Fluorescence lines in ultraviolet spectra of stars

We discuss the presence of fluorescence lines that appear in stellar spectra in the satellite ultraviolet region between 1200 and 3200 Å. Strong lines in partial multiplets result from a selective excitation, which in most cases is due to accidental wavelength coincidences. The prototype of photoexcitation by accidental resonance (PAR) is the classical Bowen mechanism. Photoexcitation by continuum radiation (PCR) is another source of selective population. Besides a brief review of ultraviolet fluorescence lines identified in spectra of symbiotic stars and cool stars we discuss in detail two emission peaks at 2506–2508 Å. We give three different explanations for the appearance of this feature in IUE spectra of three different objects: RR Tel, η Car and KQ Pup. In connection to these lines we also discuss primary and secondary cascades in the near-infrared observed in high-resolution spectra. Comments are made on the strong Fe II line at 9997 Å.