FG Sagittae: A Newborn R Coronae Borealis Star?

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.