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Preprint 14/08

UV Spectroscopy of the hybrid PG 1159-type central stars of the planetary nebulae NGC 7094 and Abell 43

M. Ziegler (1), T. Rauch (1), K. Werner (1), L. Koesterke (2), J.W. Kruk (3)

(1) Institute for Astronomy and Astrophysics, Kepler Center for Astro and Particle Physics, Eberhard Karls University, Tübingen, Germany
(2) Texas Advanced Computing Center, University of Texas, Austin, U.S.A.
(3) Department of Physics and Astronomy, Johns Hopkins University, Baltimore, U.S.A.

To be published in: Communications in Asteroseismology

Abstract. Hydrogen-deficient post-AGB stars have experienced a late helium-shell flash that mixes the hydrogen-rich envelope and the helium-rich intershell. The amount of hydrogen remaining in the stellar envelope depends on the particular moment when this late thermal pulse occurs. Previous spectral analyses of hydrogen-deficient post-AGB stars revealed strong iron de- ficiencies of up to 1 dex. A possible explanation may be neutron captures due to an efficient s-process on the AGB that transformed iron into heavier elements. An enhanced nickel abun- dance would, thus, be an indication for this scenario. We performed a detailed spectral analysis by means of NLTE model-atmosphere tech- niques based on high-resolution UV observations of the two PG 1159-type central stars of the planetary nebulae NGC 7094 and Abell 43 which are spectroscopic twins, i.e. they exhibit very similar spectra. We confirmed a strong iron-deficiency of at least one dex in both stars. The search for nickel lines in their UV spectra was entirely negative. We find that both stars are also nickel deficient by at least one dex.

Preprint (47 kb PDF file including figures)

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