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Paper: The FIVEL Nebular Modelling Package in STSDAS
Volume: 61, Astronomical Data Analysis Software and Systems III
Page: 327
Authors: Shaw, R. A.; Dufour, R. J.
Abstract: A package of IRAF tasks has been developed to derive the physical conditions in a low-density (nebular) gas given appropriate diagnostic emission line ratios; and line emissivities given appropriate emission line fluxes, the electron temperature (T ) and density (N ). The tasks in this package are based on the FIVEL program developed by DeRobertis, Dufour, and Hunt (J.Roy. Astron. Soc. Canada, 81, No. 6, 1987). These authors note that most of the common ions that dominate the nebular cooling rate have either p2,p3 , or p4 ground-state electron configurations, which have five low-lying levels. The major physical assumption within their algorithm (and within this package) is that only these five levels are physically relevant to calculating the observed emission line spectrum. This package has been extended beyond the original FIVEL program to provide diagnostics from a greater set of emission lines, most particularly those in the vacuum ultraviolet that are now available from the IUE and HST archives. Two of the tasks provide a simple IRAF-style parameter interface for calculating nebular diagnostics ( temden ); and for level populations, critical densities, detailed line emissivities, and (optionally) the ionic abundance relative to H+ ( ionic )---both for a single ion. Two additional tasks make use of a simple nebular model wherein Te and Ne are calculated in separate zones of low-, intermediate-, and high-ionization. One of these tasks, zones , calculates the physical conditions from all observed ions, while the other task, abund , derives all the ionic abundances from the line fluxes and the physical conditions derived with zones . Both of the modelling tasks make use of the TABLES layered package to provide a simple and efficient means of accessing the relevant line fluxes from many nebulae in a single structure; different emission lines are stored in separate columns, and fluxes for different nebulae are stored in separate rows. Both zones and abund can correct the line fluxes for interstellar reddening, and both provide for extensive error trapping and reasonable default conditions in case the line fluxes for some ions are unavailable.
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