Low ionization stages of iron, owing to their rich and complex structure and their relatively high abundance, dominate certain wavelength regions in the spectra of a variety of stellar and non-stellar objects, as well as the interstellar medium. Photoionization models of low excitation HII regions show Fe IV to be the dominant ionization state throughout most of the interior of the cloud, along with Fe I-III. Fe IV and Fe V have recently been identified in the EUV spectra of photospheres of hot, young white dwarfs, as recently observed by the Extreme Ultraviolet Explorer (Pradhan 1995; Vennes 1995). In both cases accurate photoionization cross sections are needed to model these objects.
The interpretation and modeling of observations rely almost entirely on theoretical values of the atomic data, which until the recent advances made under the Opacity Project (OP, Seaton etal. 1994) and the Iron Project (IP, Hummer etal. 1993) could not be obtained with sufficient accuracy and on the large scale needed to fully determine the relevant astrophysical parameters. However, the OP calculations of radiative data and photoionization cross sections for the low ionization stages of Iron, i.e. Fe I-V, are not sufficiently accurate for modeling and lead to significant discrepancies with observations (e.g. Pradhan 1995). One of the goals of the IP is to carry out improved calculations for these ions (Bautista etal. 1996). This paper presents the results for Fe IV, which completes the set of improved radiative atomic data for the Fe I-V sequence. The calculations for the other ions were reported in Bautista & Pradhan (1995) and Bautista (1997) for Fe I, Nahar & Pradhan (1994) and Nahar (1995) for Fe II, Nahar (1996) for Fe III, and Bautista (1996) for Fe V.
A full list of papers from the IRON Project published to-date is given in the references. A complete list of papers including those in press can be found at http://www.am.qub.uk/projects/iron/papers/, where abstracts are also given for each paper. Information on other works by the present authors and their collaborators, including collisional excitation and recombination of ions of iron and other elements, can be found at http://www-astronomy.mps.ohio-state.edu/pradhan/.