FLUORESCENCE LINES IN ULTRAVIOLET-SPECTRA OF STARS

We discuss the presence of fluorescence lines that appear in stellar s pectra in the satellite ultraviolet region between 1200 and 3200 angst rom. Strong lines in partial multiplets result from a selective excita tion, which in most cases is due to accidental wavelength coincidences . The prototype of photoexcitation by accidental resonance (PAR) is th e classical Bowen mechanism. Photoexcitation by continuum radiation (P CR) is another source of selective population. Besides a brief review of ultraviolet fluorescence lines identified in spectra of symbiotic s tars and cool stars we discuss in detail two emission peaks at 2506-25 08 angstrom. We give three different explanations for the appearance o f this feature in IUE spectra of three different objects: RR Tel, eta Car and KQ Pup. In connection to these lines we also discuss primary a nd secondary cascades in the near-infrared observed in high-resolution spectra. Comments are made on the strong Fe II line at 9997 angstrom.