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Session 43 - Variable Stars.
Display session, Thursday, January 08
Exhibit Hall,

[43.01] The Last Hurrahs for Dying Stars: Rapid Evolutionary Changes of the Post-AGB Stars -- FG Sge and Sakurai's Object

S. J. Margheim, E. F. Guinan, G. P. McCook (Villanova U.)

The post-AGB variable star FG Sge has provided an unique opportunity to study phenomenon which may be characteristic of the post-PN stage of stellar evolution. FG Sge is the central star in the 36-arcsec PN He 1-5 that has been undergoing rapid evolutionary changes over the last 50 yrs. Most notable are changes in temperature, luminosity, chemical composition and pulsation period. Starting in mid-1992, the star has undergone several large dimming events. These events closely resemble the dimming events from dust formation in ejected envelopes commonly observed in R CrB stars. From all the evidence, it now appears that FG Sge has evolved into a R CrB-like star. Photometry obtained during Oct. 1997 shows that the star has attained the brightness level that it had before the start of the dimming episodes.

In this paper we summarize the recent photometric history of FG Sge from UBVRI photometry obtained during 1990-1997. We combine these data with earlier photometry to define the star's light behavior over the last 100 yrs. The photometry is also combined with available spectroscopic, polarimetric, and IR observations. Particular attention is given to comparing FG Sge's post-dimming behavior with contemporaneous observations of R CrB itself, done on the same photometric system. Recent photometry of the final helium flash candidate Sakurai's Object (V4334 Sgr) is shown for comparison and its evolutionary state is discussed.

This research is supported by NSF/RUI Grants AST 93-15365 and AST 95-28506, which we gratefully acknowledge.

Program listing for Thursday