Magnetic Bubbles and Extramixing in Stars

The possible role of magnetic flux tubes in transporting matter from near the H shell through the radiative zone and into the convective envelope is explored. It is shown that the required rates of mass transport necessary to provide nuclear processed material to the envelope can be achieved if large magnetic fields are present just above the H shell in AGB and RGB stars. The required fields in this zone reach 5 MG for the AGB case and 0.5-0.05 MG for the RGB case. It may thus be possible that magnetic bouyancy play a major role in providing the extra mixing needed for these stars

In memory of Ernst Kunibert Zinner (1937–2015)

The death of Ernst Kunibert Zinner brings great sadness to the community of cosmochemists and astrophysicists. He pioneered the isotopic and chemical study of grains formed in ejecta from very diverse stars that exploded before the Solar System formed. The identification of the chemical compounds, which he found in grains of circumstellar condensates, and their isotopic compositions has permitted great advances in our understanding of stellar nucleosynthesis. Ernst Zinner was Research Professor at Washington University, where he was supported by research grants from NASA and some from the NSF

Alastair Graham Walter Cameron - Obituary

Alastair Graham Walter Cameron, one of the key discoverers of stellar nucleosynthesis and a founder of modern nuclear astrophysics, died of a heart attack in Tucson, Arizona, on 3 October 2005

Supernova-driven outflows and chemical evolution of dwarf spheroidal galaxies

We present a general phenomenological model for the metallicity distribution (MD) in terms of [Fe/H] for dwarf spheroidal galaxies (dSphs). These galaxies appear to have stopped accreting gas from the intergalactic medium and are fossilized systems with their stars undergoing slow internal evolution. For a wide variety of infall histories of unprocessed baryonic matter to feed star formation, most of the observed MDs can be well described by our model. The key requirement is that the fraction of the gas mass lost by supernova-driven outflows is close to unity. This model also predicts a relationship between the total stellar mass and the mean metallicity for dSphs in accord with properties of their dark matter halos. The model further predicts as a natural consequence that the abundance ratios [E/Fe] for elements such as O, Mg, and Si decrease for stellar populations at the higher end of the [Fe/H] range in a dSph. We show that for infall rates far below the net rate of gas loss to star formation and outflows, the MD in our model is very sharply peaked at one [Fe/H] value, similar to what is observed in most globular clusters. This suggests that globular clusters may be end members of the same family as dSphs.Comment: 8 pages, 3 figures, to be published in the Proceedings of the National Academy of Science

James B. Macelwane Award: Citation and Acceptance of Lawrence Grossman

It is a pleasure to present Lawrence Grossman for the Macelwane Award. This occasion is particularly auspicious since the AGU meeting is held here in Toronto, Canada, where Larry was born and raised. As a young man he worked in a mining company plant in northern Manitoba, fixing railroad ties and then doing copper analyses of drill cores in their assay office. On careful investigation I found that he also had some criminal connections with the nearby offices of the Ontario Provincial Police, which was located a short distance from here along the waterfront. Fortunately, this turned out to be on the legal side and not the seamy side. It appears that Larry had directed his natural investigative skills toward doing mineral and chemical analyses of soils scraped from the shoes of suspected criminals. Larry Grossman has not changed directions—just subjects. His investigative skills are now directed toward rocks and soils scraped from the sole of the early solar system. The solar nebula is a place for which scientific proofs are often circumstantial, there being few witnesses to the crimes of creation

Isotope Anomalies in the Fe-group Elements in Meteorites and Connection to Nucleosynthesis in AGB Stars

We study the effects of neutron captures in AGB stars on \oq Fe-group\cqb elements, with an emphasis on Cr, Fe, and Ni. These elements show anomalies in $^{54}$Cr, $^{58}$Fe, and $^{64}$Ni in solar-system materials, which are commonly attributed to SNe. However, as large fractions of the interstellar medium (ISM) were reprocessed in AGB stars, these elements were reprocessed, too. We calculate the effects of such reprocessing on Cr, Fe, and Ni through 1.5\msb and 3\msb AGB models, adopting solar and 1/3 solar metallicities. All cases produce excesses of $^{54}$Cr, $^{58}$Fe, and $^{64}$Ni, while the other isotopes are little altered; hence, the observations may be explained by AGB processing. The results are robust and not dependent on the detailed initial isotopic composition. Consequences for other \oq Fe group\cqb elements are then explored. They include $^{50}$Ti excesses, and some production of $^{46,47,49}$Ti. In many circumstellar condensates, Ti quantitatively reflects these effects of AGB neutron captures. Scatter in the data results from small variations (granularity) in the isotopic composition of the local ISM. For Si, the main effects are instead due to variations in the local ISM from different SNe sources. The problem of Ca is discussed, particularly with regard to $^{48}$Ca. The measured data are usually represented assuming terrestrial values for $^{42}$Ca/$^{44}$Ca. Materials processed in AGB stars or sources with variable initial $^{42}$Ca/$^{44}$Ca ratios can give apparent $^{48}$Ca excesses/deficiencies, attributed to SNe. The broader issue of Galactic Chemical Evolution is also discussed in view of the isotopic granularity in the ISM. \end{abstract

O'Connell Receives 2000 Inge Lehmann Medal

Richard J. O'Connell was awarded the Inge Lehmann Medal at the AGU Fall Meeting Honors Ceremony, which was held on December 17, 2000, in San Francisco, California. The medal recognizes outstanding contributions to the understanding of the structure, composition, and dynamics of the Earth's mantle and core

Anthony Leonid Turkevich - Obituary

Anthony Leonid Turkevich, a nuclear radiochemist and physicist and a professor of chemistry at the University of Chicago, died in his sleep on 7 September 2002 at his home in Lexington, Virginia. He was widely esteemed for his great intellectual powers, deep physical insight, and personal integrity. Tony was born in New York City on 23 July 1916, the son of a Russian Orthodox clergyman who became a head of the Russian Orthodox Church in both North America and Japan. In 1937, Tony earned a BA from Dartmouth College and, in 1940, a PhD in physical chemistry from Princeton University. His doctoral work was on diffusion determination of molecular structures and dielectric investigations of the motion of organic molecules in the solid state. Shortly after his graduation, he was invited to the University of Chicago as a research physicist in the department of physics; he worked on UV spectroscopy and the radiochemical studies of the fission products

Isotope Tracer Studies of Diffusion in Sillicates and of Geological Transport Processes Using Actinide Elements

The objectives were directed toward understanding the transport of chemical species in nature, with particular emphasis on aqueous transport in solution, in colloids, and on particles. Major improvements in measuring ultra-low concentrations of rare elements were achieved. We focused on two areas of studies: (1) Field, laboratory, and theoretical studies of the transport and deposition of U, Th isotopes and their daughter products in natural systems; and (2) Study of calcium isotope fractionation effects in marine carbonates and in carbonates precipitated in the laboratory, under controlled temperature, pH, and rates of precipitation. A major study of isotopic fractionation of Ca during calcite growth from solution has been completed and published. It was found that the isotopic shifts widely reported in the literature and attributed to biological processes are in fact due to a small equilibrium fractionation factor that is suppressed by supersaturation of the solution. These effects were demonstrated in the laboratory and with consideration of the solution conditions in natural systems, where [Ca{sup 2+}] >> [CO{sub 3}{sup 2-}] + [HCO{sub 3}{sup -}]. The controlling rate is not the diffusion of Ca, as was earlier proposed, but rather the rate of supply of [CO{sub 3}{sup 2-}] ions to the interface. This now opens the issues of isotopic fractionation of many elements to a more physical-chemical approach. The isotopic composition of Ca {Delta}({sup 44}Ca/{sup 40}Ca) in calcite crystals has been determined relative to that in the parent solutions by TIMS using a double spike. Solutions were exposed to an atmosphere of NH{sub 3} and CO{sub 2}, provided by the decomposition of (NH4)2CO3. Alkalinity, pH, and concentrations of CO{sub 3}{sup 2-}, HCO{sub 3}{sup -}, and CO{sub 2} in solution were determined. The procedures permitted us to determine {Delta}({sup 44}Ca/{sup 40}Ca) over a range of pH conditions, with the associated ranges of alkalinity. Two solutions with greatly different Ca concentrations were used, but, in all cases, the condition [Ca] >> [CO{sub 3}{sup 2-}] was met. A wide range in {Delta}({sup 44}Ca/{sup 40}Ca) was found for the calcite crystals, extending from 0.04 {+-} 0.13 to -1.34 {+-} 0.15 {per_thousand}, generally anticorrelating with the amount of Ca removed from the solution. The results show that {Delta}({sup 44}Ca/{sup 40}Ca) is a linear function of the saturation state of the solution with respect to calcite ({Omega}). The two parameters are very well correlated over a wide range in {Omega} for each solution with a given [Ca]. Solutions, which were vigorously stirred, showed a much smaller range in {Delta}({sup 44}Ca/{sup 40}Ca) and gave values of -0.42 {+-} 0.14 {per_thousand}, with the largest effect at low {Omega}. It is concluded that the diffusive flow of CO{sub 3}{sup 2-} into the immediate neighborhood of the crystal-solution interface is the rate-controlling mechanism and that diffusive transport of Ca{sup 2+} is not a significant factor. The data are simply explained by the assumptions that: (a) the immediate interface of the crystal and the solution is at equilibrium with {Delta}({sup 44}Ca/{sup 40}Ca) {approx} -1.5 {+-} 0.25 {per_thousand}, and (b) diffusive inflow of CO{sub 3}{sup 2-} causes supersaturation, thus precipitating Ca from the regions, exterior to the narrow zone of equilibrium. We consider this model to be a plausible explanation of the available data reported in the literature. The well-resolved but small and regular isotope fractionation shifts in Ca are thus not related to the diffusion of very large hydrated Ca complexes, but rather due to the ready availability of Ca in the general neighborhood of the crystal solution interface. The largest isotopic shift which occurs is a small equilibrium effect which is then subdued by supersaturation precipitation for solutions where [Ca{sup 2+}] >> [CO{sub 3}{sup 2-}] + [HCO{sub 3}{sup -}]. It is shown that there is a clear temperature dependence of the net isotopic shifts, which is simply due to changes in {Omega}, due to the equilibrium 'constants' dependence on temperature, which changes the degree of saturation and hence the amount of isotopically unequilibrated Ca precipitated. The effects that are found in natural samples, therefore, will be dependent on the degree of diffusive inflow of carbonate species at or around the crystal-liquid interface in the particular precipitating system, thus limiting the equilibrium effect. The second study treats the problem of Sr isotopic changes in aquifer waters in carbonate terrains. It was found that while dolomite dissolution, calcite precipitation, and clay exchange may govern the bulk chemistry of such waters, the major source of Sr is from the dissolution of the minor phase anhydrite

Response on Receiving the Kemp Medal

Recently Columbia University awarded Kemp medals for 'distinguished public service by a geologist' to Gerald J. Wasserburg, professor of geology and geophysics at the California Institute of Technology, and the late Paul Werner Cast, who was professor of geology at Columbia and chief of the Division of Planetary and Earth Sciences in NASA's Johnson Space Center in Houston. Professor Wasserburg's response at the presentation constitutes an appropriate tribute to the memory of Paul Cast, who at the time of his death was president of AGU's Section of Volcanology, Geochemistry, and Petrology. (Ed.