219,589 research outputs found
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, , needed for
explosion. When 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- 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,
, 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
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