520 research outputs found

    Hierarchical Structure Formation and Chemical Evolution of Damped Ly alpha Systems

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    We present a model for chemical evolution of damped Ly alpha systems considering production of metals by SNe II and infall associated with hierarchical structure formation. The growth of metallicity in these systems is a reflection of the competition between astration and infall. The apparent late turn-on of these systems is due to the late cut-off of infall. The wide range in [Fe/H] at a given redshift is explained by the range of the times for onset of star formation and the range of the times for infall cessation in different systems. The observed lower bound of [Fe/H] = -3 follows from the very rapid initial rise of [Fe/H] subsequent to onset of star formation. To reach [Fe/H] = -3 from a metal-free initial state requires only about 30 Myr so that the probability of observing lower [Fe/H] values is very small.Comment: 4 pages, 2 figures, to appear in ApJ

    Nucleosynthesis in asymptotic giant branch stars: Relevance for galactic enrichment and solar system formation

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    We present a review of nucleosynthesis in AGB stars outlining the development of theoretical models and their relationship to observations. We focus on the new high resolution codes with improved opacities, which recently succeeded in accounting for the third dredge-up. This opens the possibility of understanding low luminosity C stars (enriched in s-elements) as the normal outcome of AGB evolution, characterized by production of 12C and neutron-rich nuclei in the He intershell and by mass loss from strong stellar winds. Neutron captures in AGB stars are driven by two reactions: 13C(α,n)16O, which provides the bulk of the neutron flux at low neutron densities (Nn ≤ 107 n/cm3), and 22Ne(α,n)25Mg, which is mildly activated at higher temperatures and mainly affects the production of s-nuclei depending on reaction branchings. The first reaction is now known to occur in the radiative interpulse phase, immediately below the region previously homogenized by third dredge-up. The second reaction occurs during the convective thermal pulses. The resulting nucleosynthesis phenomena are rather complex and rule out any analytical approximation (exponential distribution of neutron fluences). Nucleosynthesis in AGB stars, modeled at different metallicities, account for several observational constraints, coming from a wide spectrum of sources: evolved red giants rich in s-elements, unevolved stars at different metallicities, presolar grains recovered from meteorites, and the abundances of s-process isotopes in the solar system. In particular, a good reproduction of the solar system main component is obtained as a result of Galactic chemical evolution that mixes the outputs of AGB stars of different stellar generations, born with different metallicities and producing different patterns of s-process nuclei. The main solar s-process pattern is thus not considered to be the result of a standard archetypal s-process occurring in all stars. Concerning the 13C neutron source, its synthesis requires penetration of small amounts of protons below the convective envelope, where they are captured by the abundant 12C forming a 13C-rich pocket. This penetration cannot be modeled in current evolutionary codes, but is treated as a free parameter. Future hydrodynamical studies of time dependent mixing will be required to attack this problem. Evidence of other insufficiencies in the current mixing algorithms is common throughout the evolution of low and intermediate mass stars, as is shown by the inadequacy of stellar models in reproducing the observations of CNO isotopes in red giants and in circumstellar dust grains. These observations require some circulation of matter between the bottom of convective envelopes and regions close to the H-burning shell (cool bottom processing). AGB stars are also discussed in the light of their possible contribution to the inventory of short-lived radioactivities that were found to be alive in the early solar system. We show that the pollution of the protosolar nebula by a close-by AGB star may account for concordant abundances of 26Al, 41Ca, 60Fe, and 107Pd. The AGB star must have undergone a very small neutron exposure, and be of small initial mass (M <= 1.5 [sols]). There is a shortage of 26Al in such models, that however remains within the large uncertainties of crucial reaction rates. The net 26Al production problem requires further investigation

    The President's Page: Apollo Belvedere [Guest Editorial]

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    It is five years since the first human beings landed on another planet and then returned to earth. It is almost two years since the last Apollo mission returned to earth from the moon. The excitement of Apollo 11 which reached a large proportion of the population of planet earth is now a quiet, majestic fact in history. The excitement and interest in scientific aspects of all the Apollo missions is active and has grown into an evolving study of our sister planet

    Somewhere Over the Rainbow

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    The study of meteorites has produced a wealth of information about these objects. Petrologic and chemical observations of meteorites have shown the presence of planetary differentiates and of undifferentiated aggregates resulting from nebular condensation and the interaction of these aggregates due to metamorphism. Equilibrium chemistry in a solar gas and in planetary environments has been an excellent guide to our studies. Studies by several groups have yielded remarkable discoveries about the early stages of planetary evolution and of precursor components from the solar nebula and from the interstellar medium (ISM)

    The Effects of H_2O in Silicate Systems

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    A theoretical discussion of the role of water in simple silicate melts is presented. It is shown that to a good approximation in the region T ≈ 1,000° C., P < 2.3 X 10^3 bars, both the solubility of water in an NaAlSi_3O_8 melt and the depression of the freezing point of albite from such a melt are grossly governed by a perfect-solution law. A more detailed theory is presented in which the entropy of mixing for an ideal mixture of the bridging oxygen atoms of the silicate with the oxygen atoms of the water is calculated. This model is shown to be in good agreement with the observed phase diagram of the NaAlSi_3O_8-H_2O system

    Origin of rare gases in natural gases

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    Studies have been made by several laboratories on the isotopic and chemical abundances of He, Ne, Ar, Kr, and Xe in natural gases from both gas wells and geothermal areas. This work has shown that the rare gases consist of a mixture of two distinct components--atmospheric and radiogenic. These investigations give information on the evolution of natural gas accumulations from considering the dissolved atmospheric components and the production of He^4, A^(40) and Xe from nuclear processes. The importance of solubility phenomena in fractionation of these elements may be approached by using a simple model. Measurements of the radiogenic gases permit estimates of the rate of evolution of the atmosphere

    Diffusion processes in lead-uranium systems

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    The ratio r_λ of radiogenic daughter to residual parent (with decay constant λ) was investigated for a continuous diffusion model for a diffusion coefficient that is an arbitrary function of time. A solution for simultaneous diffusion of both parent and daughter is given in closed form. A theory is also developed that describes the trajectories rλ, rλ′, for paired decay schemes (with decay constants λ and λ′) in the neighborhood of concordia. A wide class of systems is shown to be linear in the r_λ, r_(λ′), diagram when ∫_0^τ D(η) dη/a^2≪1. A radiation damage model is proposed foi the diffusion coefficient that relates D(τ) to the integrated irradiation damage and to the uranium (and thorium) contents. The functional relationship between r_λ and the uranium content is found for three important cases. The model D(τ) = D_(1τ) yields a r_λ, r_(λ′), curve that is very similar to that obtained by Tilton for D = D_0. The slope is somewhat greater and the linear region more extended. The theoretical curves are found to be in reasonable agreement with the existing experimental data, and they fit the existing data at least as well as the constant D model
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