Short-duration lensing events: II. Expectations and Protocols

Ongoing microlensing observations by OGLE and MOA regularly identify and conduct high-cadence sampling of lensing events with Einstein diameter crossing time, tau_E, of 16 or fewer days. Events with estimated values of tau_E of one to two days have been detected. Short duration events tend to be generated by low-mass lenses or by lenses with high transverse velocities. We compute the expected rates, demonstrate the expected ranges of parameters for lenses of different mass, and develop a protocol for observing and modeling short-duration events. Relatively minor additions to the procedures presently used will increase the rate of planet discovery, and also discover or place limits on the population of high-speed dim stars and stellar remnants in the vicinity of the Sun.Comment: 17 pages; 3 figures; submitted to ApJ 3 July 200

Spin-Up/Spin-Down models for Type Ia Supernovae

In the single degenerate scenario for Type Ia supernova (SNeIa), a white dwarf (WD) must gain a significant amount of matter from a companion star. Because the accreted mass carries angular momentum, the WD is likely to achieve fast spin periods, which can increase the critical mass, $M_{crit}$, needed for explosion. When $M_{crit}$ is higher than the maximum mass achieved by the WD, the WD must spin down before it can explode. This introduces a delay between the time at which the WD has completed its epoch of mass gain and the time of the explosion. Matter ejected from the binary during mass transfer therefore has a chance to become diffuse, and the explosion occurs in a medium with a density similar to that of typical regions of the interstellar medium. Also, either by the end of the WD's mass increase or else by the time of explosion, the donor may exhaust its stellar envelope and become a WD. This alters, generally diminishing, explosion signatures related to the donor star. Nevertheless, the spin-up/spin-down model is highly predictive. Prior to explosion, progenitors can be super-$M_{Ch}$ WDs in either wide binaries with WD companions, or else in cataclysmic variables. These systems can be discovered and studied through wide-field surveys. Post explosion, the spin-up/spin-down model predicts a population of fast-moving WDs, low-mass stars, and even brown dwarfs. In addition, the spin-up/spin-down model provides a paradigm which may be able to explain both the similarities and the diversity observed among SNeIa.Comment: Submitted to ApJ Letter

The analogue cosmological constant in Bose-Einstein condensates: a lesson for quantum gravity

For almost a century, the cosmological constant has been a mysterious object, in relation to both its origin and its very small value. By using a Bose-Einstein condensate analogue model for gravitational dynamics, we address here the cosmological constant issue from an analogue gravity standpoint. Starting from the fundamental equations describing a system of condensed bosons, we highlight the presence of a vacuum source term for the analogue gravitational field, playing the role of a cosmological constant. In this simple system it is possible to compute from scratch the value of this constant, to compare it with other characteristic energy scales and hence address the problem of its magnitude within this framework, suggesting a different path for the solution of this longstanding puzzle. We find that, even though this constant term is related with quantum vacuum effects, it is not immediately related to the ground state energy of the condensate. On the gravity side this result suggests that the interpretation and computation of the cosmological term as a form of renormalized vacuum energy might be misleading, its origin being related to the mechanism that instead produces spacetime from its pregeometric progenitor, shedding a different light on the subject and at the same time suggesting a potentially relevant role of analogue models in the understanding of quantum gravity.Comment: 24 pages, 1 figure, Proceedings of the II Amazonian Symposium on Physic

The reconstruction of Rh(001) upon oxygen adsorption

We report on a first-principles study of the structure of O/Rh(001) at half a monolayer of oxygen coverage, performed using density-functional theory. We find that oxygen atoms sit at the center of the black squares of a chess-board, $c(2\times 2)$, pattern. This structure is unstable against a rhomboid distortion of the black squares, which shortens the distance between an O atom and two of the four neighboring Rh atoms, while lengthening the distance with respect to the other two. We actually find that the surface energy is further lowered by allowing the O atom to get off the short diagonal of the rhombus so formed. We predict that the latter distortion is associated with an order-disorder transition, occurring below room temperature. The above rhomboid distortion of the square lattice may be seen as a rotation of the empty, white, squares. Our findings are at variance with recent claims based on STM images, according to which it is instead the black squares which would rotate. We argue that these images are indeed compatible with our predicted reconstruction pattern.Comment: 14 pages (inclusive of 5 figures). To appear on Surface Scienc

Economic–financial evaluation and multicriteria analysis of innovative public transport guided systems: an application to “Linea della Musica” in Rome

Trams and innovative public transport systems in historical city centres, are often not considered by public authorities, due to their strong visual impact, (e.g. in proximity of artistic-cultural places), and high project costs. The decision-makers guess that the high initial investment could not be compensated for in subsequent years. For this reason, it is very important to evaluate all the innovative solutions, not only economically, but considering all externalities, positive and negative, in a multi-criteria analysis taking into account public opinions and passengers’ behaviours essential for assessing revenues, and also return of image. On this basis, the results may be impressive: the positive economic impact may be highlighted, and they can have a result better than traditional systems. The subject of the analysis is the innovative system “Tramwave” of Ansaldo STS that combines an investment cost imperceptibly higher than the traditional tram, and a zero visual impact due to the absence of the overhead line due to the uptake of electrical energy from the ground. This system, applied to “Linea della Musica”, a planned tramway line in Rome, through a detailed financial balance of costs and revenues, shows a positive Net Present Value 30–40 years after the initial investment. The current crisis, both culturally and economically, can cause two types of reactions: an indiscriminate cutting of essential public services, or a positive crackdown, that puts ideas in motion, awakens the pride of the common good, and realizes innovations for saving money. The proposed analysis will try to encourage the second reaction

A case study on how language learning strategies are fostered in an EFL chilean classroom context

Tesis (Profesor de Inglés para la Enseñanza Básica y Media y al grado académico de Licenciado en Educación)This mixed-methods case study attempted to explore how Language Learning Strategies (LLS) are fostered within a secondary level Chilean EFL public school context, based on a teacher’s pedagogical actions. Through empirical observation, task analysis, and an interview, this thesis intended to unveil the rationale behind the predominance of certain LLS at expense of others. Previous studies and literature served as a reference to later on determine potential cohesion between background records and first-hand evidence, which proved to be in relative alignment. Accordingly, this paper looked forward to extrapolating thoughts and assumptions the teacher had about that strategy-fostering bias with objective information evidenced through the development of the tasks carried out during the observation period. The results drew a tendency placing Memory strategies among the rest as the most fostered category, whereas Compensation befell to be the least favored LLS category overall.El propósito de este estudio de caso con un enfoque de investigación mixto fue explorar el desarrollo de la promoción de Estrategias del Estudio de la Lengua (Language Learning Strategies) dentro de una clase de Inglés como Lengua Extranjera (EFL) en un curso/nivel de educación secundaria inmerso en el contexto de un establecimiento educacional público chileno basado en las acciones pedagógicas del profesor a cargo. A través de observaciones empíricas, análisis de actividades y una entrevista, la intención de esta tesis fue develar la lógica tras la predominancia de ciertas Estratégias del Estudio de la Lengua en desmedro de otras. Varios artículos, tesis y otros tipos de documentos literarios sirvieron como referencia para posteriormente determinar la cohesión entre estos precedentes y la evidencia empírica recolectada, la cual ha probado estar en relativa concordancia. Por consiguiente, este documento buscó extrapolar la subjetividad de las reflexiones y suposiciones que el profesor-muestra del caso en cuestión fue capaz de dilucidar respecto a la tendencia a parcializar la promoción de ciertas estrategias con información objetiva obtenida a través del análisis de las instrucciones y actividades desarrolladas por el profesor durante el periodo de observación. Los resultados arrojaron tendencias favoreciendo a las estrategias de Memoria por encima del resto como la categoría más fomentada, mientras que las estrategias de Compensación resultaron ser aquellas con menos rodaje