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Rio de Janeiro, Brazil

Martinez R.,Pontifical Catholic University of Rio de Janeiro | Martinez R.,Federal University of Amapa | Bordalo V.,Observatorio Nacional MCTI | Da Silveira E.F.,Pontifical Catholic University of Rio de Janeiro | Boechat-Roberty H.M.,Federal University of Rio de Janeiro
Monthly Notices of the Royal Astronomical Society | Year: 2014

In the interstellar medium, high-energy cosmic rays are able to penetrate deeply into molecular clouds, resulting in the ionization and dissociation of condensed molecules on ice-grain surfaces, which in turn leads to the desorption of a number of species. The interpretation of interstellar data requires qualitative and quantitative studies of the molecular species formed in several processes. In this work, the interaction of cosmic rays with CO-NH3 ice is simulated by the bombardment of 65-MeV heavy ions (masses of 100 u) on frozen gases. Positive and negative ions ejected from the ice surface as a consequence of the impact are analysed through time-of-flight mass spectrometry. The dependence of the ion desorption yield on the sample temperature (which varies/increases with time) has been used to discuss the emission process of the desorbed ions, especially near ice sublimation temperatures (30 K for CO at 10-8 mbar); at these temperatures, HCO+, NO+, NH3OH+ and NH3CO+ are formed and emitted. For temperatures in the 40 to 65 K range, mass spectra of positive ions are dominated by the NH4+ ion peak, similarly to what occurs for a pure frozen NH3 target. Concerning the mass spectra of negative ions, the total yield of negative ions at T=25 K is found to be about 20 times lower than that of positive ions. Before CO sublimation, CN-bearing species are formed, among which the cyanide ions CN- and OCN- are by far the most abundant species. These findings support the idea that NH4+ and OCN- ions are efficiently formed by the interaction of cosmic rays with CO-NH3 ices. © 2014 The Authors. Source

Di Bartolo L.,Observatorio Nacional MCTI | Dors C.,Federal University of Rio de Janeiro | Mansur W.J.,Federal University of Rio de Janeiro
Geophysical Prospecting | Year: 2015

The previous finite-difference numerical schemes designed for direct application to second-order elastic wave equations in terms of displacement components are strongly dependent on Poisson's ratio. This fact makes theses schemes useless for modelling in offshore regions or even in onshore regions where there is a high Poisson's ratio material. As is well known, the use of staggered-grid formulations solves this drawback. The most common staggered-grid algorithms apply central-difference operators to the first-order velocity-stress wave equations. They have been one of the most successfully applied numerical algorithms for seismic modelling, although these schemes require more computational memory than those mentioned based on second-order wave equations. The goal of the present paper is to develop a general theory that enables one to formulate equivalent staggered-grid schemes for direct application to hyperbolic second-order wave equations. All the theory necessary to formulate these schemes is presented in detail, including issues regarding source application, providing a general method to construct staggered-grid formulations to a wide range of cases. Afterwards, the equivalent staggered-grid theory is applied to anisotropic elastic wave equations in terms of only velocity components (or similar displacements) for two important cases: general anisotropic media and vertical transverse isotropy media using, respectively, the rotated and the standard staggered-grid configurations. For sake of simplicity, we present the schemes in terms of velocities in the second- and fourth-order spatial approximations, with second-order approximation in time for 2D media. However, the theory developed is general and can be applied to any set of second-order equations (in terms of only displacement, velocity, or even stress components), using any staggered-grid configuration with any spatial approximation order in 2D or 3D cases. Some of these equivalent staggered-grid schemes require less computer memory than the corresponding standard staggered-grid formulation, although the programming is more evolved. As will be shown in theory and practice, with numerical examples, the equivalent staggered-grid schemes produce results equivalent to corresponding standard staggered-grid schemes with computational advantages. Finally, it is important to emphasize that the equivalent staggered-grid theory is general and can be applied to other modelling contexts, e.g., in electrodynamical and poroelastic wave propagation problems in a systematic and simple way. © 2015 European Association of Geoscientists & Engineers. Source

De Souza A.D.,French National Center for Scientific Research | Fernandes M.B.,Observatorio Nacional MCTI | Carciofi A.C.,IAG | Chesneau O.,French National Center for Scientific Research
Proceedings of the International Astronomical Union | Year: 2014

The research of stars with the B[e] phenomenon is still in its infancy, with several unanswered questions. Physically realistic models that treat the formation and evolution of their complex circumstellar environments are rare. The code HDUST (developed by A. C. Carciofi and J. Bjorkman) is one of the few existing codes that provides a self-consistent treatment of the radiative transfer in a gaseous and dusty circumstellar environment seen around B[e] supergiant stars. In this work we used the HDUST code to study the circumstellar medium of the binary system GG Car, where the primary component is probably an evolved B[e] supergiant. This system also presents a disk (probably circumbinary), which is responsible for the molecular and dusty signatures seen in GG Car spectra. We obtained VLTI/MIDI data on GG∼Car at eight baselines, which allowed to spatially resolve the gaseous and dusty circumstellar environment. From the interferometric visibilities and SED modeling with HDUST, we confirm the presence of a compact ring, where the hot dust lies. We also show that large grains can reproduce the lack of structure in the SED and visibilities across the silicate band. We conclude the dust condensation site is much closer to the star than previously thought. This result provides stringent constraints on future theories of grain formation and growth around hot stars. Copyright © International Astronomical Union 2015. Source

Olkin C.B.,Southwest Research Institute | Young L.A.,Southwest Research Institute | Borncamp D.,Southwest Research Institute | Borncamp D.,US Space Telescope Science Institute | And 21 more authors.
Icarus | Year: 2015

Combining stellar occultation observations probing Pluto's atmosphere from 1988 to 2013, and models of energy balance between Pluto's surface and atmosphere, we find the preferred models are consistent with Pluto retaining a collisional atmosphere throughout its 248-year orbit. The occultation results show an increasing atmospheric pressure with time in the current epoch, a trend present only in models with a high thermal inertia and a permanent N2 ice cap at Pluto's north rotational pole. © 2014 The Authors. Source

Molina R.E.,National Experimental University of Tachira | Giridhar S.,Indian Institute of Astrophysics | Pereira C.B.,Observatorio Nacional MCTI | Ferro A.A.,National Autonomous University of Mexico | And 2 more authors.
Revista Mexicana de Astronomia y Astrofisica | Year: 2014

We have done a detailed abundance analysis of four unexplored candidate post- Asymptotic Giant Branch(AGB) stars IRAS 13110 - 6629, IRAS 17579 - 3121, IRAS 18321 - 1401 and IRAS 18489 - 0629 using high resolution spectra. We have constructed Spectral Energy Distributions (SED) for these objects using the existing photometric data combined with infrared (IR) fluxes. For all sample stars, the SEDs exhibit double peaked energy distribution with well separated IR peaks showing the presence of dusty circumstellar material. The CNO abundances indicate the production of N via CN cycling, but observed [C/Fe] indicates the mixing of carbon produced by He burning by third dredge up although C/O ratio remains less that 1. A moderate DG-effect is clearly seen for IRAS 18489 - 0629 and IRAS 17579 - 3121 while a large scatter observed in depletion plots for IRAS 18321 - 1401 and IRAS 13110 - 6629 indicate the presence of other processes affecting the observed abundance pattern. Source

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