Effect of Density Variations on Elemental Abundance Determinations in Gaseous Nebulae

Abstract

When there are changes in gas density within a nebula, various methods of determining the electron density Ne can give different results. Irrespective of differences in ionization structure, there will be deviations in derived values of Ne due to the physics of populating the energy levels. To focus on Ne variations, the electron temperature is held constant. For two cases presented, the values of Ne inferred range over a factor of ten from nine species (line pairs); in order of increasing Ne, they are N+(122/204 μm), O++(52/88 μm), S+(6716/6731 A), S++(18.7/33.5 μm), 0+(3726/3729 A), Ne++(15/36 μm), Ar+3 (4711/4740 A), Ar++(8.99/21.8 μm), and C++(1906/1909 A). This is basically a progression from lower to higher critical densities, Nc, for the lines involved, although other factors are involved. The above order can change somewhat for different mixes of densities.