(F)UV spectroscopy of the hybrid PG1159-type central stars of the planetary nebulae NGC 7094 and Abell43

Abstract

Hydrogen-deficient post-AGB stars have experienced a late helium-shell flash that mixes the hydrogen-rich envelope and the helium-rich intershell (between hydrogen- and helium-burning shell). 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, namely PG 1159-035 and the central stars of the planetary nebulae (CSPN) K 1-16, NGC 7094, and Abell 78, surprisingly revealed strong iron deficiencies 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 increased abundance of these would be a strong indication for this scenario. Since reliable atomic data for highly ionized species heavier than iron is only available for cobalt and nickel, we can presently determine only the nickel abundance. We performed a detailed spectral analysis by means of NLTE model-atmosphere techniques based on high-resolution UV observations of the two PG 1159-type CSPN of 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 even nickel deficient by about one dex.