Neutron capture reactions and stellar nucleosynthesis
Neutron capture reactions and stellar nucleosynthesis - heavy element buildup
STELLAR EVOLUTION↗
SEARCH · Engineering Papers
Results for “stellar nucleosynthesis”
Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
A process of stellar nucleosynthesis which mimicks mass fractionation in P-xenon
The combination of the O-shell theory of Heymann and Dziczkaniec and the supernova theory of Woosley and Howard clearly identifies the astrophysical sites for the formation of the anomalous light Xe component in carbonaceous chondrites. These sites are the O- and Ne-shells, and possibly C-shell of a massive star. Most of the Xe-124 and Xe-126 are formed in the O-shell during hydrostatic core silicon-burning, when a seed of heavy nuclei is exposed to an effective temperature near T9 = 2.0. Xe-128 is formed via Ba-128 in the O-shell, but the amounts appear too small to satisfy the deduced Xe-128/Xe-124 and Xe-128/Xe-126 yield ratios from the chondrites. However, substantial amounts of Xe-128 can be formed in the adjacent Ne- and C-shells during the explosion. The formation of Ba-128 in the O-shell would increase if the (gamma, alpha) photodisintegration rate in Ba-128 is actually smaller than calculated by Woosley and Howard. Lewis et al. have proposed that the anomalous light Xe component is mass-fractionated normal Xe. It is in this sense that the process of stellar nucleosynthesis of the present paper mimicks mass-fractionation.
Heymann, D.↗
Nucleosynthesis: Stellar and Solar Abundances and Atomic Data
Abundance observations indicate the presence of often surprisingly large amounts of neutron capture (i.e., s- and r-process) elements in old Galactic halo and globular cluster stars. These observations provide insight into the nature of the earliest generations of stars in the Galaxy the progenitors of the halo stars responsible for neutron-capture synthesis. Comparisons of abundance trends can be used to understand the chemical evolution of the Galaxy and the nature of heavy element nucleosynthesis. In addition age determinations, based upon long-lived radioactive nuclei abundances, can now be obtained. These stellar abundance determinations depend critically upon atomic data. Improved laboratory transition probabilities have been recently obtained for a number of elements. These new gf values have been used to greatly refine the abundances of neutron-capture elemental abundances in the solar photosphere and in very metal-poor Galactic halo stars. The newly determined stellar abundances are surprisingly consistent with a (relative) Solar System r-process pattern, and are also consistent with abundance predictions expected from such neutron-capture nucleosynthesis.
Cowan, John J.↗
Stellar nucleosynthesis and chemical evolution of the solar neighborhood
Current theoretical models of nucleosynthesis (N) in stars are reviewed, with an emphasis on their implications for Galactic chemical evolution. Topics addressed include the Galactic population II red giants and early N; N in the big bang; star formation, stellar evolution, and the ejection of thermonuclearly evolved debris; the chemical evolution of an idealized disk galaxy; analytical solutions for a closed-box model with continuous infall; and nuclear burning processes and yields. Consideration is given to shell N in massive stars, N related to degenerate cores, and the types of observational data used to constrain N models. Extensive diagrams, graphs, and tables of numerical data are provided.
Clayton, Donald D.↗
Oxygen isotopes in circumstellar Al203 grains from meteorites and stellar nucleosynthesis
We have found a approximately 3 micrometer Al2O3 grain (B39) in the Bishunpur LL3.1 chondrite that is enriched in 0-17 by a factor of approximately 6.8 (0-16/0-17 = 385 +/- 9) and depleted in 0-18 by almost 60% (0-16/0-18 = 853 +/- 30) relative to solar system oxygen and has an initial Al-26/Al-27 = 1.7 +/- 0.2 x 10 (exp 3), approximately 34 times greater than the initial solar system value. The isotopic compositions of B39 and two other Al2O3 grains previously reported from the Orgueil and Murchison meteorites show that these grains formed directly from the ejecta of low-mass AGB stars with C/O less than 1. A simple theoretical analysis is presented showing that the oxygen systematics of the grains are a natural consequence of main-sequence evolution followed by first dredge-up Al-26 is the result of third dredge-up. Circumstellar Al2O3 grains provide very precise isotopic data for stellar ejecta that complement spectroscopic observations of oxygen-rich stars. Isotopic differences indicate that the Al2O3 grains come from separate stars of different mass and intital oxygen composition that originated in molecular clouds different from the one in which the solar system formed.
Huss, Gary R.↗
Late emission from supernovae - A window on stellar nucleosynthesis
Monte Carlo techniques are used to study the energy deposition of gamma rays from the radioactive decay of Co-56 in Type Ib and Type II SN. It is found that above an electron fraction of about 0.1, direct excitations are negligible, and the line emission arises from thermal processes. The results indicate that the emission is dominated by neutral and singly ionized lines, including forbiden O I, forbidden Ca II, forbidden C I, and forbidden Si I. The late spectrum is shown to be useful as a diagnostic of the nucleosynthetic structure of the ejecta. The present model has been applied to observations of the Type Ib SN 1985F, and it is suggested that Type Ib SN come from stars of greater than 25 solar masses.
Fransson, Claes↗
Population II Li-6 as a probe of nucleosynthesis and stellar structure and evolution
We discuss the importance of Population II Li-6 as a diagnostic for models of primordial nucleosynthesis, cosmic-ray nucleosyntheses in the early Galaxy, and the structure and evolution of metal-poor solar-type stars. The observation of Li-6 in the subdwarf HD 84937 is shown to be consistent with the existing Population II LiBeB data within the context of a simple three-component model: (1) standard big bang nucleosynthesis, (2) Population II cosmic-ray nucleosynthesis, (3) standard (nonrotating) stellar LiBeB depletion. If this interpretation is correct, we predict a potentially detectable boron abundance for this star: about 2 x 10 exp -12. Subsequent Population II LiBeB observations, and in particular further observations of Population II Li-6, are shown to be crucial to our understanding of the primordial and early galactic creation and destruction mechanisms for light elements.
Steigman, Gary↗
Isotopically anomalous nitrogen in primitive meteorites
The extreme enrichement in N-14 (up to 48 percent) found in acid-resistant residues of the Allende and Murchison meteorites cannot be attained by normal solar system processes and must therefore be due to stellar nucleosynthesis. Consequently the xenon component enriched in the heavy isotopes associated with the light nitrogen very probably was made by stellar nucleosynthesis rather than by fission of an extinct superheavy element.
Lewis, R. S.↗
The 12C/13C Isotopic Ratio in Planetary Nebulae as Deduced From IUE Data
The relative abundances of C, N, and O and the isotopic ratio of C-12/C-13 represent tracers of nucleosynthesis in intermediate stars with main-sequence masses between 0.6 and 8.0 solar masses in our Galaxy. Determining these abundances and the isotopic C-12/C-13 ratio in planetary nebulae (PNe) represent perhaps the best means to discern exactly how the ISM is enriched by CNO stellar nucleosynthesis.
Miskey, C.L.↗
Constraints on Grain Formation Around Carbon Stars from Laboratory Studies of Presolar Graphite
We report the results of an investigation into the physical conditions in the mass outflows of asymptotic giant branch (AGB) carbon stars that are required for the formation of micron-sized presolar graphite grains, either with or without internal crystals of titanium carbide (TiC). In addition to providing detailed information about stellar nucleosynthesis, the structure and composition of presolar grains give unique information about the conditions of grain formation. In the present work we use laboratory observations of presolar graphite to gain insight into the physical conditions in circumstellar outflows from carbon AGB stars. The periodic pulsation of AGB stars enhances the gas density through shocks in the stellar atmosphere above the photosphere, promoting the condensation of dust grains. Copious mass outflow occurs largely because grains are coupled to the radiation field of the star, which accelerates them by radiation pressure; momentum is in turn transferred to gas molecules by collisions with grains. The dust/gas mixture is effectively a two-component fluid whose motion depends on atmospheric structure and which, in turn, influences that structure. In particular, the radiation pressure on the grains determines the velocity field of the outflow and thus the density distribution, while the density distribution itself determines the conditions of radiative transfer within the outflow and thus the effective radiation pressure.
Bernatowicz, T. J.↗
Quantative Measurements of Trace Elements and Three-Dimensional Atomic Scale Characterisation of Presolar O-rich Oxides and Silicates: An Atom Probe Tomography Approach
Presolar grains are recordsof a single moment in stellar evolution which have survived nebula and parent body processingwithin our Solar System. These grains condensed within a range of stellar envi-ronments including asymptotic giant branch stars, red giant branch stars, nova, supernova (SN) and hydrogen burning electron capture supernova(ECSN)[1,2]. Iso-topic and chemical compositions can be used to unravel details about environmental conditions at the time of their condensation and physical and chemical processes occurring at the time. For example,nucleosynthesis, stellar evolution, physical properties of stellar atmos-pheres, mixing from inner core to outer envelope, galac-tic chemical evolution, interstellar medium and parent body processing.NanoSIMS enables detailed characterisation of iso-topic compositions and rapid in situidentification of O-rich presolar oxides and silicates using their character-istic 17O/16O and 18O/16O isotopic ratios.Spectroscopic techniques e.g., auger spectroscopyand transmission electron microscopy, have provided additional details on major and minor chemical signatures. However, due tospatial resolution limitations,interaction volumes and interferencesfrom surrounding grains for in situtech-niques, attaining quantative characterisation of trace el-ements,hasproved challenging[1,3]. As the most sensitive geochemical tracers of envi-ronmental changes, trace elements are essential to un-ravelling the geochemical record of their parent stellar environments and evolutionary pathways[4].We car-ried out correlated in situisotopic and chemical analyses of 13 presolar grains to better understand stellar evolu-tion. In this work, we achieved this using a custom ap-proach, coordinating NanoSIMS, Scanning Electron Microscopy Energy Dispersive X-Ray Spectroscopy (SEM-EDX)and Atom Probe Tomography(APT).Our objective was to develop an approach which could ena-ble precise targeting of presolar grains for atom probe tomography and successfully execute atomic scale anal-yses of presolar oxides and silicates, to achieve quantative analysis of their trace elements for the first time.We also aimed to test the capability for Atom Probe tomography to measure isotopic compositions and stoichiometries of presolar oxide and silicate grains.
N D Nevill↗
Research reports, July 1, 1964 - June 30, 1965
Compilation of research reports in astronomy, geophysics, nucleosynthesis, stellar structure and evolution, solar system origin, and atmospheric dynamics and turbulence
STELLAR EVOLUTION↗
Carbynes - Carriers of primordial noble gases in meteorites
Five carbynes (triply bonded allotropes of carbon) have been found by electron diffraction in the Allende and Murchison carbonaceous chondrites: carbon VI, VIII, X, XI, and (tentatively) XII. From the isotopic composition of the associated noble-gas components, it appears that the carbynes in Allende (C3V chondrite) are local condensates from the solar nebula, whereas at least two carbynes in Murchison (C2 chondrite) are of exotic, presolar origin. They may be dust grains that condensed in stellar envelopes and trapped isotropically anomalous matter from stellar nucleosynthesis.
Whittaker, A. G.↗
Radio observations of molecules in the interstellar gas
Since 1968, radio astronomy has made it possible to identify nearly 50 molecules in the dense concentrations of the interstellar gas now generally termed molecular clouds. Most interstellar molecules are familiar stable compounds. However, one-fifth of the discovered species are ions, radicals, and acetylenic carbon chains so reactive in the laboratory that before being detected in space they had rarely been observed or were entirely unknown. The heavy atom backbone of the known interstellar molecules is a linear chain of C, N, O, or S. Si is found in two diatomic molecules. Rings and branched chains are missing. The most readily observed spectral lines of most interstellar molecules correspond to rotational transitions at millimeter wavelengths. These are generally excited by H2 collisions. A number of rare isotopic species are observed in interstellar molecules. Isotopic ratios differing from those on Earth exist, and can in almost all cases be attributed to stellar nucleosynthesis since the formation of the solar system.
Thaddeus, P.↗
Some key issues in isotopic anomalies - Astrophysical history and aggregation
Astrophysical history, particularly that period extending from stellar nucleosynthesis events to the formation of meteorites, is discussed as the key element for the understanding of isotopic anomalies in meteorites. The bulk homogeneity of the interstellar medium is considered, and it is argued that, despite the presence of spatial inhomogeneities due to different nucleosynthesis rates in different parts of the galaxy and supernova ejecta, a cosmic chemical memory of nucleosynthesis patterns, rather than an inhomogeneous injection, is the source of isotopic anomalies. According to this view, volatility patterns and some isotopic patterns are mapped onto a grain-size spectrum, and the FUN systematics may be explained by interstellar sputtering. Furthermore, meteoritic He and Ne abundances are inferred to be presolar, and the ubiquitous titanium isotopic anomalies are explained by processes of chemical fixation and condensation in varying environments.
Clayton, D. D.↗
Barium isotopes in Allende meteorite - Evidence against an extinct superheavy element
Carbon and chromite fractions from the Allende meteorite that contain isotopically anomalous xenon-131 to xenon-136 (carbonaceous chondrite fission or CCF xenon) at up to 5 x 10 to the 11th atoms per gram show no detectable isotopic anomalies in barium-130 to barium-138. This rules out the possibility that the CCF xenon was formed by in situ fission of an extinct superheavy element. Apparently the CCF xenon and its carbonaceous carrier are relics from stellar nucleosynthesis.
Lewis, R. S.↗
CNO isotopes in red giant stars
The production and distribution of the CNO nuclides is discussed in light of observed abundance ratios in red giants and in the interstellar medium. Isotope abundances have been measured in the atmospheres and in the recent ejecta of cool giants, including carbon stars, S-type stars and red supergiants as well as in oxygen-rich giants making their first ascent of the giant branch. Several of the observations suggest revision of currently accepted nuclear cross-sections and of the mixing processes operating in giant envelopes. By comparing red giant abundances with high-quality observations of the interstellar medium, conclusions are reached about the contribution of intermediate-mass stars to galactic nuclear evolution. The three oxygen isotopes, O-16, -17 and -18, are particularly valuable for such comparison because they reflect three different stages of stellar nucleosynthesis. One remarkable result comes from observations of O-17/O-18 in several classes of red giant stars. The observed range of values for red giants excludes the entire range of values seen in interstellar molecular clouds. Furthermore, both the observations of stars and interstellar clouds exclude the isotopic ratio found in the solar system.
Wannier, P. G.↗
Co-57 and Ti-44 production in SN 1987A
A survey of parameterized explosive silicon burning is carried out in order to limit the range of allowed masses of Co-57 produced by SN 1987A. The most likely value is between 0.5 and 2.5 times that implied by a solar ratio for Fe-57/Fe-56. Values more than four and less than one-third times the solar ratio would pose very severe problems for the theory of stellar nucleosynthesis, implying, for example, that Fe-56 was not the dominant iron group isotope or that no electron capture occurred during oxygen burning. Though dependent somewhat on the neutron excess, the Fe-57 yield is most sensitive to the occurrence of an alpha-rich freeze-out. An accurate measurement of the Fe-57 yield, as reflected observationally by the current abundance of radioactive Co-57, is thus an important constraint upon the supernova explosion mechanism. The abundance of Ti-44, another nucleus produced exclusively in the alpha-rich freeze-out, cannot be much greater than that implied by the solar ratio of Ca-44/Fe-56.
Woosley, S. E.↗