Finalized PhD theses

Número de entradas: 78

Chemical and Dynamical Galaxy Evolution

Estado: defended (16/11/2010)
Estudiante: Artigas A.
Supervisada por: Isern J.; Bravo E.

Using Smoothed Particle Hydrodynamics simulations (Gadget-2 code) we explore the evolution of an individual galaxy. In the code we have included gas cooling, star formation, gas and metal restitution from dying stars plus supernovae energy and chemical feedback (SNII and SNIa). All the ingredients have been treated statistically thus a high resolution is needed in order to achieve good results. We follow the formation and evolution of a disk-like galaxy, predict the final structure and metallicity evolution.
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Estado: defended (16/11/2010)
Estudiante: Artigas A.
Supervisada por: Isern J.; Bravo E.

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Evolució quimiodinàmica de la Galàxia

Estado: defended (16/11/2010)
Estudiante: Anna Artigas
Supervisada por: Jordi Isern Vilaboy
Universidad: Universitat de Barcelona

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Estado: defended (16/11/2010)
Estudiante: Anna Artigas
Supervisada por: Jordi Isern Vilaboy
Universidad: Universitat de Barcelona

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The effect of activity on the fundamental properties o low-mass stars

Estado: defended (14/06/2010)
Estudiante: Morales, J.C.
Supervisada por: Ignasi Ribas Canudas; Jordi, C.
Universidad: Universitat de Barcelona

Recent analyses of eclipsing binaries suggest a significant discrepancy between the observed radii of low-mass stars in binary systems and the predictions of theoretical models. This stands in contrast with the fact that theory now provides excellent agreement with the observed radii of low mass stars, either single or in wide binary systems. The observed disagreement for stars in close binary systems be associated with the generally strong magnetic activity of stars in close binaries caused by their relatively fast rotation rates. The project of this PhD thesis is to study and resolve the discrepancies by analyzing additional eclipsing binaries and modifying current stellar models to incoporate the effects of magnetic fields.
La comparación entre los modelos de estructura estelar y las observaciones de estrelles de baja masa, indican que los radios y temperaturas efectivas que predicen los modelos son aproximadamente un 10% más pequeños y un 5% más altas, respectivamente, que las observaciones. Las estrellas binarias…
Estado: defended (14/06/2010)
Estudiante: Morales, J.C.
Supervisada por: Ignasi Ribas Canudas; Jordi, C.
Universidad: Universitat de Barcelona

La comparación entre los modelos de estructura estelar y las observaciones de estrelles de baja masa, indican que los radios y temperaturas efectivas que predicen los modelos son aproximadamente un 10% más pequeños y un 5% más altas, respectivamente, que las observaciones. Las estrellas binarias eclipsantes han demostrado ser una herramienta muy útil para testear estos modelos ya que permiten obtener las masas y radios de la estrellas con precisiones del orden del 2%. En este trabajo, se han obtenido las propiedades fundamentales de los sistemas binarios CM Draconis e IM Virginis. La comparación conjunta de estos sistemas, junto con otros con propiedades muy bien determinadas, se ha utilizado para testear los modelos de estructura estelar. Los resultados indican que la actividad magnética presente en estos sistemas es la causante de las diferencias entre los modelos y las observaciones, por su efecto sobre el transporte convectivo en el interior de las estrellas y por la aparición de manchas en su superficie.

Design and evaluation of navigation and control algorithms for spacecraft formation flying missions

Estado: defended (21/05/2010)
Estudiante: Laura Perea Virgili
Supervisada por: Pedro Elosegui Larrañeta
Universidad: Universitat de Barcelona

Formation flying offers space-dependent disciplines such as astrophysics, astrodynamics, and geodesy, to name a few, the possibility of creating large spaceborne sensors from an array of small spacecraft flying in formation. This creates exciting scientific and technical opportunities as the formation…
Estado: defended (21/05/2010)
Estudiante: Laura Perea Virgili
Supervisada por: Pedro Elosegui Larrañeta
Universidad: Universitat de Barcelona

Formation flying offers space-dependent disciplines such as astrophysics, astrodynamics, and geodesy, to name a few, the possibility of creating large spaceborne sensors from an array of small spacecraft flying in formation. This creates exciting scientific and technical opportunities as the formation could be arranged to work as, for example, an interferometer, thus providing a most unlimited angular resolution or a virtual telescope, thus unrestricted focal distances. Since the first mission including formation flying technology (EO-1) was selected by NASA, some of the challenges to realize full Formation Flying (FF) capabilities has been thought to be the definition of suitable algorithms to navigate and control FF missions. The focus of this dissertation is the design and evaluation of algorithms for navigation and control for formation flying missions. Given its importance, extensive research has been already conducted to fulfill the increase of accuracy, autonomy, and other requirements of the Guidance, Navigation, and Control (GNC) systems that derive from novel applications of formation flying missions. To center the scope of present work, we have mainly focused in three of the present challenges: the difficulties of fusing different non-linear observations for relative navigation; the analysis and extension of behavioral algorithms for controlling a formation of spacecraft; and the design and validation of a control law for formation acquisition and formation keeping of a non-natural relative trajectory. These three interconnected topics cover a wide range of research in formation flying and embody the main algorithm components of formation flying algorithms from the observations to the navigation and to the control. The first challenge consisted, thus, in addressing the difficulties encountered by classical filters to estimate a state vector fusing common observations. We proposed several strategies to improve the robustness of these filters under non-linear conditions. Among these strategies, the modification of the residuals computation for the Unscented Kalman Filter (UKF) deserves special mention due to its excellent results and robustness against nonlinearities. A theoretical basis for these results became, thus, necessary regarding the new update equation of the UKF and has been developed subsequently in the frame of this thesis. This work has been published in Perea et al. (2007) and Perea and Elosegui (2008). The collective motion exhibited by some groups of animals has recently attracted the interest of many research groups who try to take advantage of the robustness and efficiency of natural patterns. With this aim, we have investigated the possibility of extending an interaction model that has shown emergent behavior. In particular, the Cucker-Smale (CS) model has been extended for its application on spacecraft formation flying. Numerical simulations of the Darwin mission have proved that this strategy is suitable for loose formation keeping. Of special relevance is the low cost of the controller, specially compared to an alternative strategy, the Zero Relative Radial Acceleration Cones (ZRRAC). The problem of tight formation keeping is addressed previous publications. In these papers, we first study the relative dynamics of a virtual telescope that follows a non-natural relative trajectory driven by the position of an observed body and not by the natural forces in space. This analysis has originated the design of several controls based on different approximations of the relative dynamics. Their performances have been tested and compared through numerical simulations of the PROBA-3 mission using, first, computer based simulations, and then, a realistic platform with GNSS hardware and operational flight software in the loop. The main conclusions show that simple control definitions, as defined by the Linear Quadratic Regulator (LQR) and Linear Quadratic Regulator with the Integral term (LQRI), can fulfill stringent requirements for formation acquisition and tight formation keeping. (Formation flying is a key, forthcoming technology with an ever increasing number of applications in space missions. This thesis faces some of the present challenges in specific navigation and control algorithms for formation flying missions: nonlinear data fusion in navigation filters; loose controls based on natural patterns; and tight controls to keep non-natural relative trajectories. The proposed algorithms have been tested under the Darwin and PROBA-3 scenarios in realistic platforms.)

Study of galactic properties and their evolution

Estado: defended (28/09/2009)
Estudiante: Violeta González Pérez
Supervisada por: Francisco Javier Castander Serentill
Universidad: Universitat de Barcelona

La tematica de la tesis de Violeta se centra en el estudio de las propiedades observacionales de las galaxias y de su evolucion, encuadrado dentro de un contexto cosmologico de formacion global de estructuras. Los elementos esenciales de la tesis son por tanto el estudio sistematico de los parametros observacionales de las galaxias como pueden ser sus luminosidades, colores, brillos superficiales, intesidades de lineas espectrales, etc y como a partir de ellos se pueden obtener parametros fisicos de las galaxias como por ejemplo su edad, metalicidad, masa estelar y tasa de formacion estelar. Ademas, se estudiara la evolucion de estos parametros en el tiempo y como se interpretan dentro de un marco cosmologico de formacion y evolucion de estructuras. Hasta la fecha Violeta se ha concentrado en el estudio de galaxias locales extraidas del SDSS que es un estudio sistematico del universo local que tiene como objetivos obtener imagenes moderadamente profundas de aproximadamente un cuarto del cielo en cinco bandas y tomar espectros de aproximadamente un millon de galaxias en esa zona del cielo seleccionadas con estrictos criterios basados en las imagenes adquiridas. La base de datos que generara el SDSS es por tanto una pieza clave para la comprension de las propiedades galacticas en el universo local. Dentro del estudio de las galaxias locales Violeta se ha centrado en las galaxias de tipo temprano, un grupo de galaxias que muestran propiedades muy homogeneas y que son muy utiles para entender el proceso de formacion y evolucion galactica. Los resultados obtenidos se centran, pero no se limitan, al estudio de los gradientes de color de estas galaxias un aspecto muy poco tratado en la literatura debido a la falta de muestras representativas. Estudiando galaxias de tipo temprano con gradientes de color anomalo que son aquellas que se encuentran mas cerca de su periodo de formacion, Violeta ha encontrado que existe una evolucion en su abundancia con el corrimiento al rojo y ha correlacionado esta propiedad con otros parametros observacionales de las galaxias.
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Estado: defended (28/09/2009)
Estudiante: Violeta González Pérez
Supervisada por: Francisco Javier Castander Serentill
Universidad: Universitat de Barcelona

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Development and validation of the thermal diagnostics instrumentation in LISA Pathfinder

Estado: defended (28/09/2009)
Estudiante: Josep Sanjuan
Supervisada por: José Alberto Lobo Gutiérrez; Ramos-Castro, J
Universidad: Universitat Politècnica de Catalunya

The thesis focuses on LISA PathFinder (LPF) temperature measurement subsystem, involving sensitivities at the micro-Kelvin level in a frequency band around one milli-Hertz. ESA's LISA Pathfinder (LPF) mission is a technology demonstrator for LISA, the first space-based Gravitational Wave observatory, designed to sense GW signals in a frequency band between 0.1 mHz and 0.1 Hz. The thesis includes both experimental and analysis work, constituting an in-depth study of the LPF thermal diagnostics subsystem: sensor selection, front-end electronics and full validation of the parts workings by means of on-ground experiments. This is done by thorough analysis of the measured noise to identify the causes of spurious components, and to fix them. Main result is the accomplishment of a fully operative LPF thermal diagnostics system, from prototype to flight model. Additional investigations (also reported) show that both LPF temperature sensors and front-end electronics can be almost directly used in LISA, down to the 0.1 mHz frequency band.
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Estado: defended (28/09/2009)
Estudiante: Josep Sanjuan
Supervisada por: José Alberto Lobo Gutiérrez; Ramos-Castro, J
Universidad: Universitat Politècnica de Catalunya

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Gamma Ray Emission from Type Ia Supernovae

Estado: defended (03/07/2009)
Estudiante: Alina Hirschmann
Supervisada por: Jordi Isern Vilaboy; Bravo, E.
Universidad: Universitat Politècnica de Catalunya

Eventhough the research on Type Ia supernovae (SNeIa) has increased considerably with the help of new observatories and data, the mechanism that triggers these explosions is still an enigma. Gamma-rays and positrons are the product of the decay of radioactive elements generated in the explosion and…
Estado: defended (03/07/2009)
Estudiante: Alina Hirschmann
Supervisada por: Jordi Isern Vilaboy; Bravo, E.
Universidad: Universitat Politècnica de Catalunya

Eventhough the research on Type Ia supernovae (SNeIa) has increased considerably with the help of new observatories and data, the mechanism that triggers these explosions is still an enigma. Gamma-rays and positrons are the product of the decay of radioactive elements generated in the explosion and therefore, play a crucial role in understanding the origin of Type Ia SN. They provide information on the internal structure of the ejecta and the effects of the burning front on it and are also the main energy source that powers the lightcurve. In addition, they can give information on the turbulence of the system, explosion asymmetries, Doppler broadening of emission lines, etc. In order to determine the appropriate mechanism of explosion which reproduces at best the observations, several theoretical models have been built taking into account that these events could lodge subsonic (deflagration), supersonic (detonation) or both (delayed detonation) flame regimes. This work focuses on understanding the transport of gamma-ray photons and positrons in the ejecta and how they can provide information on its internal structure. To study this, we have built a Monte Carlo transport code and computed gamma-ray emission simulations in 1D and 3D, using as input the set of mentioned explosion models. The Monte Carlo technique can help solve the problem of radiative transfer by studying the behavior of a representative number of photons and positrons created in SNeIa explosions. The basics of the Monte Carlo method concerns the use of random numbers with appropriate distributions to describe physical aspects. Simulations in 1D are used basically to discern between explosion models while 3D simulations provide information concerning asymmetries in the explosion, rotation and convective mixing features, and possible interaction of the ejecta with the secondary star of the system. As for the transport of gamma-ray photons, gamma-ray spectral and lightcurve simulations have shown to provide information on he ejecta through the analysis of the line shapes and widths. However, these tools alone are not determinant in the discernment of the explosion mechanism. Diagnostic tools such as the study of the different line ratios, determination of supernova age, determination of 56Ni mass, energy of the explosion, fwhm, energy deposition in the ejecta and photoelectric cut-off energy have been used as complementary tools to help constrain the model through future explosion observations. In 3D, we have analyzed the flux as a function of the direction of observation, assuming that we are capable of viewing the system from different lines of sight. The spectra and lightcurves have shown that differences in the flux as a function of the line of sight can reach 10% at the most, which means that the explosion is not very asymmetric. In addition, we have included the binary companion (a main sequence star) in an input model and have carried out simulations for such scenario to see the effects on the gamma-ray emission. The results have shown that the flux is not very sensitive to the change in the line of sight even with the presence of the main sequence companion. A future study will focus on the type of effects the emission might show considering different binary companions. Complementary to the gamma-ray emission, we have also studied the positron transport in the ejecta since these particles play an important role in powering the lightcurve at late times and they can also be closely linked to the positron-electron annihilation 511 keV line seen by INTEGRAL in the Galactic bulge and disk. The study of the positron escape fraction has allowed us to determine a needed supernova rate in the Galaxy to explain the emission INTEGRAL has measured. The rate obtained from our simulations suggests that Type Ia supernovae could be the strongest candidates since they produce the necessary positrons to explain such emission.

Gravitational wave radiation from single and binary white dwarfs

Estado: defended (30/04/2009)
Estudiante: Lorén-Aguilar, P.
Supervisada por: Jordi Isern Vilaboy; GarcÍa-Berro, E.
Universidad: Universitat Politècnica de Catalunya

En los próximos años la entrada en funcionamiento de los primeros detectores de ondas gravitatorias será una realidad, abriendo de esta manera una nueva ventana observacional de la Astronomía. Es por ello por lo que resulta de especial interés tener a disposición de la comunidad científica modelos…
Estado: defended (30/04/2009)
Estudiante: Lorén-Aguilar, P.
Supervisada por: Jordi Isern Vilaboy; GarcÍa-Berro, E.
Universidad: Universitat Politècnica de Catalunya

En los próximos años la entrada en funcionamiento de los primeros detectores de ondas gravitatorias será una realidad, abriendo de esta manera una nueva ventana observacional de la Astronomía. Es por ello por lo que resulta de especial interés tener a disposición de la comunidad científica modelos detallados de emisión de ondas gravitatorias que pudieran ser observados por estos detectores y, en particular, por el observatario espacial de ondas gravitatoras LISA, cuyo lanzamiento está previsto en 2014. Por otro lado las enanas blancas son objetos compactos y si están en sistemas binarios o si se encuentran en la zona inestable a pulsaciones podrían emitir ondas gravitatorias con la suficiente intensidad como para ser detectadas. Es por ello que en la presente tesis se ha estudiado la emisión de ondas gravitatorias de enanas blancas pulsantes y de enanas blancas en sistemas binarios. En el primer caso se ha estudiado la emisión de ondas gravitatorias procedentes de tres tipos de pulsaciones no radiales de las enanas blancas: los modos g, r y p. Se ha comprobado que para las pulsaciones de modos g, en las que la fuerza recuperadora es la gravedad, la emisión de ondas gravitatorias es demasiado débil como para ser observada mediante el satélite LISA. Por el contrario, en el caso de las pulsaciones p, donde la fuerza recuperadora es la presión, se ha podido comprobar que la emisión de radiación gravitatoria resultaría tan intensa que daría lugar a una casi inmediata amortiguación de las pulsaciones. En el segundo caso, se ha calculado la emisión de ondas gravitatorias procedente de la coalescencia de sistemas binarios. Usando un código SPH se ha simulado la coalescencia de una gran variedad de sistemas binarios, cubriendo rangos de masas y composiciones iniciales ampliamente representativos de los diferentes tipos de sistemas binarios galácticos. Con ello se ha podido calcular con detalle la forma de la emisión gravitatoria durante la fase de colisión, así como otras características de interés astrofísico de la coalescencia de enanas blancas. En particular, se ha podido demostrar que los sistemas binarios galácticos serán una fuente garantizada para el futuro detector LISA, no solo como generadores de un ruido de fondo contínuo sino como eventos aislados. Adicionalmente se han buscado las características observacionales de este tipo de sucesos, a fin de guiar las observaciones con los detectores de ondas gravitatorias mediante el uso de la emisión electromagnética de alta energía. También se ha analizado la viabilidad del escenario doblemente degenerado (DD) como progenitor de supernovas de tipo Ia, otro de los aspectos por los que la coalescencia de sistemas binarios de enanas blancas es de interés. En particular, se ha podido demostrar que la distrbución final de densidades y temperaturas de los objetos resultantes de las colisiones, en los casos en los que dos estrellas de diferentes masas están involucradas, dificulta mucho la viabilidad del escenario. Es de destacar finalmente que todo ello ha requerido un esfuerzo computacional elevado que sólo ha sido posible mejorando el código SPH que hemos utilizado. En concreto, se ha introducido una nueva y mas moderna prescripción de la viscosidad artificial y se ha paralelizado el código a fin de poder realizar simulaciones de alta resolución en un tiempo asequible.

Variable stars and eclipsing binaries in M31

Estado: defended (09/03/2009)
Estudiante: Vilardell, F.
Supervisada por: Ignasi Ribas Canudas; Jordi, C.
Universidad: Universitat de Barcelona

The main objective of the PhD thesis is the use of variable stars (Cepheids) and eclipsing binaries for an accurate determination of the distance to M31. The observational data for the project was obtained over the corse of a 4-yr campaign at the 2.5-m INT telescope (La Palma). The survey has provided photometry of some 450 eclipsing binary stars and 410 Cepheid variables. In the case of the eclipsing binaries the photometry is complemented with spectroscopy obtained from the 8-m Gemini North telescope. From the overall analysis, a distance determination to M31 with an uncertainty below 3% is antipated, thus meaning a very significant improvement over the current 10%. The results of this project will help to better establish the Cosmic Distance Scale by accurately calibrating the distance to Local Group galaxies.
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Estado: defended (09/03/2009)
Estudiante: Vilardell, F.
Supervisada por: Ignasi Ribas Canudas; Jordi, C.
Universidad: Universitat de Barcelona

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Large scale structure and dark energy

Estado: defended (30/06/2008)
Estudiante: Anna Cabre Albos
Supervisada por: Enrique Gaztañaga
Universidad: Universitat de Barcelona

Physicists currently believe that the universe is composed basically of dark energy (70%) and dark matter (25%), both unknown components. The rest is made of known (barionic) matter. The first part of the thesis studies the clustering in galaxies through the 2 and 3 point auto-correlation function in Sloan Sky Digital Survey (SDSS). This allows a direct measurement of the dark matter component and the global amplitude of fluctuations modulated by some bias. The second part of the thesis cross-correlates CMB (with WMAP data) with fluctuations in the number of galaxies (with SDSS) in order to obtain extra information about the structure of the universe. Dark energy is responsible for the accelerated expansion of the universe during late times. We can detect it through the redshift or blueshift that experiment photons coming from the Cosmic Microwave Background (CMB) when they pass through dark matter potentials (Integrated Sachs-Wolfe effect). A join analysis of both cross and auto correlations represents a novel approach to understand structure formation in the universe.
The standard cosmological model starts with Big Bang, followed by a rapid period of expansion of the universe called inflation. After that, tiny almost homogeneous fluctuations that conform the primordial universe, start to grow while universe expands now in a relatively slow rhythm. 300,000 years…
Estado: defended (30/06/2008)
Estudiante: Anna Cabre Albos
Supervisada por: Enrique Gaztañaga
Universidad: Universitat de Barcelona

The standard cosmological model starts with Big Bang, followed by a rapid period of expansion of the universe called inflation. After that, tiny almost homogeneous fluctuations that conform the primordial universe, start to grow while universe expands now in a relatively slow rhythm. 300,000 years after the Big Bang, the temperature is low enough to make the universe become neutral after the recombination of atoms with electrons. Photons are almost free of interactions since then and reach us in the form of a Cosmic Microwave Background (CMB). We can measure the spatial anisotropy spectrum of CMB temperatures and compare it to the expected spectrum of acoustic oscillations. This comparison provides a direct geometrical test from which we can deduce that universe is flat or nearly flat. This can be explained if we introduce a new constituent in the universe apart from matter, the dark energy. Dark energy acts as anti-gravity that accelerates the expansion and is also observed through standard candles Supernovae Ia. Although there is a well motivated model that can explain observations, neither dark matter nor dark energy are known elements, so it is important to use the large amount of newly available data to obtain tighter constraints on the constituents of the universe, the evolution of growth perturbations, the expansion history, and also other alternatives, such as modification of gravity at large scales. In the first part of the thesis, we study the Integrated Sachs-Wolfe effect (ISW), through the cross-correlation between large scale clustering, traced by galaxies (in our case from the catalog SDSS) and primordial temperature fluctuations from CMB (using the catalog WMAP). Photons that come from the last scattering surface can be red or blue shifted by the time evolution of fluctuations in the gravitational potentials created by large scale structures, which are traced by the large scale galaxy distribution. The ISW effect gives us information about dark energy (DE), because DE modifies the evolution of dark matter gravitational potential. There is no ISW effect in a flat universe without DE (ie in the Einstein-de-Sitter universe) because in this case the gravitational potential remains constant (in the linear regime, which corresponds to large angular scales). In principle, the ISW effect can probe dark energy independently from other observations, such as Supernovae Ia. The correlation between galaxies in redshift space can also be used to study the evolution of the dark matter gravitational potential in a way that is complementary to the cross-correlation of galaxies with CMB photons. In the second part of the thesis, we will study this effect in the luminous red galaxies of the SDSS. These galaxies trace very large volumes which is important to have more signal, and they have a known evolution which make easy to work with them.
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