ISMEUV

       Compute the continuum interstellar EUV optical depth

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

       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).

       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)

       /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
           opacities
       (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.
       Converted to IDL V5.0   W. Landsman   September 1997
       Avoid underflow messages, support double prec.  W. Landsman October 2003

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