The interpretation of emission line radiation from an ionized gas is important in a wide variety of astrophysical contexts, such as H II regions, planetary nebulae, active galactic nuclei, and nova and supernova remnants. The physical basis for line emission from a photoionized nebula has been well understood for decades, and is discussed in many excellent references (see, e.g., Osterbrock 1989; Aller 1984). It turns out that most of the common ions that dominate the nebular cooling rate have ground-state electron configurations with five low-lying levels. To fair approximation, only these five levels are relevant to calculating the observable emission line spectrum for a given ion. Transitions between these five levels span the range from the satellite ultraviolet to the infrared, and all are now observable with a combination of ground-based and space-based observing facilities.
It is relatively straightforward within this five-level atom approximation to solve the equations of statistical equilibrium (see § 2.1 ) to obtain the level populations and line emissivities. Certain ratios of these line emissivities are particularly good indicators of electron temperature (T) or density (N) (see § 2.2 ), and one can employ an iterative technique to match an observed line ratio to that computed for a given T and N . Such a technique was developed by De Robertis, Dufour & Hunt (1987; hereafter, DDH), who published a FORTRAN algorithm to derive T and N for a large number of commonly used diagnostic line ratios. Using several such diagnostics from ions with different ionization potential allows one to infer a simple physical model for an observed nebula, from which one can drive ionic abundances.
We developed a package of applications (tasks) for the IRAF/STSDAS environment to derive the physical conditions in a low-density (i.e., nebular) gas given appropriate diagnostic emission line ratios; and line emissivities given appropriate emission line fluxes, the electron temperature and density. Most tasks in this package, called nebular, are based on the FIVEL program developed by DDH, who described the equations to be solved and their method of solution. These tasks extend the functionality of the original FIVEL program, and also provide a very simple model within which to derive the nebular ionic abundances. These tasks are most useful in the fairly common instances where one desires to calculate nebular densities or temperatures directly from the traditional diagnostic line ratios, either to provide some reasonable input parameters for a more complicated physical model, or to calculate ionic abundances (or other quantities) within some simplifying assumptions.
The major reason to build this software was our need to analyze the physical conditions and ionic abundances for a large number () of nebulae relatively quickly to support an archival research program. For almost all the nebulae in that program, we used a combination of spectra from the IUE and HST archives, and/or fluxes published in the literature. As such, the data are often incomplete in the sense that only a few diagnostics may be available, and they may be different from one nebula to the next, depending upon the quality and extent of the observations, and upon the excitation level of the nebula. In cases where the data are sparse, it may not be possible to construct a very complete physical model, beyond an average temperature and density; in other cases a very detailed model may be in order, in which case we look to the traditional diagnostics as a starting point.
We chose to build upon the FIVEL library because it was fairly straightforward to do, and because it contained much of the functionality we needed. However, the user interface was rather awkward, and was very inefficient for analyzing potentially dozens of diagnostics in each of over one hundred nebulae. We chose to port this application to the IRAF/STSDAS environment in order to provide a simple, command-line interface to the various tasks, and to take advantage of an environment that was widely available and highly portable so that our collaborators could run the same software and share in the analysis. We also needed to make use of the STSDAS/TABLES data structure to provide access to our library of flux measurements, and to the derived temperatures, densities, and ionic abundances. In this way, we could update any of the values for any or all nebulae and re-run our analysis with a negligible cost in time or resources. Finally, we wanted to take advantage of the built-in graphics and error-handling capabilities that are available in IRAF.
In the sections that follow we review the equations to be solved to derive the line emissivities and other quantities of interest. Then we provide an overview of the capabilities of this software, followed by astrophysically motivated examples with line fluxes taken from the literature. We conclude with information for retrieving this software, and our plans for enhancing it in the future.