FADING AND VARIATIONS IN THE SPECTRUM OF THE CENTRAL STAR OF THE VERYYOUNG PLANETARY-NEBULA SAO-244567 (HEN-1357)

The optical spectrum of SAO 244567 obtained in 1971 shows that it was a post-AGE B1 or B2 supergiant at that time. It has turned into a plan etary nebula (PN) within the last 20 years. The IUE ultraviolet spectr a obtained during the last seven years show that the central star is r apidly evolving. It is found that the central star of this young PN ha s faded by a factor of 2.83 within the last seven years. The terminal velocity of the stellar wind has decreased from -3500 km s(-1) in 1988 to almost zero in 1994. In 1988 the CIV (1550 Angstrom) line which wa s a P-Cygni profile with strong absorption component has almost vanish ed 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 p ossibly to expansion of the nebula. These results suggest that in the central star the nuclear fuel is almost extinct as a result of post-AG E mass loss. The main stellar energy may be gravothermal energy. Typic al for hydrogen-burning AGE remnants is a very fact drop in luminosity by an order of magnitude when the burning cannot be sustained any lon ger. These results suggest that the central star of this young PN is r apidly evolving to become a DA white dwarf. An alternative interpretat ion 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 fa ctor of 2.83 from 1988 to 1995, the luminosity would remain the same i f the temperature increased from 37500K to 47500K in the same period. It may be that these changes occur in steps which are triggered by epi sodic mass loss periods during the post-AGE evolution, and that the hy drogen burning has not stopped or stopped temporarily.