Atomic Diffusion and Mixing in Old Stars V: A deeper look into the Globular Cluster NGC 6752

Abundance trends in heavier elements with evolutionary phase have been shown to exist in the globular cluster NGC 6752 [Fe/H]=-1.6. These trends are a result of atomic diffusion and additional (non-convective) mixing. Studying such trends can provide us with important constraints on the extent to which diffusion modifies the internal structure and surface abundances of solar-type, metal-poor stars. Taking advantage of a larger data sample, we investigate the reality and the size of these abundance trends and address questions and potential biases associated with the various stellar populations that make up NGC6752. Based on uvby Str\"omgren photometry, we are able to separate three stellar populations in NGC 6752 along the evolutionary sequence from the base of the red giant branch down to the turnoff point. We find weak systematic abundance trends with evolutionary phase for Ca, Ti, and Fe which are best explained by stellar-structure models including atomic diffusion with efficient additional mixing. We derive a new value for the initial lithium abundance of NGC 6752 after correcting for the effect of atomic diffusion and additional mixing which falls slightly below the predicted standard BBN value. We find three stellar populations by combining photometric and spectroscopic data of 194 stars in the globular cluster NGC 6752. Abundance trends for groups of elements, differently affected by atomic diffusion and additional mixing, are identified. Although the statistical significance of the individual trends is weak, they all support the notion that atomic diffusion is operational along the evolutionary sequence of NGC 6752.Comment: 15 pages, 11 figures, 2 online table

Atomic Diffusion and Mixing in Old Stars. III. Analysis of NGC 6397 Stars under New Constraints

We have previously reported on chemical abundance trends with evolutionary state in the globular cluster NGC 6397 discovered in analyses of spectra taken with FLAMES at the VLT. Here, we reinvestigate the FLAMES-UVES sample of 18 stars, ranging from just above the turnoff point (TOP) to the red giant branch below the bump. Inspired by new calibrations of the infrared flux method, we adopt a set of hotter temperature scales. Chemical abundances are determined for six elements (Li, Mg, Ca, Ti, Cr, and Fe). Signatures of cluster-internal pollution are identified and corrected for in the analysis of Mg. On the modified temperature scales, evolutionary trends in the abundances of Mg and Fe are found to be significant at the 2{\sigma} and 3{\sigma} levels, respectively. The detailed evolution of abundances for all six elements agrees with theoretical isochrones, calculated with effects of atomic diffusion and a weak to moderately strong efficiency of turbulent mixing. The age of these models is compatible with the external determination from the white dwarf cooling sequence. We find that the abundance analysis cannot be reconciled with the strong turbulent-mixing efficiency inferred elsewhere for halo field stars. A weak mixing efficiency reproduces observations best, indicating a diffusion-corrected primordial lithium abundance of log {\epsilon}(Li) = 2.57 +- 0.10. At 1.2{\sigma}, this value agrees well with WMAP-calibrated Big-Bang nucleosynthesis predictions.Comment: 14 pages, 5 figures, accepted by Ap

Robust Subnanometric Plasmon Ruler by Rescaling of the Nonlocal Optical Response

We present the optical response of two interacting metallic nanowires calculated for separation distances down to angstrom range. State-of-the-art local and nonlocal approaches are compared with full quantum time-dependent density functional theory calculations that give an exact account of nonlocal and tunneling effects. We find that the quantum results are equivalent to those from classical approaches when the nanoparticle separation is defined as the separation between centroids of the screening charges. This establishes a universal plasmon ruler for subnanometric distances. Such a ruler not only impacts the basis of many applications of plasmonics, but also provides a robust rule for subnanometric metrology

Magadan and the evolution of the Dal´stroi bosses in the 1930s.

RésuméMagadan et l’évolution des directeurs du Dal´stroj dans les années 1930.Bien qu’elle doive son importance à sa position centrale au sein du Goulag, Magadan est longtemps restée invisible dans l’histoire de l’URSS. Cependant, cette capitale du monde carcéral peut nous fournir un grand nombre de renseignements sur la nature et les rouages du pouvoir stalinien. Notre article traite des changements survenus dans l’administration provinciale de cette métropole subarctique pendant les années 1930, étude qui s’appuie sur des documents d’archives centrales et régionales. Bien que Magadan ait toujours tablé sur la répression, on observe des variations visibles dans sa politique qui reflètent les époques et les tendances nationales plus larges. La première génération de directeurs de camps, qui avait en partie fait ses classes sur les objectifs utopiques de la révolution bolchevique, apparaît rétrospectivement comme un groupe assez modéré qui prenait au sérieux les objectifs officiels de réhabilitation et de réinsertion sociale des détenus. C’étaient aussi des gestionnaires relativement compétents capables d’atteindre les objectifs industriels fixés par le Kremlin. Tout ceci fut transformé en 1937, année décisive, lorsque la ežovščina s’abattit de plein fouet sur Magadan, altérant définitivement sa culture bureaucratique. La production économique chuta de façon spectaculaire au fur et à mesure que la violence politique augmenta, tout ceci sous la houlette d’une équipe adaptée aux dures réalités du moment. Il y eut un difficile moment de répit en 1939 lorsque Moscou mit en place une autre hiérarchie locale qui devait relancer la production, mais qui fit preuve du même cynisme politique. Soumis aux psittacismes des discours officiels pendant les années qui suivirent, les grands idéaux d’Octobre perdirent leur sens et disparurent dans le cataclysme des grandes purges.AbstractAlthough significant as the focal point of the infamous Gulag, Magadan and its environs have long remained one of the “blank spots” of Soviet history. Yet this prison capital can tell us much about the nature and mechanism of Stalinist rule. Based upon a number of central and regional archives, this article concerns the changes in provincial administration in this subarctic metropolis throughout the Soviet 1930s. While the political calculus of Magadan always involved repression, there were obvious variations in policy that reflected both the times and larger national trends. The initial generation of camp bosses, raised at least in part on the more utopian goals of the Bolshevik Revolution, proved in retrospect to be a somewhat moderate cohort that took seriously the official goals of inmate rehabilitation and social restitution. At the same time, they were relatively competent economic managers capable of fulfilling key industrial targets set by the Kremlin. All this changed in the watershed year of 1937, when the full force of the Ezhovshchina hit Magadan and forever altered its bureaucratic culture. Economic output fell dramatically as political violence rose, all under the auspices of a new camp management team socialized by the harshening realities of the time. An uneasy truce came in 1939 with the establishment by Moscow of yet another local hierarchy, formed to reemphasize production but nevertheless cynical in its political outlook. Parroted in public discourse for years thereafter, the lofty ideals of October had lost all content and fallen victim to the earthquake of the Great Purges

Work Function Dependence of Charge Transfer in Desorption and Sputtering of Atoms from Surfaces

Using a recently developed many-electron theory, we investigate the work function dependence of charge transfer during desorption and sputtering of atoms from metal surfaces. We investigate the effects of substrate bandwidth, atomic velocity and level degeneracy on the charge transfer. We show that many-electron interactions introduce relatively small but measurable effects on the work function dependence of the charge transfer. We find that these effects can be stronger for negative ion states than for positive ion states. The reason is that for negative ions, a strongly correlated Kondo state may be formed near the surface

Nanorice Particles: Hybrid Plasmonic Nanostructures

A new hybrid nanoparticle, i.e., a nanorice particle, which combines the intense local fields of nanorods with the highly tunable plasmon resonances of nanoshells, is described herein. This geometry possesses far greater structural tunability than previous nanoparticle geometries, along with much larger local field enhancements and far greater sensitivity as a surface plasmon resonance (SPR) nanosensor than presently known dielectric-conductive material nanostructures. In an embodiment, a nanoparticle comprises a prolate spheroid-shaped core having a first aspect ratio. The nanoparticle also comprises at least one conductive shell surrounding said prolate spheroid-shaped core. The nanoparticle has a surface plasmon resonance sensitivity of at least 600 nm RIU(sup.-1). Methods of making the disclosed nanorice particles are also described herein

Resonance Lifetimes from Complex Densities

The ab-initio calculation of resonance lifetimes of metastable anions challenges modern quantum-chemical methods. The exact lifetime of the lowest-energy resonance is encoded into a complex "density" that can be obtained via complex-coordinate scaling. We illustrate this with one-electron examples and show how the lifetime can be extracted from the complex density in much the same way as the ground-state energy of bound systems is extracted from its ground-state density

Balancing near-field enhancement, absorption, and scattering for effective antenna-reactor plasmonic photocatalysis

Efficient photocatalysis requires multifunctional materials that absorb photons and generate energetic charge carriers at catalytic active sites to facilitate a desired chemical reaction. Antenna–reactor complexes are an emerging multifunctional photocatalytic structure where the strong, localized near field of the plasmonic metal nanoparticle (e.g., Ag) is coupled to the catalytic properties of the nonplasmonic metal nanoparticle (e.g., Pt) to enable chemical transformations. With an eye toward sustainable solar driven photocatalysis, we investigate how the structure of antenna–reactor complexes governs their photocatalytic activity in the light-limited regime, where all photons need to be effectively utilized. By synthesizing core@shell/satellite (Ag@SiO_2/Pt) antenna–reactor complexes with varying Ag nanoparticle diameters and performing photocatalytic CO oxidation, we observed plasmon-enhanced photocatalysis only for antenna–reactor complexes with antenna components of intermediate sizes (25 and 50 nm). Optimal photocatalytic performance was shown to be determined by a balance between maximized local field enhancements at the catalytically active Pt surface, minimized collective scattering of photons out of the catalyst bed by the complexes, and minimal light absorption in the Ag nanoparticle antenna. These results elucidate the critical aspects of local field enhancement, light scattering, and absorption in plasmonic photocatalyst design, especially under light-limited illumination conditions