PhD::SPACE Thesis

List of the 8 PhD::SPACE Thesis completed by the fellows of the Program:


8. PhD Thesis: Spatial Coherence mapping of Structured Astrophysical Sources

Author: Tiago E. C. Magalhães (fellow 2014/B005)

ESPRESSO – (Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations), is a super-stable optical high resolution spectrograph for the combined Coude’ focus of the VLT. It can be operated by either one of the UTs or collecting the light from up to 4 UTs simultaneously. In this last configuration, light from 4 UT’s is incoherently combined, in order to increase the flux delivered to the spectrograph. One of the most important goals of ESPRESSO is the detection of exo-planets through spectral changes induced by their orbiting velocity.

The Wolf Effect (or Wolf shift) – is a frequency shift in the electromagnetic spectrum, that has been described as a new redshift mechanism. The phenomenon occurs in several closely related phenomena in radiation physics, with analogous effects occurring in the scattering of light. It was first predicted by Emil Wolf in 1987 and subsequently confirmed in the laboratory by Dean Faklis and George Morris in 1988.

Wolf and others have suggested that the Wolf effect as a possible non-cosmological redshift and also relevant to gravitational lensing. In optics, two non-Lambertian sources that emit beamed energy can interact in a way that causes a shift in the spectral lines. It is analogous to a pair of tuning forks with similar frequencies (pitches), connected together mechanically with a sounding board; there is a strong coupling that results in the resonant frequencies getting “dragged down” in pitch. The Wolf Effect requires that the waves from the sources are partially coherent.

This PhD proposal will address to what extent the Wolf effect should or should not be totally discarded in ESPRESSO, in spite of the split of light from a star into 4 telescopes, with the potential to create a kind of cross-correlation equivalent to source partial coherence or other forms of source correlations required by the Wolf Effect.

Advisors: José Manuel N. V. Rebordão (IA FCUL)

Fellowship: PD/00040/2012 – PD/BD/105952/2015 - FCT

University: Lisboa

Finish date: 2020/05/08


7. PhD Thesis: Towards a Comprehensive Understanding of Tiny Stars at the Near-Infrared Domain – Determining Stellar Parameters of FGK and M Dwarfs from Their APOGEE Spectra Using the Spectral Synthesis Method

Author: Pedro I. T. K. Sarmento (fellow 2016/B012)

Abstract: The project consists in a comprehensive study of the NIR spectroscopic properties of M dwarf stars by studying a large number of binary systems composed of a solar type star and an M dwarf secondary. It involves the development and test of techniques to derive accurate and reliable effective temperatures, surface gravities and metallicities from APOGEE near infrared spectra. The goal of the project is to provide not only reliable measurements of the fundamental parameters for the tiny stars, but also provide consistency and estimate the real precision of the different methods involving photometry, spectroscopy and evolutionary models. This comprehensive study is fundamental for the proper and accurate characterization of the larger stellar component of the Galaxy, which will improve our knowledge of their nature and the chemical and dynamical evolution of the Milky Way.

Advisors: Barbara Rojas-Ayala (Universidad Andrés Bello, Chile), Elisa Delgado Mena (IA-U.Porto)

Fellowship: PD/00040/2012 – PD/BD/128050/2016 - FCT

University: Porto

Finish date: 2020/03/24


6. PhD Thesis: Accretion Versus Outflow Regions Around Young Stellar Objects

Author: Raquel M. G. de Albuquerque (fellow 2015/B006)

Abstract: The evolution of young stellar objects (YSO’s) is critically dependent on the balance between the accretion of matter coming from their surrounding disks and the collimated jets outflowing around their poles. The magnetic field plays a crucial role in both channelling these flows of plasma and controlling their dynamics and energetics.

Modelling the overall stellar formation on all scales from the global collapse and jet feedback down to the inner disk star interaction is presently out of reach. Thus, we concentrate on the central magnetospheric accretion connected to the stellar outflow in class II objects like classical T Tauri stars.

T Tauri stars are well observed objects. In such evolved protostars, observational evidences suggest that jets originate in the inner regions of the accretion disk or even closer in the magnetosphere of the central source.

Thus it is crucial to understand the connection between these two regions, one controlling accretion and the other responsible for the jet.

In this project we aim firstly, at modelling the structure of the magnetosphere of the YSO in which accretion takes place using existing numerical codes like PLUTO. This approach will enable a blending between accretion and outflow regions around YSO’s. The advantage of such numerical approach is to enable the study of time dependent solutions. Secondly, the solutions found will be used in a separate radiative transfer model to compute line profiles and test the consistency of the MHD solutions with observations.

Advisors: João Lima (IA-U.Porto & FCUP), Jorge Filipe Gameiro (IA-U.Porto & FCUP), Christophe Sauty (Observatoire de Paris)

Fellowship: PD/00040/2012 – PD/BD/113745/2015 - FCT

University: Porto

Finish date: 2020/02/17


5. PhD Thesis: Asteroseismic Characterization of Exoplanet-host Stars in Preparation for NASA’s TESS and ESA’s PLATO Space Missions

Author: Benard Nsamba (fellow 2015/B008)

Abstract: New insights on stellar evolution and stellar interiors physics are being made possible by asteroseismology, the study of stars by the observation of their natural, resonant oscillations. Asteroseismology is proving to be particularly significant for the study of solar-type stars, in great part due to the exquisite data that have been made available by NASA’s Kepler space telescope. The future looks even brighter, with NASA’s TESS and ESA’s PLATO space missions promising to revolutionize the field and increase the number of stars with detected oscillations by several orders of magnitude. The information contained in stellar oscillations allows the internal stellar structure to be constrained to unprecedented levels, while also allowing fundamental stellar properties (e.g., mass, radius, and age) to be precisely determined. In anticipation of the flood of observations from future space missions, the main goal of this project is to develop and test state-of-the-art asteroseismic techniques for the estimation of fundamental stellar properties. Particular attention will be focused on calibrating the determination of age, due to the strong dependence this quantity has on stellar physics. The goal is to address the critical requirement to obtain precise estimates of age for stars at different phases of evolution and/or with planetary systems where signatures of life are to be observed. A very important component of this project will be to understand the systematics on the derived properties that arise from changes in the input physics and the effects introduced by different evolutionary and pulsation codes. The implications of this project will be far-reaching. Not only should it provide important contributions to theories of stellar structure, stellar dynamics and evolution, but it will also play an important role in the characterization of planet-candidate host stars and their planetary systems.

Advisors: Mário J. P. F. G. Monteiro (IA-U.Porto & FCUP), Tiago L. Campante(University of Birmingham)

Fellowship: PD/00040/2012 – PD/BD/113744/2015 - FCT

University: Porto

Finish date: 2020/01/16


4. PhD ThesisThe Nature and Formation History of Bulges in Galaxies

Author: Íris P. Breda (fellow 2014/B004)

Abstract: The featureless appearance of bulges in Hubble-type galaxies has for decades sustained the view that these high-surface brightness spheroidal components are largely “simple” in terms of their assembly history, formed on a short timescale early on, and having experienced little evolution over the past several Gyr. However, our early understanding of bulges as essentially scaled ellipticals has undergone a substantial revision over the last years. It is now recognized that central luminosity components that closely resemble classical bulges can also form over much longer timescales through disk instabilities and ensuing star forming activity at the centers of galaxies. The nature and formation history of these “pseudo-bulges” is enigmatic and of considerable relevance to our understanding of the structural and spectrophotometric evolution of galaxies in general.

This PhD project aims at a spatially resolved investigation of the star formation- and chemical enrichment history of pseudo-bulges. A unique aspect of its methodology is the combined application of surface photometry and spectral population synthesis to a large sample of galaxies from the Calar Alto Legacy Integral Field spectroscopy Area survey (http://califa.caha.es) with the goal of conclusively addressing the star formation history (SFH) and the chemical abundance patterns of pseudo-bulges. One of the central questions to be investigated is whether pseudo-bulges form in a quasi-continuous manner over several Gyrs of galactic evolution and their SFH can be parametrized through a simple functional form involving integral or structural properties of their host galaxy (e.g., total stellar mass; central surface brightness and exponential scale length of the underlying disk). Additionally, this project will include a comparative study of pseudo-bulges and classical bulges with the goal of the identification of new empirical discriminators between them and yield robust observational constraints to theoretical models of pseudo-bulge formation and evolution.

Advisors: Polychronis Papaderos (IA-U.Porto), Jean Michel Gomes (IA-U.Porto)

Fellowship: PD/00040/2012 – SFRH/BD/52707/2014 - FCT

University: Porto

Finishing date: 2019/05/03


3. PhD ThesisTowards the Near-infrared Detection of Exoplanets

Author: Jason J. Neal (fellow 2014/B001)

Abstract: The search for extrasolar planets has as one of its crowns the study of exo-atmospheres. This goal has been shaping Science and scientific instrumentation for the last decade. In this project we provide a gradual approach to developing and perfecting of methodologies required for the detection and characterization of exo-atmospheres.

Building up on current in-house knowledge, we propose to develop the methodologies required for the extraction of the maximum of information, and in particular the minute signals of exoplanets from nIR spectra. The risk of this ambitions program will be minimized by a thorough and gradual pathway to this objective. The intermediary tasks and objectives have been laid out and a significant amount of observational data has already been acquired and is fully available to be explored from day one.

The expertise gained through this project will give the PhD the ability to efficiently reduce and explore nIR spectroscopic data, an emerging field in astronomy, in particular in the domain of extrasolar planets. The results of this project will be used for the science verification of two upgraded infrared instruments: CRIRES+ and VISIR (both at the VLT-ESO).

Advisors: Nuno C. Santos (CAUP), Cláudio Melo (ESO), Pedro Figueira (CAUP)

Fellowship: PD/00040/2012 – SFRH/BD/52700/2014 - FCT

University: Porto

Finish date: 2019/05/01


2. PhD Thesis: Star-Forming Galaxies’ Structural Evolution Across Cosmic Time and Environment

Author: Ana S. P. Afonso (fellow 2014/B003)

Abstract: From early times we find galaxies undergo major transformations which encode the physics of competing processes that shape the galaxy evolution (gas accretion, star formation, mergers, feedback processes, among others).

In this thesis, I study the galaxy structural evolution across cosmic time and environment. By linking the distant and local Universe, I show that the star-forming galaxies (SFGs, selected by their Ha emission) have typical disk-like morphologies and grow by a factor of 2-3 from z~2. I also attempt to measure the impact of cosmological dimming on the perceived galaxy evolution and find that it has little-to-no impact on the measured structural parameters.

By tracing Lya-selected galaxies (LAEs), I find that these galaxy population show little-to-no evolution in size (~1 kpc) across a ~3 Gyr period in the early Universe (2<z<6). However, when compared to other SFGs, this population deviates in morphological properties only at z<4. I hypothesize that LAEs and other SFGs trace the dominant galaxy population at earlier times while, later, LAEs only trace the smaller, younger, and less massive star-forming population.

By targeting a region in and around a superstructure at z~0.8 in the COSMOS field, I find that both stellar mass and environment impact the galaxy evolution. The environment has a pivotal role in the observed star formation activity in galaxies since one can witness an increase of the quiescent fraction of galaxies towards high-density regions. In the intermediate density regions, there is an increase of the star formation which can be linked with the change of the galaxy morphology from disks to ellipticals from the low- to high-density regions. The electron density decreases from low to high densities for galaxies up to 10^{10.75} M_sun. However, massive galaxies tend to point towards a different and opposite relation for which the possible ionizing mechanisms remain unclear.

Publications: Paper 1 | Paper 2 | Paper 3

Advisors: David Sobral (IA U.Lisboa), José Afonso (DF/FCUL & IA U.Lisboa)

Fellowship: PD/00040/2012 – SFRH/BD/52706/2014 - FCT

University: Lisboa

Finish date: 2019/01/07


1. PhD Thesis: Cosmic Paleontology: Searching for Superstrings

Author: Ivan Rybak (fellow 2014/B002)

Abstract: Cosmic strings as hypothetical objects passed through ups and downs of scientific community attention. They were predicted in the 1970s and reborn in the 2000s due to superstring theory implementation to the description of the early universe. The connection with superstring theory led to the new properties and a new name – cosmic superstrings, which remains the subject of theoretical and observational researchers.

This thesis is aimed at clarifying questions of the cosmic (super)string network evolution and subsequent observational predictions. The first chapter reviews the main principles and motivations to study cosmic (super)strings.

The second chapter is dedicated to the description of a semi-analytic velocity-depend one scale (VOS) model in order to study the evolution of topological defect networks. This chapter provides a thorough comparison between numerical simulations and the semi-analytic approaches. We investigate a broad range of regimes, conditions and possible errors to test the VOS model validity.

In the third chapter we develop our approach to go beyond the standard cosmic strings and include possible extensions to the strings dynamics. The developed general treatment applies to wiggly strings and strings with chiral currents. The latter is generalized for an arbitrary Lagrangian, and the exact solution in Mikowski space for such strings is found. Also we generalize the study of the scaling regimes for string networks evolution in the framework of the VOS model.

The properties of cosmic superstrings are discussed in the forth chapter. We consider dynamics and conditions for the production of string junctions both for superstrings in an expanding universe and for superstrings with currents. In addition, we suggest an approach for integrating junction dynamics in the VOS model. For this case we revisit the averaged treatment of strings collisions and averaged correlation function evolution. This leads to the formulation of the generalized VOS model with dynamical junctions.

The possible observational outcomes of this work are studied in chapter five. We review the main observational opportunities to confirm/restrict the existence of cosmic (super)strings. In particular, we revisit the cosmic microwave background anisotropies produced by cosmic strings. We generalize this study for the case of strings with non-trivial internal structure, i.e. with presence of currents. Specifically we study anisotropies caused by wiggly and superconducting (chiral) cosmic strings. In addition, we explore the stochastic gravitational background from cosmic strings and highlight the main adjustments that should be implemented in order to extend the study for non-conventional cosmic strings.

The outlook and conclusions end the thesis in chapter six.

Publications: Paper 1Paper 2

Advisors: Carlos J. A. P. Martins (IA-U.Porto), Anastasios Avgoustidis(University of Nottingham)

Fellowship: PD/00040/2012 – SFRH/BD/52699/2014 – FCT

University: Porto

Finish date: 2018/12/18