  He Triplet Modifier Reference
  David P. Huenemoerder, John C. Houck, Michael A. Nowak,
  Norbert S. Shulz, John E. Davis

  ____________________________________________________________

  Table of Contents


  1. Function Reference
     1.1 he_modifier_init
     1.2 triplet_line_em
     1.3 create_he_modifier
     1.4 W_dilution


  ______________________________________________________________________



  1.  Function Reference

  1.1.  he_modifier_init


      Synopsis
        Load He triplet modifier coefficient tables

      Usage
        he_modifier_init ([; <qualifiers>])

      Description
        This function initializes a number of internal data structures
        by reading tables of coefficients from FITS files on disk. If
        called with no arguments, the default coefficient tables are
        loaded.

        Paths to specific data files may be specified using qualifiers
        collex for collisional excitation coefficients, and photoex for
        photoexcitation coefficients.

        The coefficient tables currently included in the distribution
        are:


             he_modifier_chianti.fits:
                  Coefficients for triplet emissivities vs. density for a
                  grid of temperatures, derived from Chianti.

             he_modifier_apec.fits:
                  Coefficients for triplet emissivities vs. density for a
                  grid of temperatures, derived from a custom APEC run.

             he_photoex_modifier_chianti.fits:
                  Coefficients for triplet emissivities vs. photon density
                  for a grid of temperatures, derived from Chianti.



      Example


            he_modifier_init ( ;collex="he_modifier_chianti.fits",
                                photoex="he_photoex_modifier_chianti.fits");



      See Also
        ``create_he_modifier'', ``triplet_line_em''

  1.2.  triplet_line_em


      Synopsis
        Compute He triplet line emissivities for a given (Z,n,T)

      Usage
        Struct_Type = triplet_line_em (Z, ndens[], temp[]
        [;<qualifiers>])

      Description
        Compute the He triplet line emissivities for element, Z (an
        integer), number density, ndens, and Kelvin temperature, T.
        ndens and T may be one-dimensional arrays.  The return value is
        a structure of the form


                struct {Z, ndens, temp, w, x, y, z}



     where Z is an integer, ndens and temp are arrays of length nn and
     nT, respectively.  The struct fields w, x, y and z are arrays of
     dimension [nn,nT] containing the line emissivities.

     If the db_norm qualifier is present, the triplet line emissivities
     will be normalized so that the total emissivity w+x+y+z matches the
     value from the currently loaded spectroscopy database rather than
     the value from the relevant coefficient table. Note that the total
     emissivity has little dependence on density.

     If the photoex qualifier is present, the ndens parameter must be
     the photon energy density.

      Example


              % Compute Ne IX line emissivities on a 2D
              % density-temperature grid:

             ndens = 10^[8:15:0.1];
             temp = 10^[6:8:0.05];
             s = triplet_line_em (Ne, ndens, temp);
             plot (log10(s.ndens), s.z[*,0]);
             plot (log10(s.temp), s.z[0,*]);



      See Also
        ``create_he_modifier'', ``he_modifier_init''

  1.3.  create_he_modifier


      Synopsis
        Define a custom He triplet modifier function

      Usage
        create_he_modifier (name [, elements] [; photoex])

      Description
        Define a custom He triplet modifier function called name.  The
        modifier function can then be used in conjunction with a custom
        spectral model function created using create_aped_fun.

        The optional second argument can be used to limit the modifier
        function's effect to the elements given in a space- or comma-
        delimited string of element names. Otherwise, all elements found
        in the coefficient database will be affected.

        If the photoex qualifier is present, the new modifier function
        will represent density dependence associated with
        photoexcitation. Otherwise the modifier function will represent
        pure collisional density dependence.

      Example


        % Include collisional density dependence for
        % Ne, O, and Mg triplets only:

       create_he_modifier ("He_a", "Ne,O,Mg");
       create_aped_fun ("my_model", default_plasma_state);
       fit_fun ("my_model(1, He_a(1)) * wabs(1)");

        % Include photoexcitation density dependence for
        % all He-like species:

       create_he_modifier ("photo"; photoex);
       create_aped_fun ("myphoto_model", default_plasma_state);
       fit_fun ("my_photomodel(1, photo(1)) * wabs(1)");



      See Also
        ``create_aped_fun'', ``he_modifier_init'', ``fit_fun''

  1.4.  W_dilution


      Synopsis
        Geometric dilution factor for a uniform disk

      Usage
        Double_Type[] = W_dilution (x[])

      Description
        Compute the geometric dilution factor for a mean intensity at
        position, x=r/rstar from a uniform disk. This is used internally
        to dilute the radiation field, but may be useful for other
        purposes. The computed value is


               W = (1 - sqrt(1-1/x^2)) /2



