12,096 research outputs found
The Generation and Dissipation of Interstellar Turbulence - Results from Large Scale High Resolution Simulations
We study, by means of adaptive mesh refinement hydro- and
magnetohydrodynamical simulations that cover a wide range of scales (from kpc
to sub-parsec), the dimension of the most dissipative structures and the
injection scale of the turbulent interstellar gas, which we find to be about 75
pc, in agreement with observations. This is however smaller than the average
size of superbubbles, but consistent with significant density and pressure
changes in the ISM, which leads to the break-up of bubbles locally and hence to
injection of turbulence. The scalings of the structure functions are consistent
with log-Poisson statistics of supersonic turbulence where energy is dissipated
mainly through shocks. Our simulations are different from previous ones by
other authors as (i) we do not assume an isothermal gas, but have temperature
variations of several orders of magnitude and (ii) we have no artificial
forcing of the fluid with some ad hoc Fourier spectrum, but drive turbulence by
stellar explosions at the Galactic rate, self-regulated by density and
temperature thresholds imposed on the ISM gas.Comment: Five pages and three figures. Accepted for publication in
Astrophysical Journal (Letters
The Portuguese real exchange rate, 1995-2010 : competitiveness or price effects?
We disentangle the extent to which the real exchange rate appreciation in Por-
tugal during 1995-2010 re
ected the emergence of wage-productivity misalignments
or, instead, changes in the relative price of tradable and non-tradable goods. The
available data suggests that the latter e ect dominated at the aggregate level. The
evidence is consistent with the view that the external imbalance that character-
ized the Portuguese economy during the 1990s and early 2000s was triggered by
the liberalization of capital
ows, and not by dysfunctional wage setting institu-
tions. Using the Fundamental Equilibrium Exchange Rate approach, we nd that
recent oil price shocks played an important role in explaining the real exchange rate
overvaluation in Portugal.COMPETE; QREN; FEDER; Fundação para a Ciência e a Tecnologia (FCT
Construcciones con verbos de apoyo en español. De cómo entran los nombres en la órbita de los verbos
HT Cas - eclipsing dwarf nova during its superoutburst in 2010
We present results of a world-wide observing campaign of the eclipsing dwarf
nova - HT Cas during its superoutburst in November 2010. Using collected data
we were able to conduct analysis of the light curves and we calculated
diagrams.
The CCD photometric observations enabled us to derive the superhump period
and with the timings of eclipses the orbital period was calculated. Based on
superhump and orbital period estimations the period excess and mass ratio of
the system were obtained
IX Draconis - a curious ER UMa-type dwarf nova
We report results of an extensive world-wide observing campaign devoted to a
very active dwarf nova star - IX Draconis. We investigated photometric
behaviour of the system to derive its basic outburst properties and understand
peculiarities of IX Dra as well as other active cataclysmic variables, in
particular dwarf novae of the ER Uma-type. In order to measure fundamental
parameters of the system, we carried out analyses of the light curve, O-C
diagram, and power spectra. During over two months of observations we detected
two superoutbursts and several normal outbursts. The V magnitude of the star
varied in the range 14.6 - 18.2 mag. Superoutbursts occur regularly with the
supercycle length of 58.5+/-0.5 d. When analysing data over the past 20 years,
we found that the supercycle length is increasing at a rate of P_dot = 1.8 *
10^{-3}. Normal outbursts appear to be irregular, with typical occurrence times
in the range 3.1 - 4.1 d. We detected a double-peaked structure of superhumps
during superoutburst, with the secondary maximum becoming dominant near the end
of the superoutburst. The mean superhump period observed during superoutbursts
equals 0.066982(36) d, which is constant over the last two decades of
observations. Based on the power spectrum analysis, the evaluation of the
orbital period was problematic. We found two possible values: the first one,
0.06641(3) d, which is in agreement with previous studies and our O-C analysis
(0.06646(2) d), and the second one, 0.06482(3) d, which is less likely. The
evolutionary status of the object depends dramatically on the choice between
these two values. A spectroscopic determination of the orbital period is
needed. We updated available information on ER UMa-type stars and present a new
set of their basic statistics. Thereby, we provide evidence that this class of
stars is not uniform.Comment: Accepted for publication in MNRAS; 15 pages, 15 figures, 6 tables;
typo correcte
Examination timetabling automation using hybrid meta-heuristics
Trabalho de projeto realizado para obtenção do grau de Mestre em Engenharia Informática e de ComputadoresNos últimos anos, o tema da geração automática de horários tem sido alvo de muito estudo.
Em muitas instituições, a elaboração de horários ainda é feita manualmente, constituindo-se uma tarefa demorada e penosa para instâncias de grande dimensão. Outro problema recorrente na abordagem manual é a existência de falhas dada a dificuldade do processo de verificação, e também a qualidade final do horário produzido. Se este fosse criado por computador, o horário seria válido e seriam de esperar horários com qualidade superior dada a capacidade do computador para pesquisar o espaço de soluções.
A elaboração de horários não é uma tarefa fácil, mesmo para uma máquina. Por exemplo, horários escolares necessitam de seguir certas regras para que seja possível a criação de um horário válido. Mas como o espaço de estados (soluções) válidas é tão vasto, é impraticável criar um algoritmo que faça a enumeração completa de soluções a fim de escolher a melhor solução possível. Por outro lado, a utilização de algoritmos que realizam a enumeração implícita de soluções (por exemplo, branch and bound), não é viável para problemas de grande dimensão. A utilização de heurísticas que percorrem de uma forma guiada o espaço de estados, conseguindo assim uma solução razoável em tempo útil, constituem uma abordagem adequada para este tipo de problemas.
Um dos objetivos do projeto consiste na criação duma abordagem que siga as regras do International Timetabling Competition (ITC) 2007 incidindo na criação de horários de exames em universidades (Examination timetabling track). Este projeto utiliza uma abordagem de heurísticas híbridas. Isto significa que utiliza múltiplas heurísticas para obter a melhor solução possível. Utiliza uma variação da heurística de Graph Coloring para obter uma solução válida e as meta-heurísticas Simulated Annealing e Hill Climbing para melhorar a solução obtida.
Os resultados finais são satisfatórios, pois em algumas instâncias os resultados são melhores do que alguns dos cinco finalistas do concurso ITC 2007.Abstract: In the last few years the automatic creation of timetables is being a well-studied subject. In many institutions, the elaboration of timetables is still manual, thus being a time-consuming and difficulty task for large instances. Another current problem in the manual approach is the existence of failures given the difficulty in the process verification, and so the quality of the produced timetable. If this timetable had been created by a computer, the timetable would be valid and timetables with better quality should be obtained, given the computer’s capacity to search the solution space.
It is not easy to elaborate timetables, even for a machine. For example, scholar/university timetables need to follow certain type of constraints or rules for them to be considered valid. But since the solution space is so vast, it is highly unlikely to create an algorithm that completely enumerates the solutions in order to choose the best solution possible, considering the problem structure. The use of algorithms that perform implicit enumeration solutions (for example, an branch bound), is not feasible for large problems. Hence the use of heuristics which navigate through the solution space in a guided way, obtaining then a reasonable solution in acceptable time.
One main objective of this project consists in creating an approach that follows the International Timetabling Competition (ITC) 2007 rules, focusing on creating examination timetables. This project will use a hybrid approach. This means it will use an approach that includes multiple heuristics in order to find the best possible solution. This approach uses a variant of the Graph Coloring heuristic to find an initial valid solution, and the metaheuristics Simulated Annealing and Hill Climbing to improve that solution.
The final results are satisfactory, as in some instances the obtained results beat the results of some of the five finalists from ITC 2007
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