Density structure of the giant HII region NGC 2363     

Enrique Pérez  Rosa González Delgado  José M. Vílchez 《Astrophysics and Space Science》2001,277(1):83-86

We perform a detailed measurement of the electron density along two slit position angles in the bright, low metallicity extragalactic HII region NGC 2363. A comparison of the density structures obtained with the two independent diagnostics given by [AIV]4711/4740 and [SII]6716/6731, show that they present both different absolute values and different radial dependencies, with the [AIV] densities reaching up to 1000 cm^-3. We explore the implications for the computation of the He abundance.  相似文献   

Homogeneous Universes in Extended Inflation I     

Enrique Guzmán 《Astrophysics and Space Science》1997,249(2):179-188

We have found that for the Bianchi types I–II–III–V in the Brans-Dicke theory, the scalar field of the theory φ has the same form in the isotropic case. It is shown that the isotropization of the Universe occurs in a very short time when the Universe is dominated by vacuum energy, proving that an isotropic Robertson-Walker model is a good approximation to use in the extended inflation scenario. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

General vacuum solution for Brans-Dicke-Bianchi type-II     

Enrique Guzmán 《Astrophysics and Space Science》1991,179(2):331-334

In this paper we have obtained the general vacuum solution for Bianchi type-II in the Brans-Dicke theory for total anistropyR ₁R ₂R ₃. It is known that by use of our method, we can find the general solution for Bianchi type-II vacuum case in the general relativity theory, first given by Taub (1951). Some physical properties of this model are also discussed.  相似文献   

Cosmological Perturbation Theory and the Spherical Collapse model — I. Gaussian initial conditions     

Pablo Fosalba  & Enrique Gaztañaga 《Monthly notices of the Royal Astronomical Society》1998,301(2):503-523

We present a simple and intuitive approximation for solving the perturbation theory (PT) of small cosmic fluctuations. We consider only the spherically symmetric or monopole contribution to the PT integrals, which yields the exact result for tree-graphs (i.e. at leading order). We find that the non-linear evolution in Lagrangian space is then given by a simple local transformation over the initial conditions, although it is not local in Euler space. This transformation is found to be described by the spherical collapse (SC) dynamics, as it is the exact solution in the shearless (and therefore local) approximation in Lagrangian space. Taking advantage of this property, it is straightforward to derive the one-point cumulants, ξ_J, for both the unsmoothed and smoothed density fields to arbitrary order in the perturbative regime. To leading-order this reproduces, and provides us with a simple explanation for, the exact results obtained by Bernardeau. We then show that the SC model leads to accurate estimates for the next corrective terms when compared with the results derived in the exact perturbation theory making use of the loop calculations. The agreement is within a few per cent for the hierarchical ratios S _J  = ξ _J /ξ ^{J −1}₂. We compare our analytic results with N -body simulations, which turn out to be in very good agreement up to scales where σ ≈ 1. A similar treatment is presented to estimate higher order corrections in the Zel'dovich approximation. These results represent a powerful and readily usable tool to produce analytical predictions that describe the gravitational clustering of large-scale structure in the weakly non-linear regime.  相似文献   

Cosmological perturbation theory and the spherical collapse model — II. Non-Gaussian initial conditions     

Enrique Gaztañaga  & Pablo Fosalba 《Monthly notices of the Royal Astronomical Society》1998,301(2):524-534

In Paper I of this series, we introduced the spherical collapse (SC) approximation in Lagrangian space as a way of estimating the cumulants ξ _J of density fluctuations in cosmological perturbation theory (PT). Within this approximation, the dynamics is decoupled from the statistics of the initial conditions, so we are able to present here the cumulants for generic non-Gaussian initial conditions, which can be estimated to arbitrary order including the smoothing effects. The SC model turns out to recover the exact leading-order non-linear contributions up to terms involving non-local integrals of the J -point functions. We argue that for the hierarchical ratios S _J , these non-local terms are subdominant and tend to compensate each other. The resulting predictions show a non-trivial time evolution that can be used to discriminate between models of structure formation. We compare these analytic results with non-Gaussian N -body simulations, which turn out to be in very good agreement up to scales where σ ≲ 1.  相似文献   

Comparison of biomass and size spectra derived from optical plankton counter data and net samples: application to the assessment of mesoplankton distribution along the Northwest and North Iberian Shelf   总被引：1，自引：0，他引：1  

Nogueira  Enrique; Gonzalez-Nuevo  Gonzalo; Bode  Antonio; Varela  Manuel; Moran  Xose Anxelu G; Valdes  Luis 《ICES Journal of Marine Science》2004,61(4):508-517

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Crystal and molecular structure and spectroscopic behavior of isotypic synthetic analogs of the oxalate minerals stepanovite and zhemchuzhnikovite     

Oscar E. Piro  Gustavo A. Echeverría  Ana C. González-Baró  Enrique J. Baran 《Physics and Chemistry of Minerals》2016,43(4):287-300

The crystal structure of synthetic stepanovite, Na[Mg(H₂O)₆][Fe(C₂O₄)₃]·3H₂O, and zhemchuzhnikovite, Na[Mg(H₂O)₆][Al_0.55Fe_0.45(C₂O₄)₃]·3H₂O, has been determined by single-crystal X-ray diffraction methods. The compounds are isotypic to each other and to the previously reported Na[Mg(H₂O)₆][M(C₂O₄)₃]·3H₂O (M: Cr, Al). They crystallize in the trigonal P3c1 space group with Z = 6 molecules per unit cell and (hexagonal axes) a = 17.0483(4), c = 12.4218(4) Å for the iron compound, and a = 16.8852(5), c = 12.5368(5) Å for the Al/Fe solid solution. Comparison of our crystallographic results with previous X-ray diffraction and chemical data of type stepanovite and zhemchuzhnikovite minerals provides compelling evidence that these natural materials possess the same crystal and molecular structure as their synthetic counterparts. It is shown that the originally reported unit cell for stepanovite represents a pronounced sub-cell and that the correct unit cell and space group are based on weak superstructure reflections. The infrared and Raman spectra of both synthetic analogs were also recorded and are briefly discussed.  相似文献   

Geomechanical Characterization of Marcellus Shale     

Rafael Villamor Lora  Ehsan Ghazanfari  Enrique Asanza Izquierdo 《Rock Mechanics and Rock Engineering》2016,49(9):3403-3424

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The acidic mine pit lakes of the Iberian Pyrite Belt: An approach to their physical limnology and hydrogeochemistry     

Javier Sánchez España  Enrique López PamoEsther Santofimia Pastor  Marta Diez Ercilla 《Applied Geochemistry》2008

This study examines some relevant limnological and hydrogeochemical characteristics of 22 mine pit lakes of the Iberian Pyrite Belt (IPB). The studied pit lakes include some of the largest and historically most important mines of the IPB (including Corta Atalaya and Cerro Colorado in Riotinto, Filón Norte, Filón Centro and Filón Sur in Tharsis, Aznalcóllar, or San Telmo) and many other of minor size. As a whole, these lakes constitute, at present, a large volume of highly acidic and metal-polluted water. Some of these pit lakes are very recent (e.g., Corta Atalaya, 2.5 a; Los Frailes, 6 a) and present a continuous hydrological and geochemical evolution, although many others were abandoned decades ago and show nearly constant water volume. Depth profiles obtained in several pit lakes (e.g., San Telmo, Confesionarios, Cueva de la Mora, Concepción) indicate that many of them have developed meromixis and show, at present, a permanent thermal and chemical stratification with a well defined chemocline separating an anoxic, Fe(II)-rich monimolimnion, and a well mixed, oxygenated and Fe(III)-rich mixolimnion. In the upper layer, the bacterial oxidation of Fe(II) competes with photoreductive processes which take place in the surface water, thus provoking diel cycles of Fe(II) concentration. The observed water chemistry reflects the oxidation and dissolution of pyrite and other sulphides and gangue aluminosilicates from the country rock. The pit lakes of the IPB cover a wide range of water compositions, from circumneutral and relatively low-metal (e.g., Los Frailes, pH 7.2, 0.07 mg/L Fe, 3.8 mg/L Mn, 30 mg/L Zn), to extremely acidic and metal(loid)-rich (e.g., Corta Atalaya, pH 1.2, 36.7 g/L Fe, 6.7 g/L Zn, 1.3 g/L Cu, 159 mg/L As). Most pit lakes, however, are comprised within the pH range of 2.2–3.6 and appear to be strongly buffered by the hydrolysis and precipitation of Fe(III) in the form of schwertmannite, which forms colloids that can sorb trace elements from the aqueous phase.  相似文献   

A Dynamic Model to Simulate Water Level and Salinity in a Mediterranean Coastal Lagoon     

Biel Obrador  Enrique Moreno-Ostos  Joan Lluís Pretus 《Estuaries and Coasts》2008,31(6):1117-1129

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