The HeII Fowler lines and the OIII and NIII Bowen fluorescence lines in the symbiotic nova RR Telescopii

Aims:A detailed study of the O III and N III Bowen lines in the spectrum of RR Tel.
Methods: Absolute intensities for the He II, O III, and N III 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 He II Fowler (n to 3) series. This value has been confirmed by the STIS and IUE continuum distribution, and by the value of n_H from the damped profile of the IS H Ly-α line. We have obtained very accurate measurements for about thirty Bowen lines of O III and a precise determination of the efficiency in the O1 and O3 excitation channels (18% and 0.7%, respectively). The relative O III 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 O III 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 N III 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 ^2D_5/2 and 3d ^2D_3/2 levels of N III. We point out the important role of multiple scattering in the resonance lines of O III and N III near λ 374 and show that the observed N III line ratios and intensities can be explained in terms of line fluorescence by the three resonance lines of O III at λλ 374.432, 374.162 and 374.073 under optically thick conditions.