***********************************************************
OATS-DAUT SEMINAR
***********************************************************
Speaker: Pierluigi Selvelli (INAF-Osservatorio Astronomico di Trieste)

Title: The HeII Fowler lines and the OIII and NIII Bowen fluorescence lines in the symbiotic nova RR Tel

Date: Wednesday, April 11th, 2007
Time: 12:00 noon
Venue: Villa Bazzoni

***********************************************************
Abstract: STIS, UVES, FEROS and IUE spectra of the shell of the symbiotic nova RR Tel have been used to to perform a detailed study of the Bowen fluorescence process, in which, by a wavelength coincidence, the HeII Ly-alpha lambda 304 line excites high-lying levels of OIII and in the subsequent cascade several optical and UV lines of OIII are produced.
A new measure of reddening E(B-V) ~ 0.0 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 from STIS. 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-alpha line.
The efficiency of the OIII fluorescence mechanism is by now close to 20 % but in the past it was about as twice, as determined from IUE data. A detailed study of all possible Bowen decays in high resolution STIS and UVES spectra has lead to the detection of about eight new Bowen lines, never reported before in any astronomical object.
A detailed study of the NIII lambda 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 produce the observed line ratio. We point out the important role of multiple scattering in the resonance lines of OIII and NIII near lambda 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 lambda 374.432, 374.162 and 374.073 under optically thick conditions.
***********************************************************