FG Sagittae: A Newborn R Coronae Borealis Star?

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Abstract
We have monitored FG Sge's spectroscopic changes since the time just prior to its dramatic fading in 1992 August. The most significant qualitative changes in the spectrum include large variations in the strength of the C2 molecular bands and the gradual appearance of broad blueshifted high-velocity (~200 km s-1 relative to the photosphere) absorption components of the Na I D lines. During the deep minima of 1994 May and 1996 June, an emission-line spectrum temporarily appeared superimposed on a weak continuum; in addition to the previously reported nebular emission lines, the spectra displayed the C2 and rare earth element lines in emission. Much of the behavior exhibited by FG Sge since 1992 resembles that seen in R CrB stars, including the photometric behavior, the evolution of the Na I D line profiles, variations of the C2 band strengths, and the appearance of narrow emission lines. The results of our abundance analysis (using model atmospheres with a solar He/H ratio) indicate that the carbon abundance is currently greater than that determined by Langer et al., who reported on the dramatic increase in the abundances of the rare earth elements in FG Sge. We derive higher relative abundances of the rare earths ([Me/Fe] ~ 3) than either Langer et al. or Kipper & Kipper, which we attribute to some enhancement of these elements since ~1970. We confirm previous claims that the relative scandium abundance is high ([Sc/Fe] ~ 1) in FG Sge and suggest that it is the result of neutron captures by the light elements leading up to 45Sc. The Hα profile of FG Sge is very similar to that of V854 Cen, a R CrB star deficient in H by 2-3 dex. This is the first evidence pointing toward H-deficiency in the atmosphere of FG Sge, which further strengthens its link with the R CrB class. Additional study is required before we can say definitively whether or not its atmosphere is H deficient.

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