Exponentially growing solutions in homogeneous Rayleigh-Benard convection

It is shown that homogeneous Rayleigh-Benard flow, i.e., Rayleigh-Benard turbulence with periodic boundary conditions in all directions and a volume forcing of the temperature field by a mean gradient, has a family of exact, exponentially growing, separable solutions of the full non-linear system of equations. These solutions are clearly manifest in numerical simulations above a computable critical value of the Rayleigh number. In our numerical simulations they are subject to secondary numerical noise and resolution dependent instabilities that limit their growth to produce statistically steady turbulent transport.Comment: 4 pages, 3 figures, to be published in Phys. Rev. E - rapid communication

Colombian vanilla and its microbiota. III. Diversity and structure of the endophytic community

Endophytic fungal communities are well-recognised entities within plants worldwide. They hold species with potential in medicinal affairs, biological control of pests, industry, and more. Nevertheless, ecological data about structure and dynamics of endophytic communities are scarce. In this study we sampled root, stem and leaf tissues of Colombian vanilla species in order of both, identifying endosymbionts and characterising the community they belong to. An interesting array of endophytic species was found, including taxa that function as pathogens, saprotrophs, and dermatophytes in other plants/scenarios. Ecological parameters show a moderate diversity with a lognormal arrangement of species quite similar to communities of macroorganisms. Many species here reported belong to taxa broadly reported as sources of biologically active compounds, so they are good candidates for bioprospecting research

"El café tiene cafeína y nos despierta, nos da energía" : concepciones sobre la energía química, una buena razón para poner de acuerdo a los profesores de Física y Química y ciencias Naturales

A research was carried on students' conceptions about chemical energy. The experimental sample were students from primary level to university. Word association tasks and a paper and pencil question were used as instruments for collecting data. The test revealed that some students share certain alternative conceptions about chemical energy, and that most of them use ideas about chemical energy in a similar way to what people without formal instruction in this issue would do. Some implications for science teaching-learning are suggested

Systematic search for gamma-ray periodicity in active galactic nuclei detected by the Fermi Large Area Telescope

We use nine years of gamma-ray data provided by the Fermi Large Area Telescope (LAT) to systematically study the light curves of more than two thousand active galactic nuclei (AGN) included in recent Fermi-LAT catalogs. Ten different techniques are used, which are organized in an automatic periodicity-search pipeline, in order to search for evidence of periodic emission in gamma rays. Understanding the processes behind this puzzling phenomenon will provide a better view about the astrophysical nature of these extragalactic sources. However, the observation of temporal patterns in gamma-ray light curves of AGN is still challenging. Despite the fact that there have been efforts on characterizing the temporal emission of some individual sources, a systematic search for periodicities by means of a full likelihood analysis applied to large samples of sources was missing. Our analysis finds 11 AGN, of which 9 are identified for the first time, showing periodicity at more than 4sigma in at least four algorithms. These findings will help in solving questions related to the astrophysical origin of this periodic behavior.Comment: 16 pages, 5 figures, 4 tables. Accepted by Ap

Estudio de casos: conocimientos físicos de los estudiantes cuando terminan la escuela secundaria: una advertencia y algunas alternativas

Los profesores de física universitaria consideran usualmente que el conocimiento inicial de algunos temas básicos de matemáticas y de física es un prerrequisito para lograr un aprendizaje exitoso. Este trabajo describe el conocimiento conceptual que sobre algunos temas centrales de física básica poseen los alumnos que ingresan en la carrera de Biología de la Universidad de Alcalá, España. Para tal efecto se ha construido un test de 13 preguntas de respuestas de opción múltiple, 8 de física y 5 de matemáticas, sobre temas del currículo normal de la escuela secundaria, y relevantes para la enseñanza de la física universitaria. Se constata que esta muestra mantiene fuertemente arraigados, aún después de la instrucción secundaria, los modelos alternativos, no científicos, mas comunes. Se realiza una comparación con el aprendizaje obtenido en cursos de escuelas secundarias donde se utilizaron estrategias de enseñanza que favorecen el aprendizaje activo de la física. Se observa una diferencia sustancial en el aprendizaje, a favor de las metodologías de aprendizaje activo, las cuales muestran ser muy apropiadas para una reforma educativa exitosa. Se argumenta finalmente que este estudio puede considerarse complementario, en el campo de la física y con las restricciones de su limitada extensión, de los resultados recientemente publicados de la encuesta PISA 2006.Initial knowledge of mathematics and physics is usually regarded by university physics instructors as a prerequisite for successful estudiante learning. This article describes incoming university estudiantes’ understanding of some basic laws of physics at the Universidad de Alcalá, Spain. A 13-item single response multiple choice test has been constructed and used to measure the conceptual knowledge of physics topics normally included in high school physics. It also contains 5 items on mathematics subjects thought to be of relevance for physics instruction. The results show that these student’s sample still hold, after high school instruction, major misconceptions of the tested topics. A comparison is made between this measured understanding and the conceptual knowledge adquired by high school estudiantes enrolled in classes that followed active learning methods. The significant differences in learning gains between traditional and active learning instruction strongly indicate that traditional approaches to teaching physics in high schools should be replaced by field-tested, active learning methodologies. It is argue that this study can be considered, in the field of physics, complementary and reaffirmatory of the recently released results of the PISA 2006 survey.Fil: Benegas, Julio Ciro. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Luis. Instituto de Matemática Aplicada de San Luis ; ArgentinaFil: Perez de Landazábal, M.. Consejo Superior de Investigaciones Cientificas; España. Universidad de Alcalá; EspañaFil: Otero, J.. Universidad de Alcalá; Españ

Theoretical and experimental SERS study of thiocarbonyl compounds adsorbed on metal nanoparticles

Thiocarbonyl compounds have been reported to exhibit interesting biological and pharmacological properties but they are many often characterized by their toxicological effects. However the chemistry of thiobenzoic acid (TBA) and thiobenzamide (TB) has not been fully studied yet. Some of the biological studies of TBA are related to the tautomerism of thiocarboxylic acids and the important role that the -C(=O)-S and -C(=S)-O functional groups play in the catalytic activities of enzymes such as cysteine or serine proteases.1 From a chemical point of view, thiocarboxylates are an interesting type of molecules having two different donor atoms, a soft sulfur donor atom and a hard oxygen donor one. The presence of these unlike groups can lead to the bonding with metal surfaces. Likewise the interaction of thiobenzamide and their derivatives with metals is of great interest because both the sulfur and nitrogen atoms are also able to coordinate with the surface. Therefore the high affinity of these molecules for metal surfaces makes them suitable SERS target adsorbates. Taking advantage of the fact that SERS spectroscopy is both surface selective and highly sensitive we have attempted to determine the molecular structure of TBA and TB once they are adsorbed on the metal. The main objective of this work is focussed on discussing the observed vibrational wavenumber shifts of TBA and TB upon adsorption on silver nanoparticles. In this work the SERS substrates have been prepared by using different colloidal silver solutions according to the method described by Creighton et al.2 and Leopold and Lendl.3 The analysis of the vibrational wavenumbers shifts of the Raman and SERS spectra allow us to know the adsorption process (Figure 1). In the case of TBA, the wavenumber of the SERS band assigned to (C=O) vibrational mode shows an important blue shift up to 40 cm-1 with respect to the Raman whereas the (C-S) band undergoes a red shift up to 40 cm-1. These results suggest a unidentate coordination of TBA to the silver surface through the sulfur atom. On the other hand, the SERS band assigned in the case of TB to Amide III (mainly (CN)) exhibits a significant blueshift up to 41 cm-1, and the SERS band assigned to Amide I (mainly (CS)) shows a red shift up to 11 cm-1. These wavenumber shifts indicate that TB interacts to the silver surface through the sulfur atom. Interestingly, in previous SERS studies of pyridinecarboxamides and benzamide we have observed that some SERS bands assigned to 1;ring, Amide I (mainly (C=O)) and Amide III (mainly C-N)) show wavenumber shifts of +50, -50 and +10 cm-1, respectively, which were attributed to the deprotonation of carboxamide group.4,5 Finally, in order to verify experimental results DFT calculations have been carried out for different silver complexes of TBA and TB concluding that the unidentate coordination is the most likely interaction of both of them.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech