. For these, accurate energies of auto-ionizing levels will have to be measured in laboratory experiments, so a lot of work still needs to be done in this field. Furthermore we discuss the completeness of the line list included in Cloudy and the results of the Opacity Project and the Iron Project, which are important new sources for transition probabilities and collision strengths. Based on new SWS measurements of the [Ne v] lines of NGC 6302, we show that the suggestion of Oliva et al. (1996), that these new collision strengths would be too high for Ne v, is not correct.