N III Intercombination Lines in the IUE Spectra of Gaseous Nebulae

Abstract

Theoretical N III electron density-sensitive emission-line ratios involving intercombination transitions, derived using recent calculations of electron impact excitation rates and oscillator strengths, are presented for R<SUB>1</SUB> = I(1754.0 Å)/I(1749.7 Å), R<SUB>2</SUB> = I(1752.2 Å)/I(1749.7 Å), R<SUB>3</SUB> = I(1748.6 Å)/I(1749.7 Å), and R<SUB>4</SUB> = I(1746.8 Å)/I(1749.7 Å). The observed values of R<SUB>1</SUB>, R<SUB>2</SUB>, and R<SUB>3</SUB> for several gaseous nebulae, measured from high-resolution spectra obtained with the International Ultraviolet Explorer (IUE) satellite, imply electron densities that are compatible. However, values of N<SUB>e</SUB> derived from the R<SUB>4</SUB> ratio are up to several orders of magnitude smaller than those deduced from R<SUB>1</SUB>, R<SUB>2</SUB>, and R<SUB>3</SUB>, which is probably due to the N III 1746.8 Å line being blended with Fe II 1746.8 Å. The electron densities deduced from the N III diagnostics are, in some objects, much larger than those estimated from line ratios in nebular ions such as O III, but are in good agreement with values deduced from the chromospheric C II intercombination transitions at ∼2325 Å. These results suggest that in these nebulae the N III emission may also be chromospheric in origin.