idlastro / Astronomical Utilities: ISMEUV

[Source code]

NAME
ISMEUV
PURPOSE
Compute the continuum interstellar EUV optical depth 
EXPLANATION
The EUV optical depth is computed from the photoionization of
hydrogen and helium.
CALLING SEQUENCE
tau = ISMEUV( wave, Hcol, [ HeIcol, HeIIcol, /Fano ]
INPUTS
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.
OUTPUT
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.
EXAMPLE
(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)          
NOTES
(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)
HISTORY
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