He-like Line Emissivity Modifier - ISIS/APED density dependent emissivities


NOTE: This page will be removed soon and replaced by a greatly improved version! (which will include photoexcitation dependence for OB-star modeling) [2011.02.15]

About the emissivity modifier for He-like triplets

The He-like triplet lines which occur in the X-ray band from 5-42A are powerful diagnostics of density or photoexcitation. The APED (Astrophysical Plasma Emission Database), however, only contains emissivities for low density. To use triplet lines as a density diagnostic, one method is to fit the line strengths and from the ratio of the forbidden to intercombination line fluxes (f/i), then look up the implied density from external sources. To provide better support for more direct fitting and modeling, we have
  • made very general modifications to ISIS to provide APED-emissivity modifiers;
  • constructed a database of emissivity modifier coefficients which parameterize the density dependence;
  • written an ISIS model to implement the modifier via the coefficients table.

Further details can be found in this document.

Important Files

Two files are required to implement the emissivity modifiers within ISIS in conjunction with APED: a FITS file which tabulates emissivity modifier coefficients for He-like triplets, and an ISIS source code file which defines the modifier fit function:


Put the source file, he_modifier.sl, somewhere in your ISIS load path and put the data file in the same place. The default location for the data file is the same directory as the source code.


An ISIS test script, t_he_modifier.sl, demonstrates several ways to use the He-like triplet modifiers. The script is heavily commented, and produces the following graphics:
  • Model spectra: these are model spectra showing the intercombination and forbidden lines for a range of densities for several ions. As density increases, the forbidden line (right) weakens while the intercombination line (left) strengthens. The models were computed near the ion's temperature of maximum emissivity and included the thermal profile. Note that the intensity scale is logarithmic.

    Model flux for i and f Model flux for i and f

  • f/i ratios vs density for several ions:

    f/i ratios vs density

  • Fitting an observed O VII triplet with density dependence:

    The model spectrum and residuals:

    Fit to O VII in TW Hya

    O VII models for the best fit (black), high and low confidence limits in density (red), and at the low-density limit (gray):

    O VII models for different

    Confidence contours (68%, 90%, 99%) of density and temperature:

    Confidence map for TW Hya O VII

This page was last updated Feb 15, 2011 by David P. Huenemoerder. To comment on it or the material presented here, send email to dph@space.mit.edu.
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