13 research outputs found
Field-cycling NMR realaxation spectroscopy of poly(di-n-alkylsiloxanes in solid, mesomorphic, and isotropic liquid phases
The frequency dependence of the proton spin-lattice relaxation times T1 and T1., in the laboratory and rotating frames, respectively, is reported for solid and liquid phases of poly(diethylsiloxane) (PDES) and in melts of poly(dimethylsiloxane) (PDMS). The total frequency range is 5 X 102 -3 X 108 Hz and is mainly covered by field-cycling NMR relaxation spectroscopy. The relaxation behavior of PDES in the liquid but ordered mesophase is compared to that of isotropic melts of PDES and PDMS and also to that of nematic main-chain liquid-crystal polymers. The frequency dependences of PDES and PDMS liquids can be represented at low and high frequencies by power laws, section by section. The relaxation behavior in the isotropic melts is entirely equivalent to that previously reported for other polymer species. In the PDES mesophase, the exponents of the power laws are significantly larger and the crossover frequency between the two regimes is reduced. The dynamics in this phase are discussed with respect to the influence of chain modes and order director fluctuations. The main conclusion is, on the whole, that data of the liquid phases are determined by chain modes rather than by local segment fluctuations. The chain dynamics in the PDES mesophase resemble the chain modes in isotropic melts modified for a microstructure with reduced randomness, whereas the influence of order director fluctuations can neither be confirmed nor ruled out
V605 Aql: The Older Twin of Sakurai's Object
New optical spectra have been obtained with VLT/FORS2 of the final helium
shell flash (FF) star, V605 Aql, which peaked in brightness in 1919. New models
suggest that this star is experiencing a very late thermal pulse. The evolution
to a cool luminous giant and then back to a compact hot star takes place in
only a few years. V605 Aql, the central star of the Planetary Nebula (PN), A58,
has evolved from T5000 K in 1921 to 95,000 K today. There are
indications that the new FF star, Sakurai's Object (V4334 Sgr), which appeared
in 1996, is evolving along a similar path. The abundances of Sakurai's Object
today and V605 Aql 80 years ago mimic the hydrogen deficient R Coronae Borealis
(RCB) stars with 98% He and 1% C. The new spectra show that V605 Aql has
stellar abundances similar to those seen in Wolf-Rayet [WC] central stars of
PNe with ~55% He, and ~40% C. The stellar spectrum of V605 Aql can be seen even
though the star is not directly detected. Therefore, we may be seeing the
spectrum in light scattered around the edge of a thick torus of dust seen
edge-on. In the present state of evolution of V605 Aql, we may be seeing the
not too distant future of Sakurai's Object.Comment: 12 pages, 1 figure, ApJ Letters in pres
The Element Abundances in Bare Planetary Nebula Central Stars and the Shell Burning in AGB Stars
We review the observed properties of extremely hot hydrogen-deficient
post-AGB stars of spectral type [WC] and PG1159. Their H-deficiency is probably
caused by a (very) late helium-shell flash or a AGB final thermal pulse, laying
bare interior stellar regions which are usually kept hidden below the hydrogen
envelope. Thus, the photospheric element abundances of these stars allow to
draw conclusions about details of nuclear burning and mixing processes in the
precursor AGB stars. We summarize the state-of-the-art of stellar evolution
models which simulate AGB evolution and the occurrence of a late He-shell
flash. 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. Future
work can contribute to an even more complete picture of the nuclear processes
in AGB stars.Comment: Review, accepted for publication in PASP, Febr. 06 issue. For high
resolution versions of Figures 1 and 6 see preprint on
http://astro.uni-tuebingen.de/publications/paper_05_05.shtm