Profiles and Intensity Ratios of the C IV λ1548, 1550 Emission Lines in Planetary Nebulae

Extract

The C IV resonance doublet at λ1548, 1550 is an important diagnostic tool in the study of planetary nebulae. The predicted theoretical intensity ratio of 2:1 is, however, rarely observed in high dispersion IUE spectrograms. The observed values for a sampling of 15 objects of differing excitation class range from a low of 0.74 to a high of 1.99. Variations in optical thickness, extinctions due to nebular dust, and interstellar absorption have been proposed as the cause for the deviation. Line profiles for the C IV doublet vary for different nebulae, encompassing a wide range of shapes which include: 1) narrow symmetric (Gaussian) profiles, 2) wide symmetric, 3) asymmetric with steep blue edge and extended red wing, 4) asymmetric with steep red edge and extended blue wing, 5) P Cygni profiles, 6) split peaks due to expansion velocities, and 7) multiple peaked or chaotic line structure. In addition to the main features, occasionally weak (subsidiary or ghost?) features occur that do not agree in intensity with the main line ratios but are not attributable to lines of other ions. Ultraviolet radial velocity displacements are in good agreement with optical data. Similar diversity in profiles and intensity ratios have been observed in protoplanetary nebulae and symbiotic stars. (Paper to appear in Astron. Astrophys.)