Emission lines of [K <scp>v</scp> ] in the optical spectra of gaseous nebulae

Recent R-matrix calculations of electron impact excitation rates in K v are used to derive the nebular emission line ratio R = I (4122.6 Å)/ I (4163.3 Å) as a function of electron density ( N e ). This ratio is found to be very sensitive to changes in N e over the density range 10 3 to 10 6 cm −3 , but does not vary significantly with electron temperature, and hence in principle should provide an excellent optical N e diagnostic for the high-excitation zones of nebulae. The observed value of R for the planetary nebula NGC 7027, measured from a spectrum obtained with the Hamilton Echelle spectrograph on the 3-m Shane Telescope, implies a density in excellent agreement with that derived from [Ne iv ], formed in the same region of the nebula as [K v ]. This observation provides observational support for the accuracy of the theoretical [K v ] line ratios, and hence the atomic data on which they are based. However, the analysis of a high-resolution spectrum of the symbiotic star RR Telescopii, obtained with the University College London Echelle Spectrograph on the 3.9-m Anglo–Australian Telescope, reveals that the [K v ] 4122.6 Å line in this object is badly blended with Fe ii 4122.6 Å. Hence, the [K v ] diagnostic may not be used for astrophysical sources that show a strong Fe ii emission line spectrum.