Elemental Abundances in PG 1159 Stars

The hydrogen-deficiency in extremely hot post-AGB stars of spectral class PG1159 is probably caused by a (very) late helium-shell flash or a AGB final thermal pulse that consumes the hydrogen envelope, exposing the usuallyhidden intershell region. Thus, the photospheric elemental abundances of these stars allow to draw conclusions about details of nuclear burning and mixing processes in the precursor AGB stars. We compare predicted elemental abundances to those determined by quantitative spectral analyses performed with advanced non-LTE model atmospheres. A good qualitative and quantitative agreement is found for many species (He, C, N, O, Ne, F, Si, Ar) but discrepancies for others (P, S, Fe) point at shortcomings in stellar evolution models for AGB stars. PG1159 stars appear to be the direct progeny of [WC] stars. 1. Main characteristics of PG1159 stars The PG1159 stars are a group of 40 extremely hot hydrogen-deficient post-AGB stars. Their effective temperatures (Teff) range between 75 000 and 200 000 K. Many of them are still heating up along the constant-luminosity part of their post-AGB evolutionary path in the HR diagram (L ≈ 10L⊙) but most of them are already fading along the hot end of the white dwarf cooling sequence (with L ∼ 10 L⊙). Luminosities and masses are inferred from spectroscopically determined Teff and surface gravity (log g) by comparison with theoretical evolutionary tracks. The position of analysed PG1159 stars in the “observational HR diagram”, i.e., the Teff–g diagram, are displayed in Fig. 1. The high-luminosity stars have low log g (≈ 5.5) while the low-luminosity stars have a high surface gravity (≈ 7.5) that is typical for white dwarf (WD) stars. The derived mean mass is 0.57M⊙, a value that is practically identical to the mean mass of WDs (Bergeron et al. 2007). The PG1159 stars co-exist with hot central stars of planetary nebulae and the hottest hydrogen-rich (DA) white dwarfs in the same region of the HR diagram. About every other PG1159 star is surrounded by an old, extended planetary nebula. For a recent review with a detailed bibliography see Werner & Herwig (2006). What is the characteristic feature that discerns PG1159 stars from “usual” hot central stars and hot WDs? Spectroscopically, it is the lack of hydrogen Balmer lines, pointing at a H-deficient surface chemistry. The proof of Hdeficiency, however, is not easy: The stars are very hot, H is strongly ionized