Fading and variations in the spectrum of the central star of the very young planetary nebula SAO 244567 (Hen 1357).

Abstract

The optical spectrum of SAO 244567 obtained in 1971 shows that it was a post-AGB B1 or B2 supergiant at that time. It has turned into a planetary nebula (PN ) within the last 20 years. The IUE ultraviolet spectra obtained during the last seven years show that the central star is rapidly evolving. It is found that the central star of this young PN has faded by a factor of 2.83 within the last seven years. The terminal velocity of the stellar wind has decreased from -3500km/s in 1988 to almost zero in 1994. In 1988 the CIV (1550A) line which was a P-Cygni profile with strong absorption component has almost vanished by 1994. The UV absorption and nebular features show variations in strength. This may be due to the fading of the central star and also possibly to expansion of the nebula. These results suggest that in the central star the nuclear fuel is almost extinct as a result of post-AGB mass loss. The main stellar energy may be gravothermal energy. Typical for hydrogen-burning AGB remnants is a very fast drop in luminosity by an order of magnitude when the burning cannot be sustained any longer. These results suggest that the central star of this young PN is rapidly evolving to become a DA white dwarf. An alternative interpretation is that the present fading could be due to an episode of high mass loss, which is now just completed. If the ultraviolet fading was a factor of 2.83 from 1988 to 1995, the luminosity would remain the same if the temperature increased from 37500K to 47500K in the same period. It may b that these changes occur in steps which are triggered by episodic mass loss periods during the post-AGB evolution, and that the hydrogen burning has not stopped or stopped temporarily.