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ISMEUV

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

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