nebular: Tasks for analyzing nebular emission lines

Introduction

Tasks in this package can be used to derive various physical quantities from the emission line fluxes of a low-density (nebular) gas. These quantities include the electron temperature (T_e) and density (N_e) from various diagnostic line flux ratios, and level populations, critical densities, line emissivities and abundances for several common ions. A brief summary of the tasks is given in the table below; details may be found in the following sections.

                    Table 1. NEBULAR Tasks
+----------------------------------------------------------------+
| Task      | Description                                        |
+----------------------------------------------------------------+
| abund     | Derive ionic abundances in a 3-zone nebula         |
| at_data   | Documentation on the atomic reference data         |
| diagcols  | P-set of table column names for computed T_e & N_e |
|           |    for each zone                                   |
| fluxcols  | P-set of table column names for input emission     |
|           |    line fluxes                                     |
| ionic     | Compute level populations, critical densities,     |
|           | line emissivities & abundance for a single ion     |
| nlevel    | Documentation on the N-level atom approximation    |
| ntcontour | Plot contours of line ratios on the N_e, Te plane  |
| ntplot    | Construct a N_e, Te plot for observed diagnostic   |
|           |    line ratios                                     |
| redcorr   | Correct line flux for interstellar reddening       |
| temden    | Compute T_e or N_e from diagnostic line ratios     |
| zones     | Derive T_e & N_e in 3-zone nebula from diagnostic  |
|           |    emission line ratios                            |
+----------------------------------------------------------------+

This package is not intended to offer a full nebular modelling program, such as G. Ferland's CLOUDY. Rather, it is intended for the fairly common instances where one has somewhat incomplete information about a complicated physical system (such as a narrow- line region in an active galactic nucleus), or somewhat more information about a physically simple system, such as a fairly evolved planetary nebula. In these cases it is useful to calculate nebular densities or temperatures from the traditional diagnostic line ratios, either to provide some resonable input parameters for a more complicated physical model, or to calculate ionic abundances (or other quantities) within some simplifying assumptions.

Some of the tasks in this package make use of STSDAS binary tables for accessing potentially large lists of emission line fluxes for many nebulae. The various line fluxes are contained in different columns, and data for different nebulae (or different regions within nebulae) are contained in separate rows. The `tables.ttools' tasks provide all the needed utilities for generating, editing, and printing the table contents. Examples of test tables can be found in the directory nebular$data. Generally, the user has control of the table column names through named parameter sets.

Nebular diagnostics and abundances

Several of the tasks are based upon the FIVEL program developed by De Robertis, Dufour, and Hunt (1987). These routines make use of the fact that that most of the common ions that dominate the nebular cooling rate have either p^2, p^3, or p^4 ground-state electron configurations, which have five low-lying levels. The major physical assumption within this algorithm is that only these five levels are physically relevant for calculating the observed emission line spectrum. These tasks have been extended beyond the original FIVEL program to provide diagnostics from a greater set of emission lines, most particularly those in the vacuum ultraviolet that are now available from the IUE and HST archives.

The `temden' task will calculate N_e given T_e, or T_e given N_e, for a given ion and the associated diagnostic flux ratio. The result is displayed and stored in a task parameter. The `ionic' task will calculate the level populations, critical densities, and line emissivities for a specified ion, given N_e and T_e. It will also calculate the ionic abundance relative to H+ if the wavelength and relative flux (on the scale I[H-beta] = 100) of one of the emission lines are also specified.

The available ions from which abundances and/or diagnostics can be derived, and the ionization potential of that ion, can be found by typing "help nlevel".

3-zone nebular model

The `zones' task calculates T_e and N_e within each of 3 zones of low-, medium-, and high-ionization. It uses iteration to make simultaneous use of temperature- and density-sensitive line ratios from different ions with similar ionization potential. The `abund' task computes the abundances for several ions using T_e and N_e as computed by `zones'. The ionization potential of the ion determines which T_e and N_e is used. The input line fluxes are taken from a specified table. UV fluxes can be given on a separate scale, provided that the UV-to-optical scale factor is specified. The input line fluxes can optionally be corrected for interstellar reddening.

Future development

Additional tasks are planned for this package which will derive the interstellar extinction coefficient from (among other methods) the hydrogen Balmer decrement, as well as compute ionic abundances (relative to H+) from recombination lines.

References

The 5-level atom program was originally published by DeRobertis, Dufour & Hunt (1987). Although the nebular tasks are intended to provide all the functionality of their original "FIVEL" FORTRAN program, the code has been entirely re-engineered, and essentially all the atomic data have been updated since that code was published. These tasks also offer additional options and flexibility, including tasks for computing all available diagnostics at once within a simple physical context. Additional enhancements and a discussion of the scientific problem domain are described by Shaw & Dufour (1995). Support for this software development was provided from the NASA Astrophysics Data Program through grant NAG5-1432 to Space Telescope Science Institute, and supplemented by a grant from the STScI Director's Discretionary Research Fund.