G-band Spectral Synthesis in Solar Magnetic Concentrations

Narrow band imaging in the G-band is commonly used to trace the small magnetic field concentrations of the Sun, although the mechanism that makes them bright has remained unclear. We carry out LTE syntheses of the G-band in an assorted set of semi-empirical model magnetic concentrations. The syntheses include all CH lines as well as the main atomic lines within the band-pass. The model atmospheres produce bright G-band spectra having many properties in common with the observed G-band bright points. In particular, the contrast referred to the quiet Sun is about twice the contrast in continuum wavelengths. The agreement with observations does not depend on the specificities of the model atmosphere, rather it holds from single fluxtubes to MIcro-Structured Magnetic Atmospheres. However, the agreement requires that the real G-band bright points are not spatially resolved, even in the best observations. Since the predicted G-band intensities exceed by far the observed values, we foresee a notable increase of contrast of the G-band images upon improvement of the angular resolution. According to the LTE modeling, the G-band spectrum emerges from the deep photosphere that produces the continuum. Our syntheses also predict solar magnetic concentrations showing up in continuum images but not in the G-band . Finally, we have examined the importance of the CH photo-dissociation in setting the amount of G-band absorption. It turns out to play a minor role.Comment: To appear in ApJ, 554 n2 Jun 20, 33 pages and 9 figure

Bayesian Inversion of Stokes Profiles

[abridged] Inversion techniques are the most powerful methods to obtain information about the thermodynamical and magnetic properties of solar and stellar atmospheres. In the last years, we have witnessed the development of highly sophisticated inversion codes that are now widely applied to spectro-polarimetric observations. The majority of these inversion codes are based on the optimization of a complicated non-linear merit function. However, no reliable and statistically well-defined confidence intervals can be obtained for the parameters inferred from the inversions. A correct estimation of the confidence intervals for all the parameters that describe the model is mandatory. Additionally, it is fundamental to apply efficient techniques to assess the ability of models to reproduce the observations and to what extent the models have to be refined or can be simplified. Bayesian techniques are applied to analyze the performance of the model to fit a given observed Stokes vector. The posterior distribution, is efficiently sampled using a Markov Chain Monte Carlo method. For simplicity, we focus on the Milne-Eddington approximate solution of the radiative transfer equation and we only take into account the generation of polarization through the Zeeman effect. However, the method is extremely general and other more complex forward models can be applied. We illustrate the ability of the method with the aid of academic and realistic examples. We show that the information provided by the posterior distribution turns out to be fundamental to understand and determine the amount of information available in the Stokes profiles in these particular cases.Comment: 15 pages, 12 figures, accepted for publication in A&

Usos pedagógicos de moodle en la docencia universitaria desde la perspectiva de los estudiantes

El objetivo del artículo es conocer, analizar y comparar las percepciones de los estudiantes sobre los usos de este sistema de gestión de cursos (Moodle) en la docencia universitaria

Spotting the differences between active and non-active twin galaxies on kpc-scales. A pilot study

We present a pilot study aimed to identify large-scale galaxy properties that could play a role in activating a quiescent nucleus. To do so, we compare the properties of two isolated nearby active galaxies and their non-active twins selected from the Calar Alto Legacy Integral Field Area (CALIFA) survey. This pilot sample includes two barred and two unbarred galaxies. We characterise the stellar and ionised gas kinematics and also their stellar content. We obtain simple kinematic models by fitting the full stellar and ionised gas velocity fields and just the approaching/receding sides. We find that the analysed active galaxies present lopsided disks and higher values of the global stellar angular momentum ($\lambda_{R}$) than their non-active twins. This could be indicating that the stellar disks of the AGN gained angular momentum from the inflowing gas that triggered the nuclear activity. The inflow of gas could have been produced by a twisted disk instability in the case of the unbarred AGN, and by the bar in the case of the barred AGN. In addition, we find that the central regions of the studied active galaxies show older stellar populations than their non-active twins. The next step is to statistically explore these galaxy properties in a larger sample of twin galaxies.Comment: 24 pages, 24 figures. Accepted by MNRA

Aplicação dos Isótopos Radioactivos 210Pb e 137Cs na Avaliação de Taxas de Sedimentação em Planícies aluviais: o caso da Lezíria do Tejo

Este artigo visa divulgar os primeiros resultados obtidos na definição das taxas de sedimentação, no último século, numa área-amostra da planície aluvial do Tejo, com o recurso a métodos de datação radiométricos adequados a esta escala temporal (210Pb e 137Cs). O estudo baseou-se na sondagem SEV, efectuada numa área deprimida da planície aluvial, a 2,5km da margem direita do rio, a norte de Santarém, no âmbito do projecto GEOTARIF (Geology and Archaeology of the Tagus River Floodplain over Time, POCTI/CTA/39427/2001). Foram seleccionados apenas os 1,09m superficiais da sondagem, os quais foram seccionados em amostras de 1cm de espessura. A metodologia seguida para o tratamento das 109 amostras e para a análise do 210Pb seguiu o método de Sanchez-Cabeza et al (1998) e a cronologia da sedimentação foi obtida através do modelo CRS (Appleby & Oldfield, 1978; Sanchez-Cabeza et al, 2000). A taxa de sedimentação média para o século XX, obtida a partir do 210Pb (em excesso), foi de 8,8mm/ano, mas os dois aspectos essenciais são: (i) a grande variação na deposição de sedimentos ao longo do último século; (ii) a clara tendência do aumento das taxas de sedimentação, nesta área da planície aluvial, em especial a partir dos anos 60. Os dados obtidos para a deposição do 137Cs na planície aluvial do Tejo, entre 1954 e 1988, mostram uma maior contaminação dos sedimentos aluviais por este radionuclídeo, a partir da entrada em funcionamento da Central nuclear de Almaraz.info:eu-repo/semantics/publishedVersio

Synthesis of Fe-Au nanoparticles through phase separation using the gas aggregation technique

During the last veers different type of magnetic materials have been obtained either alloys or nanopor-ritlcs with severall metalllics shells. These particle exhibt better magnetic properties, are biocompatible and have optical properties due to their shell noble metal layer, this is possible, to synthesize heterostructured nanoparticles with care/shell structure by using sputtering targets consisting of alloys of different materials. In the case of such materials have different surface energies and atomic sizes, there are diffusion processes which lead to the formation of structured nanoparticles with a shell and core having different composition. In this work we will show the results obtained about Fe-Au nanoparticles grown by I he gas aggregation technique, using magnetron sputering sources. Colloids prepared from sputtered deposits of heterostructed nanoparticles exhibit less aggregation when compared to suspensions obtained from pure magnetic materials. Spectrophotometry measurement show the presence of gold at the surface of the nanoparticles. Magnetic properties of such particle are analyzed by VSM. Compasition end structural analysis are studied by TEM and ÉDA