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      Compute the continuum interstellar EUV optical depth


      The EUV optical depth is computed from the photoionization of
      hydrogen and helium.

Calling Sequence

      tau = ISMEUV( wave, Hcol, [ HeIcol, HeIIcol, /Fano ]


      wave - Vector of wavelength values (in Angstroms). Useful range is
              40 - 912 A; at shorter wavelengths metal opacity should be
              considered, at longer wavelengths there is no photoionization.
      Hcol - Scalar specifying interstellar hydrogen column density in cm-2.
                Typical values are 1E17 to 1E20.


      tau - Vector giving resulting optical depth, same number of elements
              as wave, non-negative values. To obtain the attenuation of
              an input spectrum, multiply by exp(-tau).

Optional Inputs

      HeIcol - Scalar specifying neutral helium column density in cm-2.
              Default is 0.1*Hcol (10% of hydrogen column)
      HeIIcol - Scalar specifying ionized helium column density in cm-2
              Default is 0 (no HeII)

Optional Input Keywords

      /FANO - If this keyword is set and non-zero, then the 4 strongest
              auto-ionizing resonances of He I are included. The shape
              of these resonances is given by a Fano profile - see Rumph,
              Bowyer, & Vennes 1994, AJ, 107, 2108. If these resonances are
              included then the input wavelength vector should have
              a fine (>~0.01 A) grid between 190 A and 210 A, since the
              resonances are very narrow.


      (1) One has a model EUV spectrum with wavelength, w (in Angstroms) and
      flux,f . Plot the model flux after attenuation by 1e18 cm-2 of HI,
      with N(HeI)/N(HI) = N(HeII)/N(HI) = 0.05
      IDL> Hcol = 1e18
      IDL> plot, w, f*exp(-ismeuv(w, Hcol, .05*Hcol, .05*Hcol))
      (2) Plot the cross-section of HeI from 180 A to 220 A for 1e18 cm-2
              of HeI, showing the auto-ionizing resonances. This is
              Figure 1 in Rumph et al. (1994)
      IDL> w = 180 + findgen(40000)*0.001 ;Need a fine wavelength grid
      IDL> plot, w, ismeuv(w, 0, 1e18, /Fano)


      (1) The more complete program ismtau.pro at
          http://hea-www.harvard.edu/PINTofALE/pro/ extends this work
          to shorter wavelengths and includes metal and molecular hydrogen
      (2) This program only compute continuum opacities, and for example,
          the He ionization edges at 504 A and 228 A are blurred by
          converging line absorptions (Dupuis et al. 1995. ApJ, 455, 574)


      Written, W. Landsman October, 1994
      Adapted from ism.c at anonymous ftp site cea-ftp.cea.berkeley.edu
      by Pat Jelinsky, Todd Rumph & others.
      Avoid underflow messages, support double prec. W. Landsman October 2003
      Fix error in He II optical Depth J. Slavin/WL Sep 2013

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