Hydrogen Dissociation and Diffusion on Transition Metal(=Ti,Zr,V,Fe,Ru,Co,Rh,Ni,Pd,Cu,Ag)-doped Mg(0001) Surfaces

The kinetics of hydrogen absorption by magnesium bulk is affected by two main activated processes: the dissociation of the H$_2$ molecule and the diffusion of atomic H into the bulk. In order to have fast absorption kinetics both activated processed need to have a low barrier. Here we report a systematic ab-initio density functional theory investigation of H$_2$ dissociation and subsequent atomic H diffusion on TM(=Ti,V,Zr,Fe,Ru,Co,Rh,Ni,Pd,Cu,Ag)-doped Mg(0001) surfaces. The calculations show that doping the surface with TM's on the left of the periodic table eliminates the barrier for the dissociation of the molecule, but the H atoms bind very strongly to the TM, therefore hindering diffusion. Conversely, TM's on the right of the periodic table don't bind H, however, they do not reduce the barrier to dissociate H$_2$ significantly. Our results show that Fe, Ni and Rh, and to some extent Co and Pd, are all exceptions, combining low activation barriers for both processes, with Ni being the best possible choice.Comment: accepted in the International Journal of Hydrogen Energ

I rapporti fra Argentina e Stati Uniti durante gli ultimi anni della Seconda Guerra Mondiale e la prima presidenza peronista (1942-1950)

Durante il periodo compreso fra gli anni 1943 e 1950, e particolarmente negli ultimi anni della Seconda Guerra Mondiale, i rapporti diplomatici tra Argentina e Stati Uniti furono caratterizzati dalla forte pressione statunitense per fare allineare l’Argentina alla posizione delle altre Repubbliche Americane in merito alla politica di sicurezza e solidarietà dell’emisfero. Si presentava però, dall’altra parte, l’ostinato desiderio di neutralità del Governo Argentino, una politica che avrebbe dovuto cambiare nel 1945, quando la dichiarazione di guerra all’Asse da parte dell’Argentina sarebbe diventata condizione indispensabile affinché la nazione sudamericana fosse ammessa alla Conferenza di San Francisco che avrebbe istituito le Nazioni Unite. Nel presente progetto di ricerca s’intende tener conto degli elementi decisivi che causarono la politica neutrale argentina. Uno di questi fu, in particolare durante gli anni della Guerra, la posizione del Paese in qualità di fornitore di carne alle potenze alleate, specialmente Gran Bretagna, il cui governo considerava conveniente che l’Argentina rimanesse non belligerante al fine di non diminuire l’ottima produzione di materia prima esportabile. Un altro elemento considerato certamente importante é il particolare rapporto che il Governo Argentino manteneva con la Germania; infatti ci sono diversi autori che segnalano la tendenza ideologica nazista del Governo Argentino, anche se noi crediamo più opportuno considerare la situazione piuttosto dal punto di vista della convenienza strategico economica, per quanto se l’Argentina avesse dichiarato Guerra all’Asse, avrebbe dovuto impiegare tutta la sua naturale energia produttiva nello sforzo della guerra. La documentazione utilizzata è stata trovata nell’Archivio Storico Diplomatico del Ministero degli Esterni della Repubblica Argentina e nell’Archivio Generale della Nazione, materiale che ha costituito il basamento di questo lavoro. Oltre a ciò, è stata di grande aiuto l’edizione del Dipartimento di Stato degli Stati Uniti Foreign Relations of the United States (F.R.U.S.), che offre in modo organizzato e dettagliato un’interessante quantità di documenti diplomatici sul tema

Transport properties for liquid silicon-oxygen-iron mixtures at Earth's core conditions

We report on the thermal and electrical conductivities of two liquid silicon-oxygen-iron mixtures (Fe$_{0.82}$Si$_{0.10}$O$_{0.08}$ and Fe$_{0.79}$Si$_{0.08}$O$_{0.13}$), representative of the composition of the Earth's outer core at the relevant pressure-temperature conditions, obtained from density functional theory calculations with the Kubo-Greenwood formulation. We find thermal conductivities $k$ =100 (160) W m$^{-1}$ K$^{-1}$, and electrical conductivities $\sigma = 1.1 (1.3) \times 10^6 \Omega^{-1}$ m$^{-1}$ at the top (bottom) of the outer core. These new values are between 2 and 3 times higher than previous estimates, and have profound implications for our understanding of the Earth's thermal history and the functioning of the Earth's magnetic field, including rapid cooling rate for the whole core or high level of radiogenic elements in the core. We also show results for a number of structural and dynamic properties of the mixtures, including the partial radial distribution functions, mean square displacements, viscosities and speeds of sound.Comment: 16 pages, 12 figure

On the binarity of Herbig Ae/Be stars

We present high resolution spectro-astrometry of a sample of 28 Herbig Ae/Be and 3 F-type pre-main sequence stars. The spectro-astrometry is shown from both empirical and simulated data to be capable of detecting binary companions that are fainter by up to 6 magnitudes at separations larger than 0.1 arcsec. The nine targets that were previously known to be a binary are all detected. In addition, we report the discovery of 6 new binaries and present 5 further possible binaries. The resulting binary fraction of 68+/-11 per cent is the largest reported for any observed sample of Herbig Ae/Be stars, presumably because of the exquisite sensitivity of spectro-astrometry for detecting binary systems. The data hint that the binary frequency of the Herbig Be stars is larger than for the Herbig Ae stars. The appendix presents model simulations to assess the capabilities of spectro-astrometry and reinforces the empirical findings. Two objects, HD 87643 and Z CMa, display evidence for asymmetric outflows. Finally, the position angles of the binary systems have been compared with available orientations of the circumprimary disc and these appear to be co-planar. The alignment between the circumprimary discs and the binary systems strongly suggests that the formation of binaries with intermediate mass primaries is due to fragmentation as the alternative, stellar capture, does not naturally predict aligned discs. The aligment extends to the most massive B-type stars in our sample. This leads us to conclude that formation mechanisms that do result in massive stars, but predict random angles beween the binaries and the circumprimary disks, such as stellar collisions, are also ruled out for the same reason.Comment: MNRAS accepted, 18 page

Thermal and electrical conductivity of solid iron and iron-silicon mixtures at Earth's core conditions

We report on the thermal and electrical conductivities of solid iron and iron–silicon mixtures (Fe0.92Si0.08 and Fe0.93Si0.07), representative of the composition of the Earth's solid inner core at the relevant pressure–temperature conditions, obtained from density functional theory calculations with the Kubo–Greenwood formulation. We find thermal conductivities View the MathML source, and electrical conductivities View the MathML source at the top of the inner core (centre of the Earth). These values are respectively about 45–56% and 18–25% higher than the corresponding conductivities in the liquid outer core. The higher conductivities are due to the solid structure and to the lower concentration of light impurities. These values are much higher than those in use for previous inner core studies, k by a factor of four and σ by a factor of three. The high thermal conductivity means that heat leaks out by conduction almost as quickly as the inner core forms, making thermal convection unlikely. The high electrical conductivity increases the magnetic decay time of the inner core by a factor of more than three, lengthening the magnetic diffusion time to 10 kyr and making it more likely that the inner core stabilises the geodynamo and reduces the frequency of reversals

Spitzer measurements of atomic and molecular abundances in the Type IIP SN 2005af

We present results based on Spitzer Space Telescope mid-infrared (3.6-30 micron) observations of the nearby IIP supernova 2005af. We report the first ever detection of the SiO molecule in a Type IIP supernova. Together with the detection of the CO fundamental, this is an exciting finding as it may signal the onset of dust condensation in the ejecta. From a wealth of fine-structure lines we provide abundance estimates for stable Ni, Ar, and Ne which, via spectral synthesis, may be used to constrain nucleosynthesis models.Comment: ApJ Letters (accepted

Atomic Undercoordination in Ag Islands on Ru(0001) Grown via Size-Selected Cluster Deposition: An Experimental and Theoretical High-Resolution Core-Level Photoemission Study

The possibility of depositing precisely mass-selected Ag clusters (Ag-1, Ag-3, and Ag-7) on Ru(0001) was instrumental in determining the importance of the in-plane coordination number (CN) and allowed us to establish a linear dependence of the Ag 3d(5/2) core-level shift on CN. The fast cluster surface diffusion at room temperature, caused by the low interaction between silver and ruthenium, leads to the formation of islands with a low degree of ordering, as evidenced by the high density of low-coordinated atomic configurations, in particular CN = 4 and 5. On the contrary, islands formed upon Ag-7 deposition show a higher density of atoms with CN = 6, thus indicating the formation of islands with a close-packed atomic arrangement. This combined experimental and theoretical approach, when applied to clusters of different elements, offers the perspective to reveal nonequivalent local configurations in two-dimensional (2D) materials grown using different building blocks, with potential implications in understanding electronic and reactivity properties at the atomic level

Thermal and electrical conductivity of iron at Earth's core conditions

The Earth acts as a gigantic heat engine driven by decay of radiogenic isotopes and slow cooling, which gives rise to plate tectonics, volcanoes, and mountain building. Another key product is the geomagnetic field, generated in the liquid iron core by a dynamo running on heat released by cooling and freezing to grow the solid inner core, and on chemical convection due to light elements expelled from the liquid on freezing. The power supplied to the geodynamo, measured by the heat-flux across the core-mantle boundary (CMB), places constraints on Earth's evolution. Estimates of CMB heat-flux depend on properties of iron mixtures under the extreme pressure and temperature conditions in the core, most critically on the thermal and electrical conductivities. These quantities remain poorly known because of inherent difficulties in experimentation and theory. Here we use density functional theory to compute these conductivities in liquid iron mixtures at core conditions from first principles- the first directly computed values that do not rely on estimates based on extrapolations. The mixtures of Fe, O, S, and Si are taken from earlier work and fit the seismologically-determined core density and inner-core boundary density jump. We find both conductivities to be 2-3 times higher than estimates in current use. The changes are so large that core thermal histories and power requirements must be reassessed. New estimates of adiabatic heat-flux give 15-16 TW at the CMB, higher than present estimates of CMB heat-flux based on mantle convection; the top of the core must be thermally stratified and any convection in the upper core driven by chemical convection against the adverse thermal buoyancy or lateral variations in CMB heat flow. Power for the geodynamo is greatly restricted and future models of mantle evolution must incorporate a high CMB heat-flux and explain recent formation of the inner core.Comment: 11 pages including supplementary information, two figures. Scheduled to appear in Nature, April 201

Signatures of delayed detonation, asymmetry, and electron capture in the mid-infrared spectra of supernovae 2003hv and 2005df

We present mid-infrared (5.2-15.2 μm) spectra of the Type Ia supernovae (SNe Ia) 2003hv and 2005df observed with the Spitzer Space Telescope. These are the first observed mid-infrared spectra of thermonuclear supernovae, and show strong emission from fine-structure lines of Ni, Co, S, and Ar. The detection of Ni emission in SN 2005df 135 days after the explosion provides direct observational evidence of high-density nuclear burning forming a significant amount of stable Ni in a SN Ia. The SN 2005df Ar lines also exhibit a two-pronged emission profile, implying that the Ar emission deviates significantly from spherical symmetry. The spectrum of SN 2003hv also shows signs of asymmetry, exhibiting blueshifted [Co III], which matches the blueshift of [Fe II ] lines in nearly coeval near-infrared spectra. Finally, local thermodynamic equilibrium abundance estimates for the yield of radioactive ^(56)Ni give M^(56)Ni ≈ 0.5 M⊙, for SN 2003hv, but only M^(56)Ni ≈ 0.13-0.22 M⊙ for the apparently subluminous SN 2005df, supporting the notion that the luminosity of SNe Ia is primarily a function of the radioactive ^(56)Ni yield. The observed emission-line profiles in the SN 2005df spectrum indicate a chemically stratified ejecta structure, which matches the predictions of delayed detonation (DD) models, but is entirely incompatible with current three-dimensional deflagration models. Furthermore, the degree that this layering persists to the innermost regions of the supernova is difficult to explain even in a DD scenario, where the innermost ejecta are still the product of deflagration burning. Thus, while these results are roughly consistent with a delayed detonation, it is clear that a key piece of physics is still missing from our understanding of the earliest phases of SN Ia explosions

Hipovitaminosis D en Adultos Mayores Habitantes de Buenos Aires

Los adultos mayores son una población de alto riesgo de hipovitaminosis D, especialmente los que habitan grandes urbes. El objetivo de este trabajo fue estudiar los niveles séricos de 25(OH)D en invierno y sus factores determinantes en ancianos sanos de Buenos Aires (34° S). Se estudiaron 169 sujetos (56 hombres), de edad promedio 71,5±5,4 años, a fines del invierno. La exposición solar, el hábitat, el tipo de vestimenta, la ingesta de vitamina D y el estatus socio-económico se evaluaron mediante cuestionarios. Se midió calcemia, parthormona, marcadores de recambio óseo y 25(OH)D séricos. Los niveles promedio de 25(OH)D circulante fueron 17,9±0,64 ng/ml. El 51% de los sujetos tenía salidas frecuentes al aire libre, y el 63% consumía alimentos ricos en vitamina D. Los niveles de 25(OH)D fueron más elevados en los hombres y en sujetos con más de 3,5 h/semana de exposición a la luz solar, en los que consumían alimentos ricos en vitamina D tres o más veces por semana, vivían en casas y vestían ropa liviana en verano. El sexo, la exposición a la luz solar y la ingesta de alimentos ricos en vitamina D predicen los niveles séricos de 25(OH)D. La clase social baja se exponía menos horas semanales al sol, consumía escasos alimentos ricos en vitamina D y presentaba niveles elevados de parathormona y marcadores del recambio óseo. Los niveles séricos de 25(OH)D fueron similares en las tres clases socioeconómicas, pero la clase alta presentaba los niveles más elevados.The urban population is prone to hypovitaminosis D due to the type of habitat and indoors lifestyle. Elderly people are susceptible to hypovitaminosis D. Furthermore, low-income elderly subjects have been identified as a target population. The aim of this study was to investigate serum levels of 25(OH)D in winter, and determinant factors in an aged population living at home, in a large overpopulated city. We studied healthy people aged 71.5±5.4 years (113 women and 56 men) living in the city of Buenos Aires (34° S) at the end of winter. Exposure to sunlight, habitat, type of clothing worn in summer, vitamin D-rich food (D-food) intake and socioeconomic status were assessed by specific questionnaires.We measured serum calcium, parathormone, markers of bone turnover and 25OHD. Mean circulating 25(OH)D levels were 17.9±0.64 ng/ml. Only 51% of subjects received sunlight exposure; 63% consumed vitamin D-rich foods. 25(OH)D levels were higher in men and in subjects with more than 3.5 hr/week of sun exposure,who ate vitamin D-rich foods three times/week or more, lived in a house, and those who wore light clothes during the summer. A multivariate linear regression model showed that sex, sunlight exposure and vitamin Drich food intake are predictors of 25(OH)D serum levels. Subjects with low income received less time of sunlight exposure per week, had poor intake of D- food, and an increased level of parathormone and bone turnover rate. Serum levels of 25(OH)D were similar in the three socioeconomic classes, but high-income subjects show a trend toward better vitamin D status.Fil: Plantalech, Luisa. Hospital Italiano; ArgentinaFil: Oliveri, María Beatriz. Universidad de Buenos Aires. Facultad de Medicina. Hospital de Clínicas General San Martín. División Osteopatías; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Inmunología, Genética y Metabolismo. Universidad de Buenos Aires. Facultad de Medicina. Instituto de Inmunología, Genética y Metabolismo; ArgentinaFil: Salerni, Helena. Gobierno de la Ciudad Autónoma de Buenos Aires. Hospital General de Agudos Carlos Durand; ArgentinaFil: Pozzo, Maria Josefina. Hospital Aleman; ArgentinaFil: Ercolano, Monica. Gobierno de la Ciudad de Buenos Aires. Hospital General de Agudos "Ramos Mejía"; ArgentinaFil: Ladizesky, Marta Graciela. Universidad de Buenos Aires. Facultad de Medicina. Hospital de Clínicas General San Martín. División Osteopatías; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Casco, Cristina. Universidad de Buenos Aires. Facultad de Medicina. Hospital de Clínicas General San Martín. División Osteopatías; ArgentinaFil: Zeni, Susana Noemi. Universidad de Buenos Aires. Facultad de Medicina. Hospital de Clínicas General San Martín. División Osteopatías; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Inmunología, Genética y Metabolismo. Universidad de Buenos Aires. Facultad de Medicina. Instituto de Inmunología, Genética y Metabolismo; ArgentinaFil: Somoza, Julia Isabel. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Inmunología, Genética y Metabolismo. Universidad de Buenos Aires. Facultad de Medicina. Instituto de Inmunología, Genética y Metabolismo; Argentina. Universidad de Buenos Aires. Facultad de Medicina. Hospital de Clínicas General San Martín. División Osteopatías; ArgentinaFil: Fassi, Juliana. Hospital Italiano; ArgentinaFil: Bagur, Alicia Cristina. Universidad de Buenos Aires. Facultad de Medicina. Hospital de Clínicas General San Martín. División Osteopatías; Argentin