Astronomy and Astrophysics, volume 310, 613-628 (1996/6-2)
Spectral analysis of the multiple-shell planetary nebula LoTr4 and its very hot hydrogen-deficient central star.
RAUCH T., KOEPPEN J. and WERNER K.
Abstract (from CDS):
We present a spectral analysis of the planetary nebula LoTr4 and its exciting star and report the discovery of a multiple-shell nebula structure. Two clearly separated shells can be detected around the central star with radii of 7.5" and 13.5" with a strong decrease of brightness 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 fragment 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 excitation with a geometrical radius of r=0.6+0.2–0.3pc and a mass of M=0.29M☉. Its helium abundance is definitely solar (±0.05dex), while the trace elements could also have slightly different abundances (±0.1dex). A stratification of the He/H abundance ratio in the nebula can be excluded. The central star had been analyzed by means of NLTE model atmospheres. From line profile fits we derive an effective temperature of Teff=120±15kK and a surface gravity of logg=5.5±0.3. The photosphere is dominated by helium and hydrogen (nH/nHe=0.5 by number). We find a solar nitrogen abundance (nN/nHe=0.001). Neither carbon nor oxygen lines are exhibited in its spectrum which sets the upper limits to nC/nHe<0.004 and nO/nHe<0.008 which are the solar abundance ratios. These abundances indicate that the central star of LoTr4 can be a "born-again post-AGB star". This scenario has also been invoked for the evolution of PG1159 stars. In this picture the O(He) stars, LoTr4 and K1-27, might either be direct progenitors of the PG1159 stars or they have experienced less drastic mass-loss events.