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zones analysis.nebular


NAME · USAGE_ · DESCRIPTION · ALGORITHM · PARAMETERS · EXAMPLES
BUGS · REFERENCES · SEE_ALSO

NAME

zones -- Derive T_e and N_e in a 3-zone nebula from diagnostic lines

USAGE

zones fluxtab outtab

DESCRIPTION

This task computes the electron temperatures (T_e) and densities (N_e) of an ionized nebular gas in separate zones of low- medium- and high-ionization from a variety of diagnostic emission line fluxes. The diagnostics for each available ion are calculated within the N-level atom approximation. (For more details about this approximation, type "help nlevel".) The output from this task can be used to compute the ionic abundances for all available ions with the `abund' task. The point of having two separate tasks is to permit the user to inspect and evaluate the results from `zones' before proceding with the abundance calculations.

The user specifies the names of an input table of emission line fluxes, and an output table for the results. If these names are identical, then new columns will be added (if necessary) to the input table to contain the calculated densitites and temperatures. The input table may contain line fluxes for many nebulae and/or many regions within nebulae, one object/region per row. The flux for each emission line or line ratio must be given in separate columns. The task locates particular emission line fluxes via names of specific columns in the input table. These columns have suggestive default names, but are entirely user-definable; type "help fluxcols" for more details. Note that the input table need not actually contain all the diagnostic lines specified in this p-set: if a named column is not found, the corresponding calculation may be skipped. HOWEVER, the target name and region name ARE required for all nebulae, and columns with that information must exist in the input table.

The diagnostic line ratios are derived from the input line fluxes, corrected for interstellar reddening. The reddening corrected line flux "I" is derived from the input line flux "F" by:

	  I(line) = F(line) * dex {-c * f(lambda)}

where "c" is the extinction constant (i.e. the logarithmic extinction at H-beta, 4861 Ang), and "f(lambda)" is derived from one of a few possible extinction functions. The choices for Galactic extinction are: Savage & Mathis (1979), Cardelli, Clayton, & Mathis (1989), and the function of Kaler (1976) which is based on Whitford (1958). The choices for extra-Galactic extinction laws are Howarth (1983) for the LMC, and Prevot et al. (1984) for the SMC. The value of "c" must be given in the input table if a correction for reddening is desired. However, the correction may be disabled if a correction flag (stored in another table column), is set to "yes". By default no reddening correction is performed unless a valid value for "c" is available, and unless the correction flag is set to "no" or is not present. The extinction law will default to that of Savage & Mathis ("gal") unless another choice is specified (one of "gal", "ccm", "jbk", "lmc", or "smc") in the input table.

The available diagnostic line ratios, the ionization potential of the associated ion, and the nebular ionization zone to which they are attributed, are listed by ion below. The line ratio is in the form I(wave1)/I(wave2), where "wave1" and "wave2" are in units of Angstroms; ratios involving sums of line strengths are given as I(wave1+wave2)/I(wave3+wave4).

    Electron density diagnostics:

	  Ion           Line Ratio	       I.P.   Zone
	---------------------------------------------------
	  C iii]       I(1907) / I(1909)       47.9   Med  
	 [O ii]        I(3726) / I(3729)       13.6   Low
	[Ne iv]        I(2423) / I(2425)       63.5   High
	 [S ii]        I(6716) / I(6731)       10.4   Low
	[Cl iii]       I(5517) / I(5537)       23.8   Med
	[Ar iv]        I(4711) / I(4740)       40.9   Med
	---------------------------------------------------

    Electron temperature diagnostics:

	  Ion           Line Ratio	       I.P.   Zone
	---------------------------------------------------
	 [N ii]   I(6548+6583) / I(5755)       14.5   Low
	 [O ii]   I(3726+3729) / I(7320+7330)  13.6   Low
	 [O iii]  I(4959+5007) / I(4363)       35.1   Med
	[Ne iii]  I(3869+3969) / I(3342)       41.1   Med
	[Ne v]    I(3426+3346) / I(2975)       97.0   High
	 [S ii]   I(6716+6731) / I(4068+4076)  10.4   Low
	 [S iii]  I(9069+9532) / I(6312)       23.4   Med
	[Ar iii]  I(7136+7751) / I(5192)       27.6   Med
	[Ar v]    I(6435+7006) / I(4626)       59.8   High
	---------------------------------------------------

These are only the most commonly used ratios, and do NOT include all of the diagnostics available in the `temden' task.

ALGORITHM

The method of computing T_e given N_e (or N_e given T_e) for given diagnostic line ratio is described in the on-line help for "nlevel", and in the references given below. This task makes use of an iterative technique to derive both the temperature AND the density within each of three zones by using simultaneous use of temperature- and density-sensitive line ratios from different ions with similar ionization potentials. The procedure is as follows:

ZONE 1:
a)
Assume N_e = 1000/cm^3 and calculate T_e from the [N ii] and [O iii] ratios.
b)
Average these two temperatures (or assume T_e = 10,000 K if unavailable) and calculate N_e from the [O ii] and [S ii] ratios.
c)
Average these two densities (or assume N_e = 1000/cm^3 if unavailable) and re-calculate T_e from the [N ii] ratio. (If the [N ii] ratio is unavailable, default back to the [O iii] ratio.)
d)
Re-calculate N_e from [O ii] and [S ii], and use the average to calculate T_e from the [O ii] and [S ii] ratios.
ZONE 2:
a)
Assume N_e = 1000/cm^3 and calculate T_e from the [O iii] ratio; if not, assume T_e = 10,000 K.
b)
Use this approximate temperature to calculate N_e from [Ar iv], [Cl iii], and C iii].
c)
Now use the average N_e from [Ar iv], [Cl iii], and C iii] (if available, use N_e = 1000 if not) to calculate T_e from [O iii], [Ne iii], [Ar iii], [S iii], and [Ar iv].
d)
Now use the average T_e (if available, use T_e = 10,000 if not) to re-calculate N_e from [Ar iv], [Cl iii], and C iii].
ZONE 3:
a)
Assume T_e = T_[O iii] (calculated in zone 2) if available, or 10,000 K if not.
b)
Calculate N_e from the [Ne iv] ratio, if available, otherwise assume N_e = 1000/cm^3.
c)
Now calculate T_e from the [Ar v] and [Ne v] ratios. Then recalculate N_e based upon the improved T_e, if either temperature ratio is available.

The electron temerature and density for all available ions are written to the output table, along with those adopted for each zone. T_e and N_e for a given zone is the weighted average of the temperatures and densities derived for all ions assigned to that zone. If a particular temperature/density diagnostic is unavailable (e.g. the relevant line fluxes are INDEF, or the line ratio does not yield a valid result), that temperature/density is excluded from the average for that zone. If there are no valid diagnostic line fluxes available for a given zone, the result is INDEF. The weights for each diagnostic are given below; the weights exceed unity when they are clearly more reliable and/or more commonly used.

		  Weights for Nebular Diagnostics

                  |     LOW      |    MEDIUM     |     HIGH
      Diagnostic  |   Ions   Wt  |   Ions    Wt  |   Ions    Wt
     -------------+--------------+---------------+--------------
         N_e      |  [O ii]   1  |  [Cl iii]  2  |  [Ne iv]   1
		  |  [S ii]   1  |  [Ar iv]   1  |  
		  |              |    C iii]  1  |  
		  |	         |               |  
         T_e      |  [N ii]   5  |   [O iii]  4  |  [Ar v]    1
		  |  [O ii]   1  |  [Ne iii]  2  |  [Ne v]    1
		  |  [S ii]   1  |  [Ar iii]  2  |  
		  |              |   [S iii]  1  |  

It is possible that one or more ratios may not be useful for a given nebula if the actual T_e or N_e lies outside the range of that diagnostic. Therefore, the derived T_e and N_e for all diagnostic ratios are written to the output table, along with the average values for each zone. Thus, the user can review the results (e.g., using `tedit') to exclude suspicious values before using `abund' to calculate the ionic abundances.

PARAMETERS

fluxtab [string]
Input table of emission line fluxes. The line fluxes for different ions are stored in separate columns, and measurements for different objects are stored in separate rows.
outtab [string]
Output table of electron temperature and density for each of three zones. If the same as input table, new columns will be appended if necessary.
(objects = "*") [string]
List of object names in input table for which to compute temperatures and densities. Separate object names by whitespace or commas. Specifying "*" will select all objects in the input table.
(fluxcols = "") [pset]
Parameter set to specify column names for certain line fluxes, the nebula name and the region code (which must both be present) in the input table. Otherwise, no error is generated if a named column does not exist in the input table; rather, the calculation proceeds as if the associated line flux is INDEF.
(faluminum = "") [pset]
Parameter set to specify column names for aluminum line fluxes.
(fargon = "") [pset]
Parameter set to specify column names for argon line fluxes.
(fcalcium = "") [pset]
Parameter set to specify column names for calcium line fluxes.
(fcarbon = "") [pset]
Parameter set to specify column names for carbon line fluxes.
(fchlorine = "") [pset]
Parameter set to specify column names for chlorine line fluxes.
(fmagnesium = "") [pset]
Parameter set to specify column names for magnesium line fluxes.
(fneon = "") [pset]
Parameter set to specify column names for neon line fluxes.
(fnitrogen = "") [pset]
Parameter set to specify column names for nitrogen line fluxes.
(foxygen = "") [pset]
Parameter set to specify column names for oxygen line fluxes.
(fpotassium = "") [pset]
Parameter set to specify column names for potassium line fluxes.
(fsilicon = "") [pset]
Parameter set to specify column names for silicon line fluxes.
(fsodium = "") [pset]
Parameter set to specify column names for sodium line fluxes.
(fsulfur = "") [pset]
Parameter set to specify column names for sulfur line fluxes.
(at_data = at_data) [string]
Atomic reference data directory name.

EXAMPLES

To see how STSDAS binary Tables are used for this task, copy the example files to your IRAF current directory and run `tcreate':

    cl> copy nebular$data/flux.dat .
    cl> copy nebular$data/flux.cols .
    cl> tcreate flux.tab flux.cols flux.dat

(Type "help tcreate" for more information about making binary tables from ascii files.) You now have a test binary table called "flux.tab" in your current directory which can be used as input for the `zones' task.

1. Find the electron temperatures/densities from various diagnostic line ratios for the object "TEST_123" in the table "flux.tab", and put the output in the table "diag.tab".

    cl> zones flux.tab diag.tab objects="TEST_123" 

You may wish to review & edit the adopted N_e and/or T_e with `tedit' after running `zones', but before running `abund'. You may view the output table with `tread', or produce a printable ASCII file with, e.g.:

    cl> tprint diag.tab > diag.ascii

2. Find the electron temperatures/densities from various diagnostic line ratios for all objects in the table "flux.tab". Obtain the extinction constant from the input column called "c_ext". Store the results in new columns in the input table.

    cl> zones flux.tab flux.tab objects="*" c_ext_col="c_ext"

BUGS

REFERENCES

The 5-level atom program, upon which this package is based, was originally written by M.M. DeRobertis, R. Dufour, and R. Hunt. This package was written by R.A. Shaw (STScI); a description was published by R.A. Shaw & R.J. Dufour (1994). Type "help nlevel" for additional information about the N-level atom approximation, and for references to the atomic parameters and the other literature references. Support for this software development was provided by the Astrophysics Data Program through NASA grant NAG5-1432, and through STScI internal research funds.

SEE ALSO

nlevel, fluxcols, ionic, temden

For general information about the `nebular' package, type "help nebular opt=sysdoc".


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