README for reference atomic data files used in NEBULAR. 	27-Aug-2008

The nebular package retrieves atomic data at run-time from 
files in this (nebular$atom_data) directory. The data are 
divided according to ion, and further into two FITS files 
per ion: a table with the temperature-dependent collision 
strengths and a table with all other data, including the 
energy levels, Einstein A-coefficients, and statistical 
weights. The names of these files are stored in the ASCII 
file "at_refer.dat"; the naming convention for the collision 
strength tables is <ion>_coll.fits, and the other tables is 
<ion>_atom.fits, where <ion> refers to the atomic symbol and 
spectrum number. An example would be "ne_v_coll.fits" for 
the collision strengths for Ne V.  

NOTE: it is essential that these atomic data tables both 
exist and be listed in the at_refer.dat file if the nebular 
tasks are to support a given ion.  However, support for new 
ions requires some (minor) changes to the nebular source code. 

FORMAT OF ATOMIC DATA

Each atomic data table is stored as a FITS BINTABLE. Keywords 
in these tables give the following information: 

  ATOM    = '       ' / name of the atom 
  SPECTRUM=           / spectrum designation (integer >= 1)
  N_LEVEL =           / no. levels supported for this ion
  SOURCE1 = '       ' / literature source for data
  NOTE1   = '       ' / comment about literature source
  GSCONFIG= '       ' / ground-state electron configuration

In the collision strength files, there is an additional keyword: 

  FIT_FUNC= '       ' / function used to fit coll. strength vs. T_e

These tables contain a column for the electron temperature, and 
one column each for the collision strength at the tabulated 
temperature. The column names are "Omega(n->m)" where n is the 
number of the upper level, and m the lower level. The column 
units specify the order of the function used to characterize 
the run of collision strength with T_e. 

For the atomic data files, there is one column each for the 
energy levels (in units of 1/Angstroms), the statistical weights, 
and spontaneous radiative transition probabilities/s for each 
level to each available lower level.  Note that values of zero 
in the table cells may mean that the transition is strictly 
forbidden or that the atomic data are not available in the 
literature. 

OTHER DATA

Besides the catalog of supported ions and the associated atomic 
data files (at_refer.dat mentioned above), the file "ntplot.dat" 
lists those diagnostic curves that are plotted by the "ntplot" 
task. This makes the application more table-driven, and allows 
the source code to be more general.  

UPDATES TO ATOMIC DATA

From time to time new/improved atomic data become available. 
When this happens the files in the atomic_data directory are 
updated with the new values, and the files that had been used 
are moved to the "/old" subdirectory. Users who wish to use the 
older files (perhaps for comparison with prior abundance 
calculations) can substitute the old files for the new ones, 
and the "nebular" package will automatically be updated. 