9,505 research outputs found
Schatten classes of integration operators on Dirichlet spaces
We address the question of describing the membership to Schatten-Von Neumann
ideals of integration operators acting on Dirichlet type spaces.
We also study this problem for multiplication, Hankel and Toeplitz operators.
In particular, we provide an extension of Luecking's result on Toeplitz
operators.Comment: Accepted for publication in Journal d' Analyse Mathem\'atique, (2012
Performance Improvements for Nuclear Reaction Network Integration
Aims: The aim of this work is to compare the performance of three reaction
network integration methods used in stellar nucleosynthesis calculations. These
are the Gear's backward differentiation method, Wagoner's method (a 2nd-order
Runge-Kutta method), and the Bader-Deuflehard semi-implicit multi-step method.
Methods: To investigate the efficiency of each of the integration methods
considered here, a test suite of temperature and density versus time profiles
is used. This suite provides a range of situations ranging from constant
temperature and density to the dramatically varying conditions present in white
dwarf mergers, novae, and x-ray bursts. Some of these profiles are obtained
separately from full hydrodynamic calculations. The integration efficiencies
are investigated with respect to input parameters that constrain the desired
accuracy and precision.
Results: Gear's backward differentiation method is found to improve accuracy,
performance, and stability in integrating nuclear reaction networks. For
temperature-density profiles that vary strongly with time, it is found to
outperform the Bader-Deuflehard method (although that method is very powerful
for more smoothly varying profiles). Wagoner's method, while relatively fast
for many scenarios, exhibits hard-to-predict inaccuracies for some choices of
integration parameters owing to its lack of error estimations.Comment: 13 pages, 12 figures, accepted to Astronomy and Astrophysics (section
15) - corrected units in Figs. 6-1
Evolutionary calculations of carbon dredge-up in helium envelope white dwarfs
We investigate the evolution of cooling helium atmosphere white dwarfs using
a full evolutionary code, specifically developed for following the effects of
element diffusion and gravitational settling on white dwarf cooling. The major
difference between this work and previous work is that we use more recent
opacity data from the OPAL project. Since, in general, these opacities are
higher than those available ten years ago, at a given effective temperature,
convection zones go deeper than in models with older opacity data. Thus
convective dredge-up of observationally detectable carbon in helium atmosphere
white dwarfs can occur for thicker helium layers than found by Pelletier et al
(1986). We find that the range of observed C to He ratios in different DQ white
dwarfs of similar effective temperature is well explained by a range of initial
helium layer mass between and , in good agreement
with stellar evolution theory, assuming a typical white dwarf mass of . We also predict that oxygen will be present in DQ white dwarf
atmospheres in detectable amounts if the helium layer mass is near the lower
limit compatible with stellar evolution theory. Determination of the oxygen
abundance has the potential of providing information on the profile of oxygen
in the core and hence on the important CO reaction
rate.Comment: Accepted for publication in MNRAS, 8 pages, 6 figure
PLEs from virtual ethnography of social web
This article presents an exploratory research based on the virtual ethnography from an environment of research and learning including new technologies. The ethnography is a method of qualitative research of social sciences that is mainly used in the socio-cultural anthropology, where it has its theoretical basis. The target was to explore the web 2.0 and its tools. The process of participant observation is by means of a blog, other tools and virtual communities. The result is a descriptive model of the web 2.0 based on a Personal Learning Environment which developed in the ethnographic experience.Postprint (published version
A linear optimization based method for data privacy in statistical tabular data
National Statistical Agencies routinely disseminate large amounts of data. Prior to dissemination these data have to be protected to avoid releasing confidential information. Controlled tabular adjustment (CTA) is one of the available methods for this purpose. CTA formulates an optimization problem that looks for the safe table which is closest to the original one. The standard CTA approach results in a mixed integer linear optimization (MILO) problem, which is very challenging for current
technology. In this work we present a much less costly variant of CTA that formulates a multiobjective linear optimization (LO) problem, where binary variables are pre-fixed, and the resulting continuous problem is solved by lexicographic optimization. Extensive computational results are reported using both commercial (CPLEX and XPRESS) and open source (Clp) solvers, with either simplex or interior-point methods, on a set of real instances. Most instances were successfully solved with
the LO-CTA variant in less than one hour, while many of them are computationally very expensive with the MILO-CTA formulation. The interior-point method outperformed simplex in this particular application.Peer ReviewedPreprin
Detonations in white dwarf dynamical interactions
In old, dense stellar systems collisions of white dwarfs are a rather
frequent phenomenon. Here we present the results of a comprehensive set of
Smoothed Particle Hydrodynamics simulations of close encounters of white dwarfs
aimed to explore the outcome of the interaction and the nature of the final
remnants for different initial conditions. Depending on the initial conditions
and the white dwarf masses, three different outcomes are possible.
Specifically, the outcome of the interaction can be either a direct or a
lateral collision or the interaction can result in the formation of an
eccentric binary system. In those cases in which a collision occurs, the
infalling material is compressed and heated such that the physical conditions
for a detonation may be reached during the most violent phases of the merger.
While we find that detonations occur in a significant number of our
simulations, in some of them the temperature increase in the shocked region
rapidly lifts degeneracy, leading to the quenching of the burning. We thus
characterize under which circumstances a detonation is likely to occur as a
result of the impact of the disrupted star on the surface of the more massive
white dwarf. Finally, we also study which interactions result in bound systems,
and in which ones the more massive white dwarf is also disrupted as a
consequence of the dynamical interaction. The sizable number of simulations
performed in this work allows to find how the outcome of the interaction
depends on the distance at closest approach, and on the masses of the colliding
white dwarfs, and which is the chemical pattern of the nuclearly processed
material. Finally, we also discuss the influence of the masses and core
chemical compositions of the interacting white dwarfs and the different kinds
of impact in the properties of the remnants.Comment: 18 pages, 6 figures. Accepted for publication in MNRA
Measuring molecular electric dipoles using trapped atomic ions and ultrafast laser pulses
We study a hybrid quantum system composed of an ion and an electric dipole.
We show how a trapped ion can be used to measure the small electric field
generated by a classical dipole. We discuss the application of this scheme to
measure the electric dipole moment of cold polar molecules, whose internal
state can be controlled with ultrafast laser pulses, by trapping them in the
vicinity of a trapped ion.Comment: 13 pages, 6 figures. Substantially modified version, with 4 new
appendices; matches published versio
Collective modes of a trapped ion-dipole system
We study a simple model consisting of an atomic ion and a polar molecule
trapped in a single setup, taking into consideration their electrostatic
interaction. We determine analytically their collective modes of excitation as
a function of their masses, trapping frequencies, distance, and the molecule's
electric dipole moment. We then discuss the application of these collective
excitations to cool molecules, to entangle molecules and ions, and to realize
two-qubit gates between them. We finally present a numerical analysis of the
possibility of applying these tools to study magnetically ordered phases of
two-dimensional arrays of polar molecules, a setup proposed to quantum-simulate
some strongly-correlated models of condensed matter.Comment: v2: 13 pages, 8 figures (from 10 figure files). Matches published
version in Appl. Phys. B, special issue "Wolfgang Paul 100
An assessment of the sea breeze energy potential using small wind turbines in peri-urban coastal areas
From wind speed data recorded hourly at 2 m high during 18 years (1993-2010) in the Llobregat Delta (15 km south of Barcelona city; northeast of the Iberian Peninsula), wind speed distributions at 10 m high were computed for the whole year and for the sea breeze period (from March 1 to September 30, from 10 to 19 local time). Weibull probability density functions fitted to the distributions were used to assess the wind energy generated by two off-grid small wind turbines: the IT-PE-100 and the HP-600W. Results from FAST and AeroDyn simulation tools were compared with those obtained by applying measured wind speeds to manufacturer power curves. Using manufacturer data, the IT-PE-100 would deliver 132 kWh during the whole year (70 kWh during the sea breeze period). From the simulations, the IT-PE-100 would deliver 155 kWh during the whole year (80 kWh during the sea breeze period). It is concluded that the sea-breeze is an interesting wind energy resource for micro-generation, not only in the Mediterranean basin but in other areas of the world with similar wind regimes, and particularly in peri-urban coastal areas where large-scale wind farms cannot be implemented.Peer ReviewedPostprint (published version
Social networks and performance in distributed learning communities
Social networks play an essential role in learning environments as a key channel for knowledge sharing and students' support. In distributed learning communities, knowledge sharing does not occur as spontaneously as when a working group shares the same physical space; knowledge sharing depends even more on student informal connections. In this study we analyse two distributed learning communities' social networks in order to understand how characteristics of the social structure can enhance students' success and performance. We used a monitoring system for social network data gathering. Results from correlation analyses showed that students' social network characteristics are related to their performancePostprint (published version
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