-- A New Package for Nebular Analysis

A New Package for Nebular Analysis

A new package is available in the STSDAS external package, called nebular. Its various tasks can derive the physical conditions in a low-density (nebular) gas given appropriate diagnostic emission line ratios; and line emissivities and ionic abundances given appropriate emission line fluxes, and the electron temperature (T_e) and density (N_e). The tasks in this package are based on the 5-level atom program developed by De Robertis, Dufour & Hunt (Jou. Roy. Astron. Soc. Canada, 81, No. 6, 195, 1987). These tasks extend the functionality of 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. 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

     diagcols       P-set of table column names for computed T_e & N_e 
                    for each zone

     fivel          Documentation on the 5-level atom approximation

     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

     ntplot         Construct log N_e vs. T_e plot for observed diagnostic 
                    line ratios

     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 photo-ionization model, such as G. Ferland's CLOUDY program. Rather, it is most useful for calculating nebular densities and 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. This package is still under development, which is why it debuts under the playpen package, rather than under analysis. We hope to develop the nebular package more fully, to include tasks for deriving interstellar reddening, calculating ionic abundances from recombination lines, and calculating the nebular continuum flux.

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 package provides all the needed utilities for generating, editing, and printing the table contents. An example of a test table 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 Abundance

The nebular tasks make use of the fact 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 from a given ion.

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. As an example, suppose we wish to find the electron density from the [S II] diagnostic ratio: I(6716)/I(6731) = 0.9, assuming an electron temperature of 10,000 K.

	cl> temden density 0.9 atom=sulfur spectrum=2 assume=10000.
	[S ii] density ratio: I(6716)/I(6731) =   0.9
	Density: 855.32 /cm^3
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(Hbeta) = 100) of one of the emission lines are also specified. For example, suppose we wanted to know the abundance of the O^+ ion, relative to ionized hydrogen. The observed flux in the [O II] 3726.1 + 3728.8 A emission line doublet (relative to I(Hbeta) = 100) is provided (along with a wavelength tolerance large enough to accommodate both lines in the pair), to relate volume emissivities to ionic abundance.

	cl> ionic oxygen 2 temper=1.e4 dens=1000. wave=3728 wv_toler=2.0 \
	>>> flxratio=0.7 verb+

	Volume Emissivities for: O^1+
	    T_e:  10000.0;  N_e: 1.000E3

	Level Populations - Critical Densities

	Level 1:   9.9E-1
	Level 2: 6.207E-3     3.314E3
	Level 3: 1.764E-3     4.491E3
	Level 4: 1.210E-7     7.731E6
	Level 5: 7.344E-8     4.362E6

	        Wavelength:   3729.85
	Upper->Lower Level:   (2-->1)
 	 Volume Emissivity: 1.263E-21

	                      3727.14      512.76
	                      (3-->1)     (3-->2)
	                    1.551E-21   8.201E-28

	                      2471.12     7322.36     7332.83
	                      (4-->1)     (4-->2)     (4-->3)
	                    5.478E-23   3.841E-23   2.013E-23

	                      2471.05     7321.77     7332.24     9089.88
	                      (5-->1)     (5-->2)     (5-->3)     (5-->4)
	                    1.370E-23   1.225E-23   2.030E-23   3.338E-37

	H-beta Volume Emissivity: 1.258E-25 N(H+) * N(e-) ergs/s

	Log10(x) =   1.000E0

	Ionic Abundance: N(O^1+) / N(H+) = 3.129E-7
The available ions from which abundances and/or diagnostics can be derived number about two dozen, are given in the online help.

3-Zone Nebular Model

The zones task calculates T_e and N_e within each of three 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, which could exceed 50 in number, are taken from a specified STSDAS table. UV fluxes can be given on a separate flux scale, provided that the UV-to-optical scale factor is specified in the table. The input line fluxes can optionally be corrected for interstellar reddening.

The complementary ntplot task produces a plot from which a physical model can be inferred. Like zones and abund, this task takes its input from an STSDAS binary table (NOT an ASCII table) and plots curves in the N_e, T_e plane that are consistent with the observed diagnostic line ratios. The output curves (which are not shown here) can optionally be stored in an STSDAS table for use as input to a presentation graphics task, such as igi in the TABLES tbplot package.

For Further Information...

A complete description of the equations to be solved and their method of solution can be found in the paper by De Robertis, Dufour & Hunt. Additional enhancements, including atomic parameters for C III] and Si III], are described by Shaw & Dufour in "Astronomical Data Analysis Software and Systems III", ASP Conf. Ser., Vol. 61, p. 327, 1994. (This paper is also available on the WWW at URL:

	http://cadcwww.dao.nrc.ca/ADASS/adass_proc/adass3/papers/shawr/shawr.html.)  
Type help fivel in the nebular package for additional information about the 5-level atom approximation, and for references to the atomic parameters.

This package is available for personal installation for users who do not have STSDAS installed, but who do have IRAF (V2.10.2 or later) and TABLES (V1.3 or later) installed. Simply retrieve the files "neb.tar" (or "neb.tar.Z") and "neb.README" in the directory /software/stsdas/outgoing from node ftp.stsci.edu and follow the installation instructions. Additional updates to this task are described in the "neb.README" file, and in postings to the adass.iraf.applications newsgroup. Funding to develop this package was provided by the NASA Astrophysics Data Program, through grant NAG5-1432 to the Space Telescope Science Institute.

This article, with additional tables and figures, appeared in the STSDAS Newsletter (Spring 1994 issue). All of the STSDAS Newsletters are available via WWW through URL: http://ra.stsci.edu/Newsletter.html. You may obtain hardcopies of the Newsletter by sending an email request to hotseat@stsci.edu.

Dick Shaw
shaw@stsci.edu


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