Fe VII lines in the spectrum of RR Telescopii

¹ CCLRC Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK e-mail: p.r.young@rl.ac.uk
² School of Science and Mathematics, Sheffield Hallam University, Sheffield S1 1WB, UK
³ Smithsonian Astrophysical Observatory, 60 Garden Street, Cambridge, MA 02138, USA

Received: 21 September 2004
Accepted: 15 November 2004

Abstract

Thirteen transitions within the ground  configuration of Fe vii are identified in ultraviolet and optical spectra of the symbiotic star RR Telescopii obtained with the STIS instrument of the Hubble Space Telescope. The line fluxes are compared with theoretical data computed with the recent atomic data of Berrington et al., and high resolution optical spectra from VLT/UVES are used to identify blends. Seven branching ratios are measured, with three in good agreement with theory and one affected by blending. The λ5277/λ4943 branching ratio is discrepant by >, indicating errors in the atomic data for the λ5277 line. A least-squares minimization scheme is used to simultaneously derive the temperature, T, and density, N_e, of the RR Tel nebula, and the interstellar extinction, , towards RR Tel from the complete set of emission lines. The derived values are:  ± 0.23,  ± 0.05, and . The extinction is not well-constrained by the Fe vii lines, but is consistent with the more accurate value derived here from the Ne v λ2974/λ1574 ratio in the STIS spectrum. Large differences between the K. A. Berrington et al. electron excitation data and the earlier F. P. Keenan & P. H. Norrington data-set are demonstrated, and the latter is shown to give worse agreement with observations.