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Brown D.,Specola Vaticana
Astrophysics and Space Science

One of the ways by which subdwarf B stars are thought to form is through binary star interactions. The metallicity of the sdB progenitor stars in such binary systems should not seem to be a major factor in the formation of sdB stars. However, given the different environments in which sdB stars are found, binary population synthesis simulations have been conducted in order to examine how metallicity might be a subtle factor in the formation of sdB stars in such environments. This is then applied to clusters of stars and to the UV Upturn phenomenon. © 2010 Springer Science+Business Media B.V. Source

Turrini D.,National institute for astrophysics | Combe J.-P.,Bear Fight Institute | McCord T.B.,Bear Fight Institute | Oklay N.,Max Planck Institute for Solar System Research | And 9 more authors.

The Dawn spacecraft recently observed the presence of dark material, which in turn proved to be associated with the presence of OH and H-rich material, on the surface of Vesta. The source of this dark material has been almost unanimously identified with the low albedo asteroids, likely analogous to the carbonaceous chondrites found on Earth, that impacted on Vesta over its lifetime. However, it is still a matter of debate whether the delivery of the dark material is associated with a few large impact events, to micrometeorites or to the continuous, secular flux of impactors on Vesta. The "continuous flux" scenario, in particular, predicts that a significant fraction of the exogenous material accreted by Vesta should be due to non-dark impactors likely analogous to ordinary chondrites, which instead represent only a minor contaminant in the Howardite-Eucrite-Diogenite meteorites. In this work, we explored the "continuous flux" scenario and its implications for the composition of the vestan regolith, taking advantage of the data from the Dawn mission and the Howardite-Eucrite-Diogenite meteorites to constrain the contamination history of Vesta. We developed a model for the delivery of the exogenous material to Vesta and verified how the results it supplies are sensitive to the different parameters we consider. We calibrated the flux of impactors predicted by our model with the number of dark craters observed inside the Rheasilvia basin and we tested the assumptions on the impact conditions by studying the formation of Cornelia crater and of its dark deposits with a hydrocode simulation. We used our calibrated model to show that the "stochastic events" scenario and the "micrometeoritic flux" scenario are just natural consequences of the "continuous flux" scenario. We then used the model to estimate the amounts of dark and hydroxylate materials that were delivered on Vesta since the Late Heavy Bombardment and we showed how our results match well with the values estimated by the Dawn mission. We finally used our model to assess the amount of Fe and siderophile elements that the continuous flux of impactors would mix in the vestan regolith: concerning the siderophile elements, we focused our attention on the role of Ni. The results we obtained are in agreement with the data available on the Fe and Ni content of the Howardite-Eucrite-Diogenite meteorites and can be used as a reference frame in future studies of the data from the Dawn mission and of the Howardite-Eucrite-Diogenite meteorites. Our model cannot yet provide an answer to the conundrum of the fate of the missing non-carbonaceous contaminants, but we discuss some possible reasons for this discrepancy with the otherwise coherent picture described by our results. © 2014 Elsevier Inc. All rights reserved. Source

Gabriele Gionti S.J.,Specola Vaticana | Gabriele Gionti S.J.,University of Arizona
International Journal of Geometric Methods in Modern Physics

Recent results in Local Regge Calculus are confronted with Spin-Foam Formalism. Introducing BarrettCrane Quantization in Local Regge Calculus makes it possible to associate a unique Spin j h with an hinge h, fulfilling one of the requirements of Spin-Foam definition. It is shown that inter-twiner terms of Spin-Foam can follow from the closure constraint in Local Regge Calculus. © 2012 World Scientific Publishing Company. Source

Capozziello S.,University of Naples Federico II | Capozziello S.,Compl University Of Monte gelo | Capozziello S.,National Institute of Nuclear Physics, Italy | Gabriele Gionti S.J.,Specola Vaticana | And 3 more authors.
Journal of Cosmology and Astroparticle Physics

We select f(R) gravity models that undergo scale factor duality transformations. As a starting point, we consider the tree-level effective gravitational action of bosonic String Theory coupled with the dilaton field. This theory inherits the Busher's duality of its parent String Theory. Using conformal transformations of the metric tensor, it is possible to map the tree-level dilaton-graviton string effective action into f(R) gravity, relating the dilaton field to the Ricci scalar curvature. Furthermore, the duality can be framed under the standard of Noether symmetries and exact cosmological solutions are derived. Using suitable changes of variables, the string-based f(R) Lagrangians are shown in cases where the duality transformation becomes a parity inversion. © 2016 IOP Publishing Ltd and Sissa Medialab srl . Source

Opeil C.P.,Boston College | Consolmagno G.J.,Specola Vaticana | Safarik D.J.,Los Alamos National Laboratory | Britt D.T.,University of Central Florida
Meteoritics and Planetary Science

In our ongoing survey of meteorite physical properties, we have to date measured the thermal conductivity for seventeen stony meteorites at temperatures ranging from 5K to 300K. Here, we report new results for nine ordinary chondrites, one enstatite chondrite, and the basaltic achondrites Frankfort (howardite) and Los Angeles (shergottite). We find that thermal conductivity is significantly lower than would be expected from averaging the laboratory conductivities of their constituent minerals, with a dependence on temperature different from the expected conductivity of pure minerals. In addition, we find a linear relationship between the inverse of the porosity of the samples measured and their thermal conductivity, regardless of meteorite composition or type. We conclude that thermal conductivity is controlled by the presence of shock-induced microcracks within the meteorites, which provide a barrier to the transmission of thermal energy via phonons. In contrast to conductivity, our first measurement of heat capacity as a function of temperature (on Los Angeles) suggests that heat capacity is primarily a function of oxide composition and is not strongly affected by the physical state of the sample. © 2012 The Meteoritical Society. Source

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