Global ETD Search
Search theses and dissertations gathered from participating repositories worldwide. Every result links back to the library that holds it. No account is needed.
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Showing 1 to 20 of 21 for “"AGB stars"”.
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Mixing and Nucleosynthesis in Low- and Intermediate-Mass AGB Stars
The existence of carbon stars brighter than M$\sb {\rm bol}$ = $-4$ can be understood in terms of dredge up in thermally pulsing asymptotic giant branch (AGB) stars. Models in this study include the effect that carbon recombination has upon opacity, and it is shown that a low-metallicity (Z = …
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Testing stellar models with observations from AGB and post-AGB stars
Low- to intermediate-mass stars (0.8 - 7 M{u2299}) are interesting in a broad astronomical context because they produce large amounts of electromagnetic radiation, they eject large amounts of mass, and the ejected mass contains many products of internal nucleosynthesis. However, theoretical studies …
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Testing stellar models with observations from AGB and post-AGB stars
Low- to intermediate-mass stars (0.8 - 7 M{u2299}) are interesting in a broad astronomical context because they produce large amounts of electromagnetic radiation, they eject large amounts of mass, and the ejected mass contains many products of internal nucleosynthesis. However, theoretical studies …
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Carbon stars and silicon carbide
Stars between about 0.8 and 8 times the mass of the Sun will eventually evolve become asymptotic giant branch (AGB) stars, where they pulsate and throw off mass from their atmospheres, forming dust shells in the space around them. AGB stars between about 2 to 4 solar masses can develop atmospheres …
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Hot planetary nebulae nuclei and related objects
The hottest central stars found in planetary nebulae excite the O VI atomic transition in their atmospheres. Presumably the central stars of all planetary nebulae become O VI stars during some phase of their evolution. Observational studies show that all post-asymptotic giant branch (P-AGB) stars …
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Mapping the interstellar magnetic field: Observations of the Goldreich-Kylafis effect towards late-type evolved stars and star-forming regions
… magnetic fields in both late-type evolved stars and star-forming regions. Observations of linearly polarized molecular line transitions arising from the Goldreich-Kylafis (G-K) effect provide valuable insight into the magnetic field geometry in these sources. Through observing with CARMA, …
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The chemistry of metal-containing molecules in the circumstellar envelopes of late-type stars: Millimeter-wave observations and abundance modeling
… program of Asymptotic Giant Branch (AGB) and post-AGB stars has been conducted in order to investigate the role of metal-containing compounds in the circumstellar envelopes surrounding these objects. The role of metal bearing molecules in the chemical evolution of these envelopes was …
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Following Carbon's Evolutionary Path: From Nucleosynthesis to the Solar System
… ratios in the envelopes of various types of stars were also measured from both CO and CN isotopologs. Such objects as carbon and oxygen-rich asymptotic giant branch (AGB) stars, supergiants, planetary nebulae, and S-type stars were observed. Results from this study indicate 12C/13C values for …
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Study of the $^{19}$F(p,$\alpha$)$^{16}$O reaction through the Trojan Horse Method and its astrophysical enviroment
… intershell region of Asymptotic Giant Branch (AGB) stars and in the same region also the $s$ elements are produced. Because fluorine is produced in the He-intershell and then dredged up to the surface together with $s$ process elements, its abundance is used as probe for AGB models and …
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Post-common-envelope binary central stars of Planetary Nebulae in the OGLE-IV survey
… complex, non-spherical shapes. Mass loss in AGB stars is presumed as the shaping mechanism but how it results in different PNe morphologies is still unclear. Binary central stars that have undergone common envelope evolution are thought to be a possible solution to this longstanding problem. …
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Isotopic and Elemental Compositions of Stardust and Protosolar Dust Grains in Primitive Meteorites
… Red Giant or Asymptotic Giant Branch stars. The second most abundant fraction of grains show large <super>18<super>O excesses and come from core collapse supernovae. The next most abundant fraction of grains comes from high metallicity AGB stars of approximately solar mass. A minor …
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Microanalytical Investigations of Presolar SiC Grains as Probes of Condensation Conditions in Astrophysical Environments
… environments, including asymptotic giant branch: AGB) stars, J stars, born-again AGB stars, and Type II supernovae: SNe). The isotopic compositions and microstructural properties of a large number of presolar grains have previously been measured, but the two techniques have been combined in the …
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The galactic evolution ofs-process elements via asymptotic giant branch stars
… to study the ability of asymptotic giant branch stars to produce s-process elements as a function of initial mass and metallicity. The normal methods of extracting information from spectroscopic observations of the surfaces of these stars are reviewed and it is shown how misleading some of the …
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Neutron induced reactions on 26Al, 36Cl and 41Ca and their astrophysical implications
… indeed severely depleted by neutron captures in AGB stars. In the (n,p) and (n,alpha) measurements on 36Cl, eighteen resonances are observed in the energy region up to 250 keV, whereas eight were identified before. Only the lowest energy resonance shows a significant (n,alpha)-contribution of …
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Circumstellar Distribution of Dust Grains Around Proto -Planetary Nebulae: Observational Evidence for Axisymmetry and 2.5-D Radiative Transfer Model Calculations
… intermediate initial mass (0.8--8 solar masses) stars that are in rapid transition from the asymptotic giant branch (AGB) phase into the planetary nebula (PN) phase. Largely spherically symmetric circumstellar dust shells of AGB stars are known to evolve into mostly axisymmetric PNs. Since the …
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Dust disks around hot white dwarfs and central stars of planetary nebulae
… 24 um emission, seven of which are still central stars of planetary nebulae (CSPNs). We have therefore used archival Spitzer IRAC and MIPS observations of PNe to search for CSPNs with excess IR emission, and found five additional IR excesses likely originating from dust disks. For some of these …
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Evolution of the Circumstellar Atomic and Molecular Envelopes Around Evolved Stars
The asymptotic giant branch (AGB) mass loss history of stars is recorded in the structure of their circumstellar envelope (CSE). For the more evolved protoplanetary nebula (PPN) and planetary nebula (PN) phases, the CSE also chronicles its interaction with fast winds and …
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Observational and theoretical studies of SiO maser polarization toward late-type evolved stars: Insights from EVPA reversal features
… of SiO masers toward late-type evolved stars continue to be a subject of debate. Possible primary sources include the local magnetic field or anisotropic pumping, while additional polarization may arise due to conversion from linear to circular modes through scattering or Faraday …
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Multi-element isotopic analyses of presolar graphite grains from the Orgueil meteorite
… from low-metallicity asymptotic giant branch stars. Evidence for this comes from correlated <super>12</super>C and <super>30</super>Si enrichments. In low-metallicity asymptotic giant branch stars, <super>12</super>C and <super>29,30</super>Si that are produced in the He shell are mixed into …
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