3.4 A Caution About the Wavelengths



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3.4 A Caution About the Wavelengths

Please note that the wavelengths used throughout this paper are those commonly used in the astronomical literature. However, the wavelengths used in the nebular package are derived from the published atomic data for each ion, which are vacuum wavelengths that result from the theoretically computed energy level separations derived in the various papers from which the atomic data (usually the collision strengths) were taken. These derived wavelengths are used partly for consistency with the models, and partly because there is as yet no good reference for all the wavelengths of all the ions used in these tasks. But be aware that there are differences with the accepted values (usually around +1 Å). The wavelength discrepancy is only likely to cause confusion when using the ionic task to compute an ionic abundance from a particular line. In this case, be sure the wave or wv_toler parameters are set appropriately. These wavelength discrepancies (in the fourth decimal place) are a reminder of the imperfections inherent in all the models from which the atomic data are derived, although the uncertainties in the cross-sections can range from 5% to 50%.



Rocio Katsanis
Thu Aug 8 17:23:06 EDT 1996