SPECTRAL-ANALYSIS OF THE MULTIPLE-SHELL PLANETARY-NEBULA LOTR4 AND ITS VERY HOT HYDROGEN-DEFICIENT CENTRAL STAR

We present a spectral analysis of the planetary nebula LoTr4 and its e xciting star and report the discovery of a multiple-shell nebula struc ture. Two clearly separated shells can be detected around the central star with radii of 7.5'' and 13.5'' with a strong decrease of brightne ss from south-east to north-west. Relative to these shells the central star appears to be off-centered by about 2'' to the south-east. A fra gment of a third shell is found with a radius of 22.5'' south-east of the central star. This and the high degree of asymmetry indicate that the nebula ejection was a complicated process. LoTr4 is of high excita tion with a geometrical radius of r = 0.6(-0.3)(+0.2) pc and a mass of M = 0.29 M.. Its helium abundance is definitely solar (+/-0.05dex), w hile the trace elements could also have slightly different abundances (+/-0.1dex). A stratification of the He/H abundance ratio in the nebul a can be excluded. The central star had been analyzed by means of NLTE model atmospheres. From line profile fits we derive an effective temp erature of T-eff = 120+/-15kK and a surface gravity of log g=5.5+/-0.3 . The photosphere is dominated by helium and hydrogen (n(H)/n(He) = 0. 5 by number). We find a solar nitrogen abundance (n(N)/n(He) = 0.001). Neither carbon nor oxygen lines are exhibited in its spectrum which s ets the upper limits to n(C)/n(He) < 0.004 and n(O)/n(He) < 0.008 whic h are the solar abundance ratios. These abundances indicate that the c entral star of LoTr4 can be a ''born-again post-AGB star''. This scena rio has also been invoked for the evolution of PG 1158 stars. In this picture the O(He) stars, LoTr4 and K 1-27, might either be direct prog enitors of the PG 1159 stars or they have experienced less drastic mas s-loss events.