Other
Scientific paper
Nov 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996mnras.283..141i&link_type=abstract
Monthly Notices of the Royal Astronomical Society, Vol. Vol. 283, No. 1, p. 141 - 148
Other
6
Stars: Agb And Post-Agb -- Stars: Carbon -- Stars: Individual: Fg Sge -- Stars: Variables: Other, Post-Agb Stars: Spectral Variations, Post-Agb Stars: Peculiar Variables, Post-Agb Stars: Circumstellar Shells, Post-Agb Stars: Planetary Nebulae, Peculiar Variables: Spectral Variations, Peculiar Variables: Circumstellar Shells, Peculiar Variables: Planetary Nebulae
Scientific paper
The spectral variation of the post-AGB peculiar variable star FG Sge has been monitored at Asiago Observatory since the 1970s. One medium-dispersion spectrum covering the range from 3950 to 5050 Å was taken on 1992 August 9, just when a photometric decline started. The spectrum was different from those of this object reported in some previous works; moreover it was different from spectra of any known type of star. The Swan band of C2 molecules was seen at 4737 Å and atomic absorption lines of Ba II, Sr II and La II, and of the other s-process elements were unusually deep. On the other hand, no trace of CN bands was seen. The absorption lines of the iron group were weak, and the absorption line of Mg II 4481 Å was not seen. Since the C2 band was detected, this object could be a kind of carbon star. Therefore, the photometric decline could be, as suggested by some authors, an R CrB type phenomenon. The C2 bands nearly disappeared in 1993 when the star was recovering its luminosity by about 3 mag in V from the first minimum. All absorption lines also weakened in 1993, and the spectra resembled that of an F-G type supergiant. This phenomenon suggests that the brightening in 1993 was not a simple dilution of the obscure matter. In 1994 April the C2 bands reappeared and weak traces of CN molecular bands were detected, while the absorption lines remained weak. The depths of the absorption lines increased in the latter half of 1994 when the star faded again.
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