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Longland R.,Polytechnic University of Catalonia | Longland R.,Institute Destudis Espacials Of Catalonia Ieec
Astronomy and Astrophysics | Year: 2012

Context. Recent reaction rate evaluations include reaction rate uncertainties that have been determined in a statistically meaningful manner. Furthermore, reaction rate probability density distributions have been determined and published in the form of lognormal parameters with the specific goal of pursuing Monte Carlo nucleosynthesis studies. Aims. A variety of methods is available for randomly sampling over reaction rate probability densities. The aim of this work is to investigate these methods and determine the most accurate method for estimating elemental abundance uncertainties. Methods. Experimental Monte Carlo reaction rates are first computed for the 22Ne + α, 20Ne(p, γ)21Na, 25Mg(p, γ)26Al, and 18F(p, α) 15O reactions, which are used to calculate reference nucleosynthesis yields for 16 nuclei affected by nucleosynthesis in massive stars and classical novae. Five different methods of randomly sampling over these reaction rate probability distributions are then developed, tested, and compared with the reference nucleosynthesis yields. Results. Given that the reaction rate probability density distributions can be described accurately with a lognormal distribution, Monte Carlo nucleosynthesis variations arising from the parametrised estimates for the reaction rate variations agree remarkably well with those obtained from the true rate samples. Most significantly, the most simple parametrisation agrees within just a few percent, meaning that Monte Carlo nucleosynthesis studies can be performed reliably using lognormal parametrisations of reaction rate probability density functions. © 2012 ESO.

Nzioki A.M.,University of Cape Town | Carloni S.,Institute Destudis Espacials Of Catalonia Ieec | Goswami R.,University of Cape Town | Dunsby P.K.S.,University of Cape Town
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2010

We develop a new covariant formalism to treat spherically symmetric spacetimes in metric f(R) theories of gravity. Using this formalism we derive the general equations for a static and spherically symmetric metric in a general f(R) gravity. These equations are used to determine the conditions for which the Schwarzschild metric is the only vacuum solution with vanishing Ricci scalar. We also show that our general framework provides a clear way of showing that the Schwarzschild solution is not a unique static spherically symmetric solution, providing some insight into how the current form of Birkhoff's theorem breaks down for these theories. © 2010 The American Physical Society.

Trigo-Rodriguez J.M.,Institute of Space science CSIC | Trigo-Rodriguez J.M.,Institute Destudis Espacials Of Catalonia Ieec | Javier Martin-Torres F.,CSIC - National Institute of Aerospace Technology
Planetary and Space Science | Year: 2012

Earth and Titan are two planetary bodies formed far from each other. Nevertheless the chemical composition of their atmospheres exhibits common indications of being produced by the accretion, plus ulterior in-situ processing of cometary materials. This is remarkable because while the Earth formed in the inner part of the disk, presumably from the accretion of rocky planetesimals depleted in oxygen and exhibiting a chemical similitude with enstatite chondrites, Titan formed within Saturns sub-nebula from oxygen- and volatile-rich bodies, called cometesimals. From a cosmochemical and astrobiological perspective, the study of the H, C, N, and O isotopes on Earth and Titan could be the key to decipher the processes occurred in the early stages of formation of both planetary bodies. The main goal of this paper is to quantify the presumable ways of chemical evolution of both planetary bodies, in particular the abundance of CO and N 2 in their early atmospheres. In order to do that the primeval atmospheres and evolution of Titan and Earth have been analyzed from a thermodynamic point of view. The most relevant chemical reactions involving these species and presumably important at their early stages are discussed. Then, we have interpreted the results of this study in light of the results obtained by the Cassini-Huygens mission on these species and their isotopes. Given that H, C, N, and O were preferentially depleted from inner disk materials that formed our planet, the observed similitude of their isotopic fractionation, and subsequent close evolution of Earths and Titans atmospheres points towards a cometary origin of Earth atmosphere. Consequently, our scenario also supports the key role of late veneers (comets and water-rich carbonaceous asteroids) enriching the volatile content of the Earth at the time of the late heavy bombardment of terrestrial planets. © 2011 Elsevier Ltd. All rights reserved.

Jose J.,Polytechnic University of Catalonia | Jose J.,Institute Destudis Espacials Of Catalonia Ieec | Moreno F.,Polytechnic University of Catalonia | Parikh A.,TU Munich | And 2 more authors.
Astrophysical Journal, Supplement Series | Year: 2010

Type I X-ray bursts (XRBs) are thermonuclear stellar explosions driven by charged-particle reactions. In the regime for combined H/He-ignition, the main nuclear flow is dominated by the rp-process (rapid proton-captures and β+-decays), the 3α-reaction, and the αp-process (a suite of (α, p) and (p, γ) reactions). The main flow is expected to proceed away from the valley of stability, eventually reaching the proton drip line beyond A = 38. Detailed analysis of the relevant reactions along the main path has only been scarcely addressed, mainly in the context of parameterized one-zone models. In this paper, we present a detailed study of the nucleosynthesis and nuclear processes powering type I XRBs. The reported 11 bursts have been computed by means of a spherically symmetric (one-dimensional), Lagrangian, hydrodynamic code, linked to a nuclear reaction network that contains 325 isotopes (from 1H to 107Te), and 1392 nuclear processes. These evolutionary sequences, followed from the onset of accretion up to the explosion and expansion stages, have been performed for two different metallicities to explore the dependence between the extension of the main nuclear flow and the initial metal content. We carefully analyze the dominant reactions and the products of nucleosynthesis, together with the physical parameters that determine the light curve (including recurrence times, ratios between persistent and burst luminosities, or the extent of the envelope expansion). Results are in qualitative agreement with the observed properties of some well-studied bursting sources. Leakage from the predicted SbSnTe cycle cannot be discarded in some of our models. Production of 12C (and implications for the mechanism that powers superbursts), light p-nuclei, and the amount of H left over after the bursting episodes will also be discussed. © 2010. The American Astronomical Society.

Nzioki A.M.,University of Cape Town | Dunsby P.K.S.,University of Cape Town | Goswami R.,University of Cape Town | Carloni S.,Institute Destudis Espacials Of Catalonia Ieec
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2011

We present a framework for the study of lensing in spherically symmetric spacetimes within the context of f(R) gravity. Equations for the propagation of null geodesics, together with an expression for the bending angle, are derived for any f(R) theory and then applied to an exact spherically symmetric solution of Rn gravity. We find that for this case more bending is expected for Rn gravity theories in comparison to general relativity and is dependent on the value of n and the value of the distance of closest approach of the incident null geodesic. © 2011 The American Physical Society.

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