Precursor AGB Research Articles

AGB star intershell abundances inferred from UV spectra of extremely hot post-AGB 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 usually-hidden intershell region. Thus, the photospheric element abundances of these stars allow us to draw conclusions about details of nuclear burning and mixing processes in the precursor AGB stars. We compare predicted element 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.

High-resolution ultraviolet spectroscopy of PG 1159-035 with HST and FUSE

Context. PG 1159-035 is the prototype of the PG 1159 spectral class which consists of extremely hot hydrogen-deficient (pre-) white dwarfs. It is also the prototype of the GW Vir variables, which are non-radial g-mode pulsators. The study of PG 1159 stars reveals insight into stellar evolution and nucleosynthesis during AGB and post-AGB phases. Aims. We perform a quantitative spectral analysis of PG 1159-035 focusing on the abundance determination of trace elements. Methods. We have taken high-resolution ultraviolet spectra of PG 1159-035 with the Hubble Space Telescope and the Far Ultraviolet Spectroscopic Explorer. They are analysed with non-LTE line blanketed model atmospheres. Results. We confirm the high effective temperature with high precision (T eff = 140000 ± 5000 K) and the surface gravity of log g = 7. For the first time we assess the abundances of silicon, phosphorus, sulfur, and iron. Silicon is about solar. For phosphorus we find an upper limit of solar abundance. A surprisingly strong depletion of sulfur (2% solar) is discovered. Iron is not detected, suggesting an upper limit of 30% solar. This coincides with the Fe deficiency found in other PG 1159 stars. We redetermine the nitrogen abundance and find it to be lower by one dex compared to previous analyses. Conclusions. The sulfur depletion is in contradiction with current models of AGB star intershell nucleosynthesis. The iron deficiency confirms similar results for other PG 1159 stars and is explained by the conversion of iron into heavier elements by n-capture in the s-processing environment of the precursor AGB star. However, the extent of the iron depletion is stronger than predicted by evolutionary models. The relatively low nitrogen abundance compared to other pulsating PG 1159 stars weakens the role of nitrogen as a distinctive feature of pulsators and non-pulsators in the GW Vir instability strip.

High-gravity central stars

NLTE spectral analyses of high-gravity central stars by means of state-of-the-art model atmosphere techniques provide information about the precursor AGB stars. The hydrogen-deficient post-AGB stars allow investigations on the intershell matter which is apparently exhibited at the stellar surface. We summarize recent results from imaging, spectroscopy and spectropolarimetry.

The formation of globules in planetary nebulae

We discuss the formation of globules in planetary nebulae, typified by those observed in the Helix Nebula. We show that the properties of the globules, their number, mass, separation, and overall geometry strongly support a scenario in which globules are formed by the fragmentation of a swept-up shell as opposed to models in which the knots form in the AGB wind. We show that the RT or other instabilities which lead to the break-up of shells formed in the nebulae by fast winds or ionization fronts can produce arrays of globules with the overall geometry and within the mass range observed. We also show that the presence of a magnetic field in the circumstellar gas may play an important role in controlling the fragmentation process. Using field strengths measured in the precursor AGB envelopes, we find that close to the central star where the fields are relatively strong, the wavelengths of unstable MRT modes are larger than the shell dimensions, and the fragmentation of the shell is suppressed. The wavelength of the most unstable MRT mode decreases with increasing distance from the star, and when it becomes comparable to the shell thickness, it can lead to the sudden, rapid break-up of an accelerating shell. For typical nebula parameters, the model results in numerous fragments with a mass scale and a separation scale similar to those observed. Our results provide a link between global models of PN shaping in which shells form via winds and ionization fronts, and the formation of small scale structures in the nebulae.

Identification of neon in FUSE and VLT spectra of extremely hot hydrogen-deficient (pre-) white dwarfs

One of the strongest absorption lines observed in far-ultraviolet FUSE spectra of many PG 1159 stars remained unidentified up to now. We show that this line, located at 973.3 A, stems from Ne . We also present new optical high- resolution spectra of PG 1159 stars, obtained with the ESO VLT, which cover the Ne  3644 A line and a newly identified Ne  multiplet in the 3850-3910 A region. We compare the observed neon lines with NLTE models and conclude a substantial neon overabundance in a number of objects. Although a detailed analysis is still to be performed in order to compare quantita- tively the abundances with evolutionary theory predictions, this corroborates the idea that the PG 1159 stars and their immediate progenitors, the (WC)-type nuclei of planetary nebulae, display intershell matter of their precursor AGB stars. Possibly as the consequence of a late He-shell flash, H-deficient and (s-processed) Fe-depleted matter, that is strongly enriched by 3α-processed elements (C, O, Ne), is dredged up to the surface. Hence, a detailed study of the element abundance patterns in these peculiar stars gives the unique possibility to probe mixing and nucleosynthesis processes in the precursor AGB stars.

The molecular envelopes of Planetary Nebulae

We report the results of an extensive survey of molecular gas in planetary nebulae (PNe) using the millimeter lines of CO. The observations provide a range of information on the kinematics, the structure, and the mass of PN envelopes, which are important for understanding the formation and evolution of the ionized nebulae from the molecular envelopes ejected during the precursor AGB and proto-PNe phases.

New evolutionary relationships for Magellanic Cloud planetary nebulae

view Abstract Citations (24) References (23) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS New Evolutionary Relationships for Magellanic Cloud Planetary Nebulae Dopita, Michael A. ; Meatheringham, Stephen J. ; Wood, Peter R. ; Webster, B. Louise ; Morgan, David H. ; Ford, Holland C. Abstract Two new evolutionary correlations have been discovered to apply to the population of planetary nebulae (PN) in the Magellanic Clouds. First, the age of the nebular shell is found to follow a relationship, which is shown to be consistent with a model in which the total energy of the ionized and swept-up gas drives the expansion down the density gradient in the precursor AGB wind. Second, a tight correlation is found between the expansion velocity and a combination of the excitation class and the Hβ flux. This appears to be determined by the mass of the planetary nebula nuclear star. These correlations provide strong observational support for the idea that the PN shells are ejected at low velocity during the asymptotic giant branch phase of evolution, and that they are continually accelerated during their nebular lifetimes. Publication: The Astrophysical Journal Pub Date: April 1987 DOI: 10.1086/184870 Bibcode: 1987ApJ...315L.107D Keywords: Ionized Gases; Magellanic Clouds; Planetary Nebulae; Stellar Evolution; Stellar Mass; White Dwarf Stars; Asymptotic Giant Branch Stars; H Beta Line; Hertzsprung-Russell Diagram; High Resolution; Line Of Sight; Astrophysics; GALAXIES: MAGELLANIC CLOUDS; NEBULAE: PLANETARY; STARS: EVOLUTION; STARS: WHITE DWARFS full text sources ADS | data products SIMBAD (2) NED (2)