705 research outputs found
Photoerosion and the abundances of the light elements
The abundances of the rare light elements H-2, He-3, Li-7, and B-11 are shown to be potentially affected by photoerosion. That process, involving the interaction of high energy photons from galactic centers with atomic nuclei, will increase the abundances of H-2, He-3, and B-11 while lowering slightly those of Li-7 and He-4. In some regions of galaxies the effects may be large enough to impact their chemical evolution. In particular this process may have enhanced the H-2 and He-3 abundances near the center of our galaxy over and above those from the big bang, as well as the galactic B-11 abundance over that from cosmic-ray spallation
Natural Kinds and Ceteris Paratis Generalizations: In Praise of Hunches
According to stereotypical logical empiricist conceptions, scientific findings are approximately true (or perhaps true ceteris paribus) law-like generalizations used to predict natural phenomena. They are deployed using topic-neutral, generally reliable inferential principles like deductive or statistical inferences. Natural kinds are the kinds in such generalizations. Chemical examples show that such conceptions are seriously incomplete. Some important chemical generalizations are true often enough, even though not usually true, and they are applied using esoteric topic- and discipline-specific inference rules. Their important methodological role is to underwrite often-enough reliable, often socially implemented, scientifically informed guessing about chemical phenomena. Some chemical natural kinds earn their naturalness mainly from participating in such generalizations. These results generalize: many scientific generalizations, inference rules, and natural kinds function to inform guessing, that is, to underwrite the generation of hunches
Natural Kinds and Ceteris Paratis Generalizations: In Praise of Hunches
According to stereotypical logical empiricist conceptions, scientific findings are approximately true (or perhaps true ceteris paribus) law-like generalizations used to predict natural phenomena. They are deployed using topic-neutral, generally reliable inferential principles like deductive or statistical inferences. Natural kinds are the kinds in such generalizations. Chemical examples show that such conceptions are seriously incomplete. Some important chemical generalizations are true often enough, even though not usually true, and they are applied using esoteric topic- and discipline-specific inference rules. Their important methodological role is to underwrite often-enough reliable, often socially implemented, scientifically informed guessing about chemical phenomena. Some chemical natural kinds earn their naturalness mainly from participating in such generalizations. These results generalize: many scientific generalizations, inference rules, and natural kinds function to inform guessing, that is, to underwrite the generation of hunches
A Simultaneous Solution to the ^6Li and ^7Li Big Bang Nucleosynthesis Problems from a Long-Lived Negatively-Charged Leptonic Particle
The Li abundance observed in metal poor halo stars exhibits a plateau
similar to that for Li suggesting a primordial origin. However, the
observed abundance of Li is a factor of larger and that of Li is
a factor of 3 lower than the abundances predicted in the standard big bang when
the baryon-to-photon ratio is fixed by WMAP. Here we show that both of these
abundance anomalies can be explained by the existence of a long-lived massive,
negatively-charged leptonic particle during nucleosynthesis. Such particles
would capture onto the synthesized nuclei thereby reducing the reaction Coulomb
barriers and opening new transfer reaction possibilities, and catalyzing a
second round of big bang nucleosynthesis. This novel solution to both of the Li
problems can be achieved with or without the additional effects of stellar
destruction.Comment: 6 pages, 2 figures, to be published in Physical Review
New Constraints on Dispersive Form Factor Parameterizations from the Timelike Region
We generalize a recent model-independent form factor parameterization derived
from rigorous dispersion relations to include constraints from data in the
timelike region. These constraints dictate the convergence properties of the
parameterization and appear as sum rules on the parameters. We further develop
a new parameterization that takes into account finiteness and asymptotic
conditions on the form factor, and use it to fit to the elastic \pi
electromagnetic form factor. We find that the existing world sample of timelike
data gives only loose bounds on the form factor in the spacelike region, but
explain how the acquisition of additional timelike data or fits to other form
factors are expected to give much better results. The same parameterization is
seen to fit spacelike data extremely well.Comment: 24 pages, latex (revtex), 3 eps figure
The X^- Solution to the ^6Li and ^7Li Big Bang Nucleosynthesis Problems
The Li abundance observed in metal poor halo stars appears to exhibit a
plateau as a function of metallicity similar to that for Li, suggesting a
big bang origin. However, the inferred primordial abundance of Li is
1000 times larger than that predicted by standard big bang
nucleosynthesis for the baryon-to-photon ratio inferred from the WMAP data.
Also, the inferred Li primordial abundance is 3 times smaller than the big
bang prediction. We here describe in detail a possible simultaneous solution to
both the problems of underproduction of Li and overproduction of Li in
big bang nucleosynthesis. This solution involves a hypothetical massive,
negatively-charged leptonic particle that would bind to the light nuclei
produced in big bang nucleosynthesis, but would decay long before it could be
detected. We consider only the -nuclear reactions and assume that the effect
of decay products is negligible, as would be the case if lifetime were large or
the mass difference between the charged particle and its daughter were small.
An interesting feature of this paradigm is that, because the particle remains
bound to the existing nuclei after the cessation of the usual big bang nuclear
reactions, a second longer epoch of nucleosynthesis can occur among -nuclei.
We confirm that reactions in which the hypothetical particle is transferred can
occur that greatly enhance the production of Li while depleting Li. We
also identify a new reaction that destroys large amounts of Be, and hence
reduces the ultimate Li abundance. Thus, big-bang nucleosynthesis in the
presence of these hypothetical particles, together with or without an event of
stellar processing, can simultaneously solve the two Li abundance problems.Comment: 18 pages, 7 figures, minor changes and references added, ApJ accepte
Synthesis of the elements in stars: forty years of progress
Forty years ago Burbidge, Burbidge, Fowler, and Hoyle combined what we would now call fragmentary evidence from nuclear physics, stellar evolution and the abundances of elements and isotopes in the solar system as well as a few stars into a synthesis of remarkable ingenuity. Their review provided a foundation for forty years of research in all of the aspects of low energy nuclear experiments and theory, stellar modeling over a wide range of mass and composition, and abundance studies of many hundreds of stars, many of which have shown distinct evidence of the processes suggested by B2FH. In this review we summarize progress in each of these fields with emphasis on the most recent developments
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