ismeuv: Compute the continuum interstellar extreme ultraviolet (EUV)...
In astrolibR: Astronomy Users Library

Description Usage Arguments Details Value Author(s) References Examples

Compute the continuum interstellar extreme ultraviolet (EUV) optical depth

1	ismeuv(wave, hcol, heicol=0.1hcol, heiicol=0hcol, fano=F)

`wave`	vector of wavelength values, in Angstroms
`hcol`	scalar specifying interstellar hydrogen column density, in atoms cm-2
`heicol`	scalar specifying neutral helium column density, in atoms cm-2 (default = 0.1*hcol)
`heiicol`	scalar specifying ionized helium column density, in atoms cm-2 (default = 0.0)
`fano`	If =TRUE, then the 4 strongest auto-ionizing resonances of He I are included (default = FALSE)

The EUV optical depth is computed from the photoionization of hydrogen and helium. The useful range for wave is 40 - 912 A; at shorter wavelengths, metal opacity should be considered, and at longer wavelengths there is no photoionization.To obtain the attenuation of an input spectrum, multiply by exp(-tau).

This function only computes 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). 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.

Typical values for hcol range from 1E17 to 1E20. For fano=TRUE, the shape of th auto-ionizing resonances of He I is given by a Fano profile (Rumph et al. 1994). 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.

tau

vector giving resulting optical depth for each element of wave

Written by W. Landsman 1994

R adaptation by Arnab Chakraborty June 2013

Dupuis, J., Vennes, S., Bowyer, S., Pradhan, A. K. and Thejll, P., 1995, Hot White Dwarfs in the Local Interstellar Medium: Hydrogen and Helium Interstellar Column Densities and Stellar Effective Temperatures from Extreme-Ultraviolet Explorer Spectroscopy, Astrophys. J. 455, 574 http://adsabs.harvard.edu/abs/1995ApJ...455..574D

Rumph, T., Bowyer, S. and Vennes, S. 1994, Interstellar medium continuum, autoionization, and line absorption in the extreme ultraviolet, Astron. J. 107, 2108-2114 http://adsabs.harvard.edu/abs/1994AJ....107.2108R

	
# 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

hcol = 1e18
w = seq(100,900,length=801)
ismeuv(w, hcol)

# f = rep(1,length=8*20)
# plot(w, f*exp(-ismeuv(w, hcol, .05*hcol, .05*hcol)), pch=20)

#  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)

# w = 180 + seq(0,40,length=40000        # create a fine wavelength grid
# plot(w, ismeuv(w, 0, 1e18, fano=TRUE), pch=20)