830 research outputs found
Post-AGB stars in the Magellanic Clouds and neutron-capture processes in AGB stars
We explore modifications to the current scenario for the slow neutron capture
process in asymptotic giant branch (AGB) stars to account for the Pb deficiency
observed in post-AGB stars of low metallicity ([Fe/H] ~ -1.2) and low initial
mass (~ 1 - 1.5 Msun) in the Large and Small Magellanic Clouds. We calculated
the stellar evolution and nucleosynthesis for a 1.3 Msun star with [Fe/H]=-1.3
and tested different amounts and distributions of protons leading to the
production of the main neutron source within the 13C-pocket and proton
ingestion scenarios. No s-process models can fully reproduce the abundance
patterns observed in the post-AGB stars. When the Pb production is lowered the
abundances of the elements between Eu and Pb, such as Er, Yb, W, and Hf, are
also lowered to below those observed. Neutron-capture processes with neutron
densities intermediate between the s and the rapid neutron-capture processes
may provide a solution to this problem and be a common occurrence in low-mass,
low-metallicity AGB stars.Comment: 6 pages, 4 figures. To be published in Astronomy and Astrophysic
The lead discrepancy in intrinsically s-process enriched post-AGB stars in the Magellanic Clouds
Context: Our understanding of the s-process nucleosynthesis in asymptotic
giant branch (AGB) stars is incomplete. AGB models predict, for example, large
overabundances of lead (Pb) compared to other s-process elements in metal-poor
low-mass AGB stars. This is indeed observed in some extrinsically enhanced
metal-poor stars, but not in all. An extensive study of intrinsically s-process
enriched objects is essential for improving our knowledge of the AGB third
dredge-up and associated s-process nucleosynthesis. Aims: We compare the
spectral abundance analysis of the SMC post-AGB star J004441.04-732136.4 with
state-of-the-art AGB model predictions with a main focus on Pb. The low S/N in
the Pb line region made the result of our previous study inconclusive. We
acquired additional data covering the region of the strongest Pb line. Methods:
By carefully complementing re-reduced previous data, with newly acquired UVES
optical spectra, we improve the S/N of the spectrum around the strongest Pb
line. Therefore, an upper limit for the Pb abundance is estimated from a merged
weighted mean spectrum using synthetic spectral modeling. We then compare the
abundance results from the combined spectra to predictions of tailored AGB
evolutionary models from two independent evolution codes. In addition, we
determine upper limits for Pb abundances for three previously studied LMC
post-AGB objects. Results: Although theoretical predictions for
J004441.04-732136.4 match the s-process distribution up to tungsten (W), the
predicted very high Pb abundance is clearly not detected. The three additional
LMC post-AGB stars show a similar lack of a very high Pb abundance. Conclusion:
From our study, we conclude that none of these low-mass, low-metallicity
post-AGB stars of the LMC and SMC are strong Pb producers. This conflicts with
current theoretical predictions.Comment: 4 pages, 3 figure
The Chemical Evolution of Magnesium Isotopic Abundances in the Solar Neighbourhood
The abundance of the neutron-rich magnesium isotopes observed in metal-poor
stars is explained quantitatively with a chemical evolution model of the local
Galaxy that considers - for the first time - the metallicity-dependent
contribution from intermediate mass stars. Previous models that simulate the
variation of Mg isotopic ratios with metallicity in the solar neighbourhood
have attributed the production of Mg25 and Mg26 exclusively to hydrostatic
burning in massive stars.
These models match the data well for [Fe/H]>-1.0 but severely underestimate
Mg25/Mg24 and Mg26/Mg24 at lower metallicities. Earlier studies have noted that
this discrepancy may indicate a significant role played by intermediate-mass
stars. Only recently have detailed calculations of intermediate-mass stellar
yields of Mg25 and Mg26 become available with which to test this hypothesis. In
an extension of previous work, we present a model that successfully matches the
Mg isotopic abundances in nearby Galactic disk stars through the incorporation
of nucleosynthesis predictions of Mg isotopic production in asymptotic giant
branch stars.Comment: 9 pages, 6 figures, to appear in Publications of the Astronomical
Society of Australia (PASA) in 2003, vol. 20, No.
Crystal structure of a heterotetrameric NMDA receptor ion channel
N-Methyl-D-aspartate (NMDA) receptors belong to the family of ionotropic glutamate receptors, which mediate most excitatory synaptic transmission in mammalian brains. Calcium permeation triggered by activation of NMDA receptors is the pivotal event for initiation of neuronal plasticity. Here, we show the crystal structure of the intact heterotetrameric GluN1-GluN2B NMDA receptor ion channel at 4 angstroms. The NMDA receptors are arranged as a dimer of GluN1-GluN2B heterodimers with the twofold symmetry axis running through the entire molecule composed of an amino terminal domain (ATD), a ligand-binding domain (LBD), and a transmembrane domain (TMD). The ATD and LBD are much more highly packed in the NMDA receptors than non-NMDA receptors, which may explain why ATD regulates ion channel activity in NMDA receptors but not in non-NMDA receptors
Evaluation of left renal vein and inferior vena cava variations through routine abdominal multi-slice computed tomography
Background: The aim of this study was to evaluate the frequency of left renal vein (LRV) and inferior vena cava (IVC) variations and the effect of gender on this frequency, as well as the presence of associated abdominal pathologies.Materials and methods: Multi-slice computed tomography (MSCT) images from 746 patients were evaluated retrospectively.Results: Left renal vein variations were identified in 9.8% of cases, while retroaortic LRV (RLRV) and circumaortic LRV (CLRV) were found in 7.4% and 2.4% of cases, respectively. No significant correlation was found between gender and LRV variations (p = 0.797). Of the cases with LRV and IVC variations, 73% had abdominal pathologies, the most common of which were nephrolithiasis, which appeared in 18 (32%) cases, and renal cysts, which appeared in 14 (25.4%) cases.Conclusions: MSCT is a rapid and reliable method of identifying LRV and IVC variations and associated abdominal pathologies
Abundances in intermediate-mass AGB stars undergoing third dredge-up and hot-bottom burning
High dispersion near-infrared spectra have been taken of seven
highly-evolved, variable, intermediate-mass (4-6 Msun) AGB stars in the LMC and
SMC in order to look for C, N and O variations that are expected to arise from
third dredge-up and hot-bottom burning. The pulsation of the objects has been
modelled, yielding stellar masses, and spectral synthesis calculations have
been performed in order to derive abundances from the observed spectra. For two
stars, abundances of C, N, O, Na, Al, Ti, Sc and Fe were derived and compared
with the abundances predicted by detailed AGB models. Both stars show very
large N enhancements and C deficiencies. These results provide the first
observational confirmation of the long-predicted production of primary nitrogen
by the combination of third dredge-up and hot-bottom burning in
intermediate-mass AGB stars. It was not possible to derive abundances for the
remaining five stars: three were too cool to model, while another two had
strong shocks in their atmospheres which caused strong emission to fill the
line cores and made abundance determination impossible. The latter occurrence
allows us to predict the pulsation phase interval during which observations
should be made if successful abundance analysis is to be possible.Comment: Accepted for publication in MNRA
The Investigation of third-order hyperpolarizabilities and susceptibilities of push-pull azobenzene polymers
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