13,697 research outputs found
Spectral type, temperature and evolutionary stage in cool supergiants
In recent years, temperature scales in cool supergiants (CSGs) have been
disputed, and the possibility that spectral types (SpTs) do not depend
primarily on temperature has been raised. We explore the relations between
different observed parameters and the capability of deriving accurate intrinsic
stellar parameters from them through the analysis of the largest spectroscopic
sample of CSGs to date from SMC and LMC. We explore possible correlations
between different observational parameters, also making use of near- and
mid-infrared colours and literature on photometric variability. Direct
comparison between the behaviour of atomic lines (Fe I, Ti I, and Ca II) in the
observed spectra and synthetic atmospheric models provides compelling evidence
that effective temperature is the prime underlying variable driving the SpT
sequence in CSGs. However, there is a clear correlation between SpT and
luminosity, with later ones tending to correspond to more luminous stars with
heavier mass loss. The population of CSGs in the SMC is characterised by a
higher degree of spectral variability, early spectral types (centred on type
K1) and low mass-loss rates (at least as measured by dust-sensitive
mid-infrared colours). The population in the LMC displays less spectroscopic
variability and later spectral types. The distribution of spectral types is not
single-peaked. Instead, the brightest CSGs have a significantly different
distribution from less luminous objects, presenting mostly M subtypes (centred
on M2), and increasing mass-loss rates for later types. In conclusion, the
observed properties of CSGs in the SMC and the LMC cannot be described
correctly by standard evolutionary models. The very strong correlation between
spectral type and bolometric luminosity, supported by all data from the Milky
Way, cannot be reproduced at all by current evolutionary tracks.Comment: 25 pages, 24 figure
Analysis of the Behaviour of Biofuel-Fired Gas Turbine Power Plants
The utilisation of biofuels in gas turbines is a promising alternative to fossil fuels for power generation. It would lead to significant reduction of CO2 emissions using an existing combustion technology, although significant changes seem to be needed and further technological development is necessary. The goal of this work is to perform energy and exergy analyses of the behaviour of gas turbines fired with biogas, ethanol and synthesis gas (bio-syngas), compared with natural gas. The global energy transformation process (i.e. from biomass to electricity) has also been studied. Furthermore, the potential reduction of CO2 emissions attained by the use of biofuels has been determined, considering the restrictions regarding biomass availability. Two different simulation tools have been used to accomplish the aims of this work. The results suggest a high interest and the technical viability of the use of Biomass Integrated Gasification Combined Cycle (BIGCC) systems for large scale power generation
A wide family of singularity-free cosmological models
In this paper a family of non-singular cylindrical perfect fluid cosmologies
is derived. The equation of state corresponds to a stiff fluid. The family
depends on two independent functions under very simple conditions. A sufficient
condition for geodesic completeness is provided.Comment: 7 pages, RevTeX
Estrategia didáctica para favorecer la formación y el desarrollo de la competencia para gestionar el conocimiento matemático en los estudiantes universitarios
Nota: Artículo derivado de tesis doctoral de igual título aprobada en mayo de 2009.La investigación tuvo como objeto el proceso docente-educativo de la matemática en la carrera de Ingeniería en Sistemas de Computación de la Universidad APEC (UNAPEC). Estuvo orientada al diseño de una estrategia didáctica para favorecer la formación y el desarrollo de la competencia para gestionar el conocimiento matemático en los estudiantes, la que se sustenta en un modelo teórico que se explica a partir de tres subsistemas que permiten develar las relaciones que se dan en el proceso de formación y desarrollo de la competencia declarada en el contexto del proceso docente-educativo (PDE) de la matemática.
Para determinar la factibilidad y pertinencia se realizó la ejemplificación y valoración de la estrategia didáctica a través del método de expertos, y la constatación empírica a través de un pre-experimento pedagógico. La novedad científica está dada al develar las relaciones existentes entre los subsistemas que componen el modelo y expresan la concepción de la competencia para gestionar el conocimiento matemático
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