Finalized PhD theses

Number of entries: 76

High energy emission from classical and recurrent novae

Status: defended (09/10/2017)
Student: Laura Delgado
Supervised by: Margarita Hernanz
University: Universitat Autònoma de Barcelona

In recent years, several nova explosions - eight classical novae and two symbiotic recurrent novae - have been detected by Fermi/LAT at E>100 MeV. In most cases, this emission has been observed early after the explosion, around the optical maximum, and for a short period of time. The high-energy gamma-ray…
Status: defended (09/10/2017)
Student: Laura Delgado
Supervised by: Margarita Hernanz
University: Universitat Autònoma de Barcelona

In recent years, several nova explosions - eight classical novae and two symbiotic recurrent novae - have been detected by Fermi/LAT at E>100 MeV. In most cases, this emission has been observed early after the explosion, around the optical maximum, and for a short period of time. The high-energy gamma-ray emission is a consequence of neutral pion decay and/or Inverse Compton, which are related to particle (p and e-) acceleration in the strong shock between the nova ejecta and the circumstellar matter. RS Ophiuchi (2006) was the first nova for which particle acceleration was predicted. This prediction showed that the blast wave decelerated faster than expected as a consequence of the acceleration of particles in the shock and their escape.  The aim of this thesis is to study the evolution of the symbiotic recurrent novae the first days after the outburst through a multiwavelength study, mainly X-ray emission, and its relation with the acceleration of particles. In particular, we present a comprehensive multiwavelength study of the last outburst of RS Oph and V745 Sco.
RS Oph is a recurrent nova in a symbiotic system composed of a white dwarf and a red giant with a recurrence period of ~21 years. In this work, we present a new analysis of XMM-Newton's observations of RS Oph early after its 2006 outburst both with RGS and EPIC-MOS. We compare these results with those obtained for RXTE, Swift and Chandra observations, and previous studies of RGS observations. The evolution of the radio and IR emissions during the first days after the outburst was studied. The multiwavelength studies allow us to get a global picture of the shocked plasma and its relationship with the particle acceleration.

V745 Sco is also a symbiotic recurrent nova with a recurrence period of ~25 years. We present the analysis of the Swift/XRT observations of V745 Sco early after its 2014 outburst simultaneous to Fermi detection. We combine our results with Chandra and NuStar observations to get a global picture of the evolution of the nova ejecta. As in RS Oph we compiled all the radio and IR information about V745 Sco the first days after the explosion. Finally, particle acceleration in V745 Sco can be explained by a diffusive shock model at the blast wave and the subsequent escape of the very high-energy particles as in RS Oph.

With the study of these two novae, and its comparison, we demonstrate common features in the early evolution of a nova remnant and their relationship with particle acceleration in the symbiotic recurrent novae. Multiwavelength results provide new insights into the evolution of the shocked plasma and the interaction with the circumstellar material, being a powerful tool to understand the gamma-ray emission. 

The accretion flow onto white dwarfs and its X-ray emission properties

Status: defended (21/09/2017)
Student: Nataly Ospina
Supervised by: Margarita Hernanz
University: Universitat Autònoma de Barcelona

Explosive burning of hydrogen on top of accreting white dwarfs causes nova outbursts. The binary system where classical novae occur is a cataclysmic variable whereas, some, recurrent novae occur in symbiotic binaries. The analysis of the X-ray emission from novae in their post outburst stages provides…
Status: defended (21/09/2017)
Student: Nataly Ospina
Supervised by: Margarita Hernanz
University: Universitat Autònoma de Barcelona

Explosive burning of hydrogen on top of accreting white dwarfs causes nova outbursts.
The binary system where classical novae occur is a cataclysmic variable whereas, some,
recurrent novae occur in symbiotic binaries. The analysis of the X-ray emission from
novae in their post outburst stages provides important information about the nova explosion
mechanism and the reestablishment of accretion. In some cases, like V2487 Oph
1998, observations with XMM-Newton a few years after outburst indicate that accretion
has been re-established and its X-ray spectra look like those of magnetic cataclysmic variables,
of the intermediate polar class.
In this work a numerical model of accretion flow onto magnetic white dwarfs and their
corresponding X-ray emission has been developed to be compared with observations
of post outburst novae where accretion is active again. Distributions of the different
physical quantities that describe the emission region have been obtained for different
masses of white dwarf and different accretion rates. The associated X-ray spectrum has
been also obtained with the main objective of studying accretion in the emission region.
These results have been applied to the nova V2487 Oph 1998 with the aim to obtain the
mass of the white dwarf since this nova has been identified as a recurrent nova, with
a previous eruption in 1900, and therefore as a good candidate for a type Ia Supernova
progenitor.

Numerical relativity studies in Anti-de Sitter spacetimes: Gravitational Collapse and the AdS/CFT correspondence

Status: defended (12/07/2017)
Student: Daniel Santos Olivan
Supervised by: Carlos Fernandez Sopuerta
University: Universitat de Barcelona

In this thesis, we study three different problems using Numerical Relativity on asymptotically Anti-de Sitter (AdS) spacetimes. The first is our research on the gravitational collapse of massless scalar fields on asymptotically AdS spacetimes. We have developed a new method that combines two different…
Status: defended (12/07/2017)
Student: Daniel Santos Olivan
Supervised by: Carlos Fernandez Sopuerta
University: Universitat de Barcelona

In this thesis, we study three different problems using Numerical Relativity on asymptotically Anti-de Sitter (AdS) spacetimes.

The first is our research on the gravitational collapse of massless scalar fields on asymptotically AdS spacetimes. We have developed a new method that combines two different formulations of the Einstein Field Equations to get closer and with more accuracy to the collapse. We have numerical evidence that in the separation of the branches there is a universal power law in the mass of the formed apparent horizons for subcritical configurations in addition to the one for supercritical ones. This new power law confirms that there is a gap in the mass of the apparent horizon.

In the second part, we introduce a shock waves model in AdS to study the far-from-equilibrium regime in the heavy ion collisions through the holographic correspondence in a non-conformal theory. In the models used until now, the shock waves correspond to conformal gauge theories while QCD is not conformal. In order to get closer to a description of the actual physical collisions, we present the first shock waves collision in a non-conformal theory. With this, we show how the non-conformality increases the hydrodynamisation time and also that this can happen before the equation of state is fulfilled.

In the last part, we propose the use of spectral methods for high precision computations. The exponential convergence of spectral methods can approximate functions with very high accuracy with few hundred terms in our spectral expansion while in other numerical methods it would be a few orders of magnitude larger. This makes spectral methods very attractive because they facilitate the accessibility to very small error simulations, removes the bottleneck of the memory demand and also help in the computational speed because fewer points are needed for the computation. We have tested this idea with the ANETO library for simulations in AdS spacetimes and the gravitational collapse in an asymptotically flat spacetime with very promising results. This library has been developed as a direct result of this thesis and that can be downloaded as Free Software.

Physico-chemical properties of chondritic meteorites: clues on the origin and evolution of their parent bodies

Status: defended (06/07/2017)
Student: Carles Eduard Moyano Cambero
Supervised by: Josep Maria Trigo Rodríguez
University: Universitat Autònoma de Barcelona

In the first million years of the solar nebula, aggregation and melting of dust and presolar grains triggered the formation of the first solid materials of the Solar System. Among them, a variety of igneous glassy spherules known as chondrules can be found. These materials progressively aggregated…
Status: defended (06/07/2017)
Student: Carles Eduard Moyano Cambero
Supervised by: Josep Maria Trigo Rodríguez
University: Universitat Autònoma de Barcelona


In the first million years of the solar nebula, aggregation and melting of dust and presolar grains triggered the formation of the first solid materials of the Solar System. Among them, a variety of igneous glassy spherules known as chondrules can be found. These materials progressively aggregated together to form larger bodies, such as asteroids, planetesimals, and finally planets. Those that did not experience chemical segregation due to heating and melting of their materials, called undifferentiated bodies, still conserve some very primordial materials of the Solar System. The meteorites coming from these objects, known as chondrites from the chondrules that they contain, are studied in this thesis with a multidisciplinary approach, using several instrumental techniques to analyze their physico-chemical properties. Since retrieving samples directly from asteroids is a very complex concept, the terrestrial collections of meteorites become an available source of samples from these bodies. The information obtained from chondrites can be extrapolated to better understand the composition, structure, and physical properties of asteroids. Thus, the study of chondrites can be very helpful to practical applications such as the deflection through solid projectiles of asteroids that suppose a potential thread to life on Earth.

Collapse scenarios in magnetized star-forming regions

Status: defended (26/06/2017)
Student: Carmen Juárez Rodríguez
Supervised by: Josep M. Girart; Aina Palau
University: Universitat de Barcelona

Turbulence, magnetic fields and gravity driven flows are important for the formation of new stars. Although magnetic fields have been proven to be important in the formation of stars, only a few works have been done combining magnetic field and kinematic information.  Such studies are important to analyze…
Status: defended (26/06/2017)
Student: Carmen Juárez Rodríguez
Supervised by: Josep M. Girart; Aina Palau
University: Universitat de Barcelona

Turbulence, magnetic fields and gravity driven flows are important for the formation of new stars. Although magnetic fields have been proven to be important in the formation of stars, only a few works have been done combining magnetic field and kinematic information.  Such studies are important to analyze both gravity and gas dynamics and be able to compare them with the magnetic field. In this thesis we will combine dust polarization studies with kinematic analysis towards different star-forming regions. The aim of this thesis has been to study the physical properties of dense cores (at scales < 0.1 pc) from molecular line and dust emission, and to study the role of the magnetic field in their dynamic evolution. For this, we have used millimeter and submillimeter observational data.
 
The studies have been performed towards 3 different star-forming regions. The pre-stellar core FeSt 1-457, located in an isolated and magnetized environment in the Pipe nebula. The high-mass star-forming region NGC6334V, in a more advanced evolutionary state and in an environment surrounded by other massive star-forming regions. And L1287, a lower-mass region but with similar characteristics to NGC6334V, with presence of high-velocity gas and several centimeter and infrared sources.
 
The studies of the pre-stellar core FeSt1-457 and the massive region NGC6334V, show how the magnetic field has been overcome by gravity and is not enough to avoid the gravitational collapse. In addition, NGC6334V and the lower-mass region L1287 present very similar scenarios with the material converging from large scales (~0.1 pc) to the potential wells of both regions at smaller scales (~0.02 pc) through two dense gas flows separated by 2-3 km/s. In a similar scenario, FeSt1-457 is located just in the region where two dense gas structures, separated by 3 km/s, appear to converge.

Precipitation measurements with polarimetric GNSS Radio Occultations

Status: defended (22/06/2017)
Student: Ramon Padullés Rulló
Supervised by: Estel Cardellach Galí
University: Universitat de Barcelona

In 2009, the Spanish Ministry of Science and Innovation approved a proposal to modify the Global Positioning System (GPS) receiver and to allocate a Polarimetric (Pol) Radio Occultation (RO) antenna in the Spanish PAZ satellite. PAZ became an opportunity to test the new Pol-RO concept, which aims…
Status: defended (22/06/2017)
Student: Ramon Padullés Rulló
Supervised by: Estel Cardellach Galí
University: Universitat de Barcelona

In 2009, the Spanish Ministry of Science and Innovation approved a proposal to modify the Global Positioning System (GPS) receiver and to allocate a Polarimetric
(Pol) Radio Occultation (RO) antenna in the Spanish PAZ satellite. PAZ became an opportunity to test the new Pol-RO concept, which aims to capture ROs using a
two orthogonal linear polarization antenna. The experiment has been named Radio Occultations and Heavy Precipitation with PAZ (ROHP-PAZ). The objective is to
measure the phase difference between the horizontal and the vertical components of the incoming electromagnetic field that is induced by heavy precipitation flattened
raindrops. This effect, widely studied in the weather radar community, will be measured from space using GNSS signals for the first time with PAZ, which is
planned to be launched in 2017 (date yet to be confirmed).

The main objective of this new concept is to enhance the RO capabilities by providing vertical precipitation information along with the current standard RO
thermodynamic products (i. e. temperature, pressure and moisture). Until now, no other observing system has been able to provide simultaneous thermodynamic and
precipitation information under extreme conditions. The high vertical resolution, global coverage and all-weather capability properties of the RO observations com-
bined with vertical indication of precipitation intensity can be of great value for heavy rain characterization, and therefore for climate and weather forecasting and
research.

Within this context, the theoretical background for the technique, its feasibility and applications have been assessed in this dissertation. The theoretical basis
has been developed combining electromagnetic propagation theory and cloud and precipitation microphysics. Very detailed forward scattering simulations at L-band
have been obtained in order to relate the microphysical parameters with the expected Pol-RO observables. Feasibility has been addressed using coincident (in
space and time) RO profiles and space-based precipitation observations. Such simultaneous observations allow for the characterization of actual RO measurements
according to the coincident precipitation information, and allow us to obtain, for example, the noise level under precipitating scenarios. Finally, the applications have
been investigated through realistic end-to-end simulations of the Pol-RO observations, which provide the anticipated Pol-RO products for different precipitation
situations, regions, and seasons.

Before the launch of the satellite, a field campaign has been conducted with the aim of starting the characterization of the polarimetric measurements. The engi-
neering model of the PAZ antenna was placed at the top of a mountain peak in order to capture, for the first time, linear polarimetric GNSS signals at low graz-
ing angles. Although the geometry and the scenario are different from those that PAZ will be studying from space, this campaign has been useful to start identifying
the hardware internal effects and unexpected precipitation features that will affect the Pol-RO observations. These effects have been incorporated into the simulations,
providing valuable feedback to obtain more realistic Pol-RO products.

These exercises yielded several relevant results. The noise level analysis from actual RO observations sensing precipitation scenarios has allowed us to set a de-
tectability threshold for the technique, indicating that a high percentage of moderate to heavy precipitation events will be detected with PAZ. Nevertheless, the inte-
grated nature of the Pol-RO observable does not allow us to distinguish between the contributions from the rain’s intensity and extension, leaving an ambiguity in the
provided product. In an attempt to solve such ambiguity, a tomographic approach has been proposed, which has yielded promising theoretical results. Moreover, it
has been shown how the Pol-RO observables can be linked to physical precipitation parameters, such as the along-ray averaged rain rate, in a probabilistic way.
The end-to-end simulation has also revealed that the ionosphere will induce a non-negligible depolarization that will require calibration.

Besides providing feedback for the simulations, the data from the field campaign have also shown the first observational evidence that precipitation and other hy-
drometeors have a noticeable effect on the GNSS polarimetric signals. These effects have been compared with the simulations, showing agreement within an order of
magnitude. The collocated data has also been used to show the potential applications of Pol-ROs products. Comparison of model outputs with RO retrievals, in the presence of heavy rain, has shown discrepancies that will need further investigation, and Pol-RO data appears to be a well-fitted dataset for such studies.

Precipitation measurements with Polarimetric Radio Occultations

Status: defended (22/06/2017)
Student: Ramon Padullés
Supervised by: Estel Cardellach
University: Universitat Autònoma de Barcelona

The new Polarimetric Radio Occultation technique will be tested from space for the first time aboard the Spanish satellite PAZ. This concept aims to capture Radio Occultations using a two orthogonal linear polarization antenna, with the objective to measure the phase difference between the horizontal…
Status: defended (22/06/2017)
Student: Ramon Padullés
Supervised by: Estel Cardellach
University: Universitat Autònoma de Barcelona


The new Polarimetric Radio Occultation technique will be tested from space for the first time aboard the Spanish satellite PAZ. This concept aims to capture Radio Occultations using a two orthogonal linear polarization antenna, with the objective to measure the phase difference between the horizontal and the vertical components of the incoming electromagnetic field that is induced when heavy precipitation flattened raindrops are present in the ray-path. The theoretical background for the technique, its feasibility and applications have been assessed in this dissertation. Forward scattering simulations at L-band have been obtained combining electromagnetic propagation theory and cloud and precipitation microphysics. The feasibility has been proven using coincident (in space and time) RO profiles and space-based precipitation observations, indicating that a high percentage of moderate to heavy precipitation events will be detected with PAZ. The applications have been investigated through realistic end-to-end simulations. Using them, it has been shown how the Pol-RO observables can be linked to physical precipitation parameters, and some potential applications have been proposed. Furthermore, the ionosphere has been identified as a potential threat that will require calibration.

The Sun as a laboratory of particle physics

Status: defended (29/05/2017)
Student: Vinyoles, N.
Supervised by: Aldo Serenelli ; Jordi Isern Vilaboy
University: Universitat Autònoma de Barcelona

The Sun is by far the most studied and best-known star. The solar struc- ture, revealed by helioseismology and solar neutrinos, is well determined, and accurate solar models give information about the past, present and the future of the Sun. These solar models, or Standard Solar Models (SSM), are…
Status: defended (29/05/2017)
Student: Vinyoles, N.
Supervised by: Aldo Serenelli ; Jordi Isern Vilaboy
University: Universitat Autònoma de Barcelona

The Sun is by far the most studied and best-known star. The solar struc- ture, revealed by helioseismology and solar neutrinos, is well determined, and accurate solar models give information about the past, present and the future of the Sun. These solar models, or Standard Solar Models (SSM), are useful for describing the solar interior, and at the same time provide a deeper knowledge on other disciplines, such as stellar structure and evolution, particle physics and even non-standard particle physics. In fact, the large density of the solar core, its temperature and sheer size allow studying physics in environments that are hard to reproduce in earth-based experiments. Consequently, the Sun is a powerful laboratory to test non-standard particle physics. In particular, the Sun offers very interesting possibilities for studying weakly interacting light particles that arise from extensions of the Standard Model of particles to address some of the most pressing open questions in fundamental physics, such as the nature of dark matter. The main goal of this thesis is to use solar models to study the impact of different types of weakly interacting particles on the solar structure. Then, based on the structural changes they produce, the goal is to set the most restrictive bounds to properties of these particles using solar data from helioseismology and neutrinos. In order to pursue this goal, it is important to have realistic solar models that reproduce, as best as possible, the available observations. Motivated by this fact, this thesis presents a new generation of SSMs that includes recent updates on some important nuclear reaction rates and a more con- sistent treatment of the equation of state. Models also include updated errors computed using Monte-Carlo simulations and a novel and flexible treatment of opacity uncertainties based on opacity kernels, required in the light of recent theoretical and experimental works on radiative opacity. In fact, radiative opacities are proposed as one of the possible solutions to the solar abundance problem, that is the conflict between helioseismic predictions from the models and observations when new releases of so- lar composition are used to construct the SSM, in contrast to the good agreement obtained with the older compositions based on more simplistic one-dimensional model atmospheres. Therefore, to have an understand- ing of the current status of the radiative opacities and the corresponding uncertainties is important. For that reason, an exhaustive study of the available radiative opacity tables is presented in this thesis in the context of SSMs. Current uncertainties in the solar composition and opacities can be over- come for studies of particle physics that do not depend on a detailed knowledge of the solar interior composition. For this purpose, the Best Fit Model, a solar model that better reproduces the observations using realistic evolutionary inputs, is introduced. Finally, a new statistical analysis that combines helioseismology and solar neutrino observations (the 8B and 7Be fluxes) is presented, and it is used to place upper limits to the properties of non standard weakly interacting particles, and in particular, to axions, hidden photons and minicharged particles. This statistical analysis is based on the Best Fit Model and it includes theoretical and observational errors, accounts for tensions be- tween input parameters of solar models and can be easily extended to include other observational constraints. For the fist time, constraints on the properties of these particles are placed by using a method that com- bines both helioseismology and solar neutrino observations. Additionally, the fact that Best Fit Models are the basis of the statistical analysis results in more robust bounds independent on the solar abundance problem. The bounds obtained are: for the axion-photon coupling constant g10 < 4.1 at 3 CL, for the product of the kinetic mixing and mass of hidden photons, χm < 1.8·10E-12 V at 3 CL and for the chage of the minicharged particles, ε = 2.2·10−14 at 2 CL for mf = 0−25 eV. For all the cases, the results are the most restrictive solar bounds, being a factor of 2 better for axions and hidden photons. Moreover, the results obtained for hidden photons and minicharged particles are globally the most restrictive bounds.  

The Sun as a laboratory of particle physics

Status: defended (29/05/2017)
Student: Núria Vinyoles Vergés
Supervised by: Aldo Serenelli
University: Universitat Autònoma de Barcelona

The main goal of this thesis is to use solar models to study the impact of different types of weakly interacting particles on the solar structure. Then, based on the structural changes they produce, the goal is to set the most restrictive bounds to properties of these particles using solar data from…
Status: defended (29/05/2017)
Student: Núria Vinyoles Vergés
Supervised by: Aldo Serenelli
University: Universitat Autònoma de Barcelona

The main goal of this thesis is to use solar models to study the impact of different types of weakly interacting particles on the solar structure. Then, based on the structural changes they produce, the goal is to set the most restrictive bounds to properties of these particles using solar data from helioseismology and neutrinos. With that purpose, a new statistical analysis that combines helioseismology and solar neutrino observations is presented and it is used to place upper limits to the properties of non standard weakly interacting particles, and in particular, to axions, hidden photons and minicharged particles. The results are the most restrictive solar bounds, being approximately a factor two better than previous ones. Moreover, the results obtained for hidden photons and minicharged particles are globally the most restrictive bounds.

Cosmology with galaxy surveys

Status: defended (26/05/2017)
Student: Pujol, Arnau
Supervised by: Enrique Gaztañaga
University: Universitat Autònoma de Barcelona

Galaxy surveys are an important tool for cosmology. The distribution of galaxies allow us to study the formation of structures and their evolution, which are needed ingredients to study the evolution and content of the Universe. However, according to the standard model of cosmology, the so-called ΛCDM model,…
Status: defended (26/05/2017)
Student: Pujol, Arnau
Supervised by: Enrique Gaztañaga
University: Universitat Autònoma de Barcelona

Galaxy surveys are an important tool for cosmology. The distribution of galaxies allow us to study the formation of structures and their evolution, which are needed ingredients to study the evolution and content of the Universe. However, according to the standard model of cosmology, the so-called ΛCDM model, most of the matter is made of dark matter, which gravitates but does not interact with light. Hence, the galaxies that we observe from our telescopes only represent a small fraction of the total mass of the Universe. Because of this, we need to understand the connection between galaxies and dark matter in order to infer the total mass distribution of the Universe from galaxy surveys.  At large scales, galaxies trace the matter distribution. In particular, the galaxy density fluctuations at large scales are proportional to the underlying matter fluctuations by a factor that is called galaxy bias. This factor allows us to infer the total matter distribution from the distribution of galaxies, and hence knowledge of galaxy bias has a very important impact on our cosmological studies. This PhD thesis is focused on the study of galaxy and halo bias at large scales.  There are several techniques to study galaxy bias, in this thesis we focus on two of them. The first technique uses the fact that galaxy bias can be modelled from a galaxy formation model. One of the most common models is the Halo Occupation Distribution (HOD) model, that assumes that galaxies populate dark matter haloes depending only on the halo mass. With this hypothesis, and assuming a halo bias model, we can relate galaxy clustering with matter clustering and halo occupation. However, this hypothesis is not always accurate enough. We use the Millennium Simulation to study galaxy and halo bias, the halo mass dependence of halo bias, and its effects on galaxy bias prediction. We also study the local density dependence of halo bias, and we show that density constrains much more bias than mass. Another technique to study galaxy bias is by using weak gravitational lensing to directly measure mass in observations. Weak gravitational lensing is the field that studies the weak image distortions of galaxies due to the light deflections produced by the presence of a foreground mass distribution. Theses distortions can then be used to infer the total mass (baryonic and dark) distribution at large scales. We develop and study a new method to measure galaxy bias from the combination of weak lensing and galaxy density fields. The method consists on reconstructing the weak lensing maps from the distribution of the foreground galaxies. Bias is then measured from the correlations between the reconstructed and real weak lensing fields. We test the different systematics of the method and the regimes where this method is consistent with other methods to measure linear bias.
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