astrolibR (version 0.1)

ismeuv: Compute the continuum interstellar extreme ultraviolet (EUV) optical depth

Description

Compute the continuum interstellar extreme ultraviolet (EUV) optical depth

Usage

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

Arguments

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)

Value

tau

vector giving resulting optical depth for each element of wave

Details

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.

References

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

Examples

	
# 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)