Lines of OIV and SIV in the Goddard High-Resolution Spectrograph spectrum of RR Tel: constraints on atomic data

Abstract

High signal-to-noise ratio spectra of RR Tel obtained at medium resolution with the Goddard High-Resolution Spectrograph (GHRS) on the Hubble Space Telescope (HST) are used to test available atomic data for the OIV 2s^22p ^2P-2s2p^2 ^4P multiplet (UV 0.01). The fine-structure intervals of the 2s2p^2 ^4P term given by Moore (1983) appear to need revision. The flux ratios of lines within multiplet UV (0.01), which have a common upper level, depend only on transition probabilities. The observed flux ratio of lines from the ^4P_3/2 level differs from that predicted by theory, but this difference cannot be attributed to a blend with a line of SIV]. At the electron densities in the RR Tel nebula, other flux ratios give information on the relative electron excitation rates between the ^2P and ^4P fine-structure levels. Using the collision strengths calculated by Zhang, Graziani & Pradhan, the rate to the ^4P_5/2 level, relative to the rates to the other J states, appears to be underestimated by ~ 10 per cent, which is within the expected uncertainty of 20 per cent. We also discuss the SIV 3s^23p ^2P-3s3p^2 ^4P multiplet.