The OIV and SIV intercombination lines in the ultraviolet spectra of astrophysical sources
Doyle, J. G.; Madjarska, M. S.; Mathioudakis, M.; Keenan, F. P.; Espey, B. R.; Exter, K. M.; Hibbert, A.; Pollacco, D. L.; Ahmed, S.; Brage, T.; Keenan, M. T. C.
Abstract
New electron density diagnostic line ratios are presented for the OIV 2s22p 2P-2s2p24P and SIV 3s23p 2P-3s3p24P intercombination lines around 1400Å. A comparison of these with observational data for the symbiotic star RR Telescopii (RR Tel), obtained with the Space Telescope Imaging Spectrograph (STIS), reveals generally very good agreement between theory and observation. However the SIV2P3/2-4P1/2 transition at 1423.824Å is found to be blended with an unknown feature at 1423.774Å. The linewidth for the latter indicates that the feature arises from a species with a large ionization potential. In addition, the SIV2P1/2-4P3/2 transition at 1398.044Å is identified for the first time (to our knowledge) in an astrophysical source other than the Sun, and an improved wavelength of 1397.166 Å is measured for the OIV2P1/2-4P3/2 line. The OIV and SIV line ratios in a sunspot plume spectrum, obtained with the Solar Ultraviolet Measurements of the Emitted Radiation (SUMER) instrument on the Solar and Heliospheric Observatory, are found to be consistent, and remove discrepancies noted in previous comparisons of these two ions.