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Post Info TOPIC: PG1159 Central Stars
Blobrana


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PG1159 Stars
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Title: Discovery of Iron in PG1159 Stars
Authors: K. Werner, T. Rauch, J. W. Kruk

The lack of FeVII lines in PG1159 stars had led to the conclusion that in some objects iron must be strongly depleted. We have now detected FeX lines in FUSE spectra of the very hottest PG1159 stars Teff=150,000 - 200,000 K; RXJ2117.1+3412, K1-16, NGC 246, Longmore 4). Surprisingly, we derive a solar iron abundance. It is conspicuous that they are among the most massive PG1159 stars (0.71-0.82 Msun), in contrast to those objects for which strongest Fe-deficiency was claimed (0.53-0.56 Msun). Based on new FeVIII line identifications in SOHO/SUMER UV spectra of the Sun, we were able to detect these lines in FUSE spectra of several "cooler" (Teff<150,000 K) objects, among them is the prototype PG1159-035. An abundance analysis is in progress.

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Blobrana


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RE: PG1159 Central Stars
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Title: O(He) Stars
Authors: T. Rauch (1), E.Reiff (1), K. Werner (1), J. W. Kruk (2) ((1) Institute for Astronomy and Astrophysics, Kepler Center for Astro and Particle Physics, Eberhard Karls University, Tuebingen, Germany, (2) Department of Physics and Astronomy, Johns Hopkins University, Baltimore, U.S.A.)

Spectral analyses of H-deficient post-AGB stars have shown that a small group of four extremely hot objects exists which have almost pure He absorption-line spectra in the optical. These are classified as O(He) stars. For their evolution there are two scenarios: They could be the long-sought hot successors of RCrB stars, which have not been identified up to now. If this turns out to be true, then a third post-AGB evolutionary sequence is revealed, which is probably the result of a double-degenerate merging process. An alternative explanation might be that O(He) stars are post early-AGB stars. These depart from the AGB just before they experience their first thermal pulse (TP) which will then occur as a late thermal pulse (LTP). This would be a link to the low-mass He-enriched sdO stars and low-mass, particularly He-rich PG1159 stars.

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Blobrana


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Title: Search For Iron, Nickel, and Fluorine in PG1159 Stars
Authors: E. Reiff, T. Rauch, K. Werner, J.W. Kruk, L. Koesterke

A possible origin of the iron-deficiency in PG1159 stars could be neutron captures on Fe nuclei. A nickel overabundance would corroborate this idea. Consequently we are looking for nickel lines in PG1159 stars. Prime targets are relatively cool objects, because Ni VI is the dominant ionisation stage and the spectral lines of this ion are accessible with UV observations. We do not find such lines in the coolest PG1159 star observed by FUSE (PG1707+427, Teff = 85,000 K) and conclude that the nickel abundance is not enhanced. Hence, the Fe-deficiency in PG1159 stars remains unexplained. In addition, we present results of a wind analysis of the hybrid-PG1159 star NGC 7094 and the [WC]-PG1159 transition-type object Abell 78 in order to derive F abundances from the F VI 1139.5 Angstrom line. In both cases, we find F overabundances, in agreement with results of photospheric analyses of many PG1159 stars. Surprisingly, we find indications for a very low O abundance in NGC 7094.

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Blobrana


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PG1159 stars
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Title: Evidence of thin helium envelopes in PG1159 stars
Authors: L. G. Althaus, A. H. Córsico, M. M. Miller Bertolami, E. García--Berro, S. O. Kepler

We present evidence that PG1159 stars could harbour He--rich envelopes substantially thinner than those predicted by current evolutionary models with current estimates of mass loss, which may be attributable to an extensive mass--loss episode during the born--again AGB phase. Specifically, we show that the models with thin He--rich envelopes predict remarkably large magnitudes of the rates of period change of the trapped and untrapped modes observed in the pulsating star PG 1159$-$035. This is a consequence of the much shorter evolutionary timescale of the models with thin He--rich envelopes during the low--gravity PG1159 regime. Our findings are particularly interesting in view of the suggestion of an evolutionary link between the helium--deficient PG1159 star H1504+65 and the recently discovered white dwarfs with almost pure carbon atmospheres.

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Blobrana


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Posts: 131433
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PG1159 Central Stars
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Title: [WC] and PG1159 Central Stars of Planetary Nebula: the Need for an Alternative to the Born-Again Scenario
Authors: Orsola De Marco

Hydrogen-deficient central stars of planetary nebula such as Wolf-Rayet and PG1159 central stars and some weak emission line stars are primarily composed of helium and carbon. This abundance is well explained by a scenario where a single post-AGB star experiences a last helium shell flash which ingests and burns, or simply dilutes, the remaining hydrogen atmosphere. But despite its success in matching the photospheric abundances of these stars, this scenario is faced with several observational challenges. A binary scenario is proposed here as a more natural way to face some of the most stringent observational constraints. In this scenario the H-rich primary in a close binary formed during a common envelope on the AGB, suffers a last helium shell flash, which results in a H-deficient primary with some of the characteristics needed to match the observations.

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