The He II Fowler lines and the O III and N III Bowen fluorescence lines in the symbiotic nova RR Telescopii

Aims. A detailed study of the Oiii and Niii Bowen lines in the spectrum of RR Tel. Methods. Absolute intensities for the Heii ,O iii ,a nd Niii emission lines have been obtained from STIS, UVES, FEROS and IUE data, after re-calibration of UVES and FEROS on the STIS scale. Results. A new measure of reddening (E(B−V) ∼ 0.00) has been obtained from the comparison between the observed and the theoretical intensity decrement for 20 emission lines of the Heii Fowler (n → 3) series. This value has been confirmed by the STIS and IUE continuum distribution, and by the value of nH from the damped profile of the IS H Ly-α line. We have obtained very accurate measurements for about thirty Bowen lines of Oiii and a precise determination of the efficiency in the O1 and O3 excitation channels (18% and 0.7%, respectively). The relative Oiii intensities are in good agreement with the predictions by Froese Fischer (1994). A detailed study of the decays from all levels involved in the Bowen mechanism has lead to the detection of two new Oiii Bowen lines near λ 2190. High resolution IUE data have shown a nearly linear decline with time, from 1978 to 1995, in the efficiency of the O1 and O3 processes, with a steeper slope for the O3 channel. A detailed study of the Niii λ 4640 lines and of their excitation mechanism has shown that, recombination and continuum fluorescence being ruled out, line fluorescence remains the only viable mechanism to pump the 3d 2 D5/2 and 3d 2 D3/2 levels of Niii. We point out the important role of multiple scattering in the resonance lines of Oiii and Niii near λ 374 and show that the observed Niii line ratios and intensities can be explained in terms of line fluorescence by the three resonance lines of Oiii at λλ 374.432, 374.162 and 374.073 under optically thick conditions.