Symbiotic stars frequently exhibit anomalous C IV λλ1548.2, 1550.8 doublet flux ratios, in which the intensity ratio of the blue and red doublet members is below the optically thick limit of unity. Three symbiotic stars (RR Tel, RX Pup, and AG Peg) have been investigated in detail with high-resolution spectra obtained from the International Ultraviolet Explorer (IUE) archives. We have examined the deficit of C IV λ1548.2 emission relative to λ1550.8 in context with the fluorescent pumping of the Fe II multiplet (45.01) λ1548.204 by the C IV λ1548.2 line. In RR Tel the C IV λ1548.2 flux is correlated with the intensities of fluorescent-excited Fe II lines formed by the downward cascades from the y^4H0112 upper level. This suggests that a circumstellar Fe II forming region in RR Tel responds to variations of the C IV line strength, which provides further evidence in support of Fe II fluorescence in UV spectra of symbiotic stars first identified by Johansson. During periods when the C IV I(λ1548.2)/I(λ1550.8) ratio in RX Pup is significantly less than the optically thick limit of unity, we find that the flux deficit of C IV λ1548.2 is approximately equal to the combined flux of the Fe II Bowenpumped a^4 F 911 -> y^4 H1 112 transition. This could explain the anomalous C IV ratios observed in this system and several deep absorption features in the C IV λ1548 line profile, when the C IV flux ratio was substantially less than the optically thick limit of unity. Unlike RR Tel and AG Peg in which the velocity centroids of the Fe II Bowen lines correspond to the peak emission of the C IV λ1548.2 line, the Bowen-pumped Fe II lines in RX Pup exhibit a velocity range 0 s vFeii s + 80 km s^-1. This could reflect the differential motion of individual gas parcels within an orbiting cloud that obscured the Mira and hot ionizing object, consistent with the model proposed by Whitelock et al. to explain the photometric behavior of RX Pup between 1982 and 1983. The cloud would be the source of Fe II fluorescent emission and would explain why the anomalous C IV doublet intensities were observed during a 3 yr period when the V and J photometric bands indicated a decrease in light.
Michalitsianos, A.G., Kafatos, M., Meier, S.R. (1992). Fe ii Fluorescence and Anomalous C iv Doublet Intensities in Symbiotic Novae, Astrophysical Journal, 389:649-656. doi: 10.1086/171237