Tracing the element enrichment in the Universe requires to understand the element production in stellar models which is not well understood, in particular at low metallicity. In this thesis a variety of nucleosynthesis processes in stellar models across initial masses and metallicities is investigated and their relevance for chemical evolution explored. Stellar nucleosynthesis is investigated in asymptotic giant branch (AGB) models and massive star models with initial masses between 1 M⊙ and 25 M⊙ for metal fractions of Z = 0.02, 0.01, 0.006, 0.001, 0.0001. A yield grid with elements from H to Bi is calculated. It serves as an input for chemical evolution simulations. AGB models are computed towards the end of the AGB phase and massive star models are calculated until core collapse followed by explosive core-collapse nucleosynthesis. The simulations include convective boundary mixing in all AGB star models and feature efficient hot-bottom burning and hot dredge-up in AGB models as well the predictions of both heavy elements and CNO species under hot-bottom burning conditions. H-ingestion events in the low-mass low-Z AGB model with initial mass of 1M⊙ at Z = 0.0001 result in the production of large amounts of heavy elements. In super-AGB models H ingestion could potentially lead to the intermediate neutron-capture process. To model the chemical enrichment and feedback of simple stellar populations in hydrodynamic simulations and semi-analytic models of galaxy formation the SYGMA module is created and its functionality is verified through a comparison with a widely adopted code. A comparison of ejecta of simple stellar populations based on yields of this work with a commonly adopted yield set shows up to a factor of 3.5 and 4.8 less C and N enrichment from AGB stars at low metallicity which is attributed to complete stellar models, the modeling of the AGB stage and hot-bottom burning in super- AGB stars. Analysis of two different core-collapse supernova fallback prescriptions show that the total amount of Fe enrichment by massive stars differs by up to two at Z = 0.02. Insights into the chemical evolution at very low metallicity as motivated by the observations of extremely metal poor stars require to understand the H-ingestion events common in stellar models of low metallicity. The occurrence of H ingestion events in super-AGB stars is investigated and identified as a possible site for the production of heavy elements through the intermediate neutron capture process. The peculiar abundance of some C-Enhanced Metal Poor stars are explained with simple models of the intermediate neutron capture process. Initial efforts to model this heavy element production in 3D hydrodynamic simulations are presented. For the first time the nucleosynthesis of interacting convective O and C shells in massive star models is investigated in detail. 1D calculations based on input from 3D hydrodynamic simulations of the O shell show that such interactions can boost the production of odd-Z elements P, Cl, K and Sc if large entrainment rates associated with O-C shell merger are assumed. Such shell merger lead in stellar evolution models to overproduction factors beyond 1 dex and p-process overproduction factors above 1 dex for 130,132Ba and heavier isotopes. Chemical evolution models are able to reproduce the Galactic abundance trends of these odd-Z elements if O-C shell merger occur in more than 50% of all massive stars. / Graduate

Submitted by Jadson Francisco de Jesus SILVA (jadson@uefs.br) on 2018-07-12T22:44:48Z No. of bitstreams: 1 Disserta??o Andr? VErs?o Final + Ficha Catalogr?fica (1).pdf: 5597865 bytes, checksum: 47b14bdb3b0cc0a2e5b68f0252c589ff (MD5) / Made available in DSpace on 2018-07-12T22:44:48Z (GMT). No. of bitstreams: 1 Disserta??o Andr? VErs?o Final + Ficha Catalogr?fica (1).pdf: 5597865 bytes, checksum: 47b14bdb3b0cc0a2e5b68f0252c589ff (MD5) Previous issue date: 2017-12-20 / Funda??o de Amparo ? Pesquisa do Estado da Bahia - FAPEB / In this dissertation we present as proposal the teaching of Star Evolution in the discipline sciences for students of the 9th year of elementary school. From this series, students have their first contact with study of the atomic structure, periodic table, chemical reactions, among other topics of physics and chemistry. Using Stellar Evolution, in an interdisciplinary way, we were able to discuss these obligatory contents and, in addition, we present new knowledge such as: the synthesis of chemical elements, nuclear reactions and other topics of Astronomy and Astrophysics. This approach allows students to gain access to modern physics content guided by NCPs, which in turn has been neglected in school curricula. Based on Chevallard's Didactic Transposition Theory, we analyze the aspects that make it possible to adapt the Wise Knowledge, Star Evolution, to the reality of basic education. To do so, we did initially a quantitative survey in textbooks, evaluating how the knowledge related to Astronomy has been presented. The second step was to perform a pre-test to evaluate students' prior knowledge, and then compare them with a post-test. The transposition of these contents indicated that the Star Evolution in conjunction with the science discipline makes learning more playful and meaningful, being a perfectly feasible proposal, as well as showing the results. In addition, we constructed a didactic game, an educational product that can be used as a tool that helps students learn the evolutionary processes of the stars. / Nesta disserta??o apresentamos como proposta o ensino de Evolu??o Estelar na disciplina ci?ncias para alunos do 9? ano do ensino fundamental. A partir desta s?rie, os alunos tem seu primeiro contato com estudo da estrutura at?mica, tabela peri?dica, rea??es qu?micas, entre outros t?picos de f?sica e qu?mica. Usando a Evolu??o Estelar, de um modo interdisciplinar, pudemos discutir estes conte?dos obrigat?rios e, al?m disso, apresentamos novos conhecimentos como: a s?ntese dos elementos qu?micos, rea??es nucleares e outros temas de Astronomia e Astrof?sica. Esta abordagem permite que os alunos tenham acesso a conte?dos de f?sica moderna orientados pelos PCN, que por sua vez, tem sido ainda negligenciado nos curr?culos escolares. Com base na Teoria da Transposi??o Did?tica de Chevallard, analisamos os aspectos que tornam poss?vel adequar o Saber S?bio e a Evolu??o Estelar, ? realidade do ensino b?sico. Para tanto, fizemos inicialmente um levantamento quantitativo em livros did?ticos, avaliando como os Saberes ligados a Astronomia tem sido apresentados. O segundo passo foi a realiza??o de um pr?-teste para avaliarmos o conhecimento pr?vio dos estudantes, e posteriormente compar?-los com um p?s-teste. A transposi??o destes conte?dos indicou que a Evolu??o Estelar em conjunto com a disciplina ci?ncias torna o aprendizado mais l?dico e significativo, sendo uma proposta perfeitamente vi?vel, assim como mostram os resultados. Ademais, constru?mos um jogo did?tico, produto educacional que pode ser usado como ferramenta que auxilie os estudantes no aprendizado dos processos evolutivos das estrelas.

<p>Chemical evolution addresses the problem of the formation of the chemical elements and their evolution throughout the history of the universe. This thesis discusses in particular the chemical evolution in the young universe and what we may learn from the observations of the oldest stars. The present day production of carbon in the Galaxy is also discussed. Interstellar media of young, metal-poor, star-forming systems are expected to show large chemical abundance inhomogeneities due to local supernova explosions. These inhomogeneities are reflected in the surface abundances of the population of longlived, low-mass stars. A stochastic model of the chemical evolution in such systems is presented and used to study the metallicity distribution and the scatter in chemical abundance ratios. The model takes into account mixing of the enriched material by turbulent motions and cloud collisions in the interstellar medium as well as infall of pristine matter. The predicted metallicity distribution shows, in accordance with observations of extreme Pop II strars in the Galactic halo, a distinct cut-off at [Fe/H]~-4. However, the fraction of stars below [Fe/H]=-4 agrees with observatrion only if a population of metal-free stars (Pop III) was never able to form. The predicted scatter in abundance ratios is demonstrated to be crucially dependent on the as yet uncertain supernova yields and the relatively small star-to-star scatter is tentatively explained by the averaging of a large number of contributing supernovae and by the selection effects favouring contributions from supernovae in a certain mass range for the most metal-poor stars. Furthermore, stars enriched by one single supernova are predicted to be found in very narrow sequences in the abundance ratio diagrams (so called A/A diagrams). Verification of the existence of such features, called single supernova sequences, is observationally challenging. Abundance analysis of carbon was performed in a large sample of solar-type stars in the Galactic disk using the forbidden [C I] line at 8727 Å. A comparison between the relation of [C/O] with metallicity for the Galactic stars and that of dwarf irregular galaxies suggests that large amounts of carbon are produced today by massive, so called Wolf-Rayet stars. Low-mass stars are less important. This was also demonstrated by modelling the chemical evolution of carbon.</p>

Chemical evolution addresses the problem of the formation of the chemical elements and their evolution throughout the history of the universe. This thesis discusses in particular the chemical evolution in the young universe and what we may learn from the observations of the oldest stars. The present day production of carbon in the Galaxy is also discussed. Interstellar media of young, metal-poor, star-forming systems are expected to show large chemical abundance inhomogeneities due to local supernova explosions. These inhomogeneities are reflected in the surface abundances of the population of longlived, low-mass stars. A stochastic model of the chemical evolution in such systems is presented and used to study the metallicity distribution and the scatter in chemical abundance ratios. The model takes into account mixing of the enriched material by turbulent motions and cloud collisions in the interstellar medium as well as infall of pristine matter. The predicted metallicity distribution shows, in accordance with observations of extreme Pop II strars in the Galactic halo, a distinct cut-off at [Fe/H]~-4. However, the fraction of stars below [Fe/H]=-4 agrees with observatrion only if a population of metal-free stars (Pop III) was never able to form. The predicted scatter in abundance ratios is demonstrated to be crucially dependent on the as yet uncertain supernova yields and the relatively small star-to-star scatter is tentatively explained by the averaging of a large number of contributing supernovae and by the selection effects favouring contributions from supernovae in a certain mass range for the most metal-poor stars. Furthermore, stars enriched by one single supernova are predicted to be found in very narrow sequences in the abundance ratio diagrams (so called A/A diagrams). Verification of the existence of such features, called single supernova sequences, is observationally challenging. Abundance analysis of carbon was performed in a large sample of solar-type stars in the Galactic disk using the forbidden [C I] line at 8727 Å. A comparison between the relation of [C/O] with metallicity for the Galactic stars and that of dwarf irregular galaxies suggests that large amounts of carbon are produced today by massive, so called Wolf-Rayet stars. Low-mass stars are less important. This was also demonstrated by modelling the chemical evolution of carbon.

The work of this dissertation will study various aspects of the dynamics of compact objects using numerical simulations.We consider BH dynamics within two modified or alternative theories of gravity. Within a family of Einstein-Maxwell-Dilaton-Axion theories, we find that the GW waveforms from binary black hole (BBH) mergers differ from the standard GW waveform prediction of GR for especially large axion values. For more astrophysically realistic (i.e. smaller) values, the differences become negligible and undetectable. Weestablish the existence of a well-posed initial value problem for a second alternative theory fo gravity (quadratic gravity) and demonstrate in spherical symmetry that a linear instability is effectively removed on consideration of the full nonlinear theory.We describe the key components and development of a code for studying BBH mergers for which the mass ratio of the binaries is not close to one. Such intermediate mass ratio inspirals (IMRIs) are much more difficult to simulate and present greater demands on resolution, distributed computing, accuracy and efficiency. To this end, we present a highly-scalable framework that combines a parallel octree-refined adaptive mesh with a wavelet adaptive multiresolution approach. We give results for IMRIs with mass ratios up to 100:1. We study the ejecta from BNS in Newtonian gravity. Using smoothed particle hydrodynamics we develop and present the highly scalable FleCSPH code to simulate such mergers. As part of the ejecta analysis, we consider these mergers and their aftermath as prime candidates for heavy element creation and calculate r-process nucleosynthesis within the post-merger ejecta. Lastly we consider a non-standard, yet increasingly explored, interaction between a BH and a NS that serves as a toy model for primordial black holes (PBH) and their possible role as dark matter candidates. We present results from a study of such systems in which a small BH forms at the center of a NS. Evolving the spherically symmetric system in full GR, we follow the complete dynamics as the small BH consumes the NS from within. Using numerical simulations, we examine the time scale for the NS to collapse into the PBH and show that essentially nothing remains behind. As a result, and in contradiction to other claims in the literature, we conclude that thisis an unlikely site for ejecta and nucleosynthesis, at least in spherical symmetry.

The ejecta following the collision and merging of two neutron stars (kilonova) are currently considered promising sites for nucleosynthesis of r-process elements. Since the observed kilonova in 2017, GW2017817, astrophysicists have been working to analyze the collected electromagnetic spectra, trying to identify r-process elements. However, a lack of fundamental atomic data has been holding the efforts back. Motivated by spectral modelling of kilonovae out of equilibrium, this project aims to create a tool that uses line lists of spectroscopic accuracy from the Vienna Atomic Line Database (VALD) to generate energy level lists automatically for any ion. VALD in particular is used because it has wavelengths accurate enough for line identification purposes. The resulting level lists are compared to equivalent level lists from the database managed by the National Institute for Standards and Technology (NIST), in order to both ensure that the tool worked, and identify discrepancies between the databases. A number of problems with the VALD data were identified, mostly resulting in duplicate and missing energy levels. Finally, we also test the data in computations of kilonova expansion opacities in a complete solar r-process abundance mixture. Further work is needed to evaluate how damaging these problems are when modelling kilonovae, and when necessary remedy said problems. / Ejektat från en kollision och sammanslagning av två neutronstjärnor (kilonova) betraktas som lovande platser för nukleosyntes av r-processämnena. Sedan den observerade kilonovan år 2017, GW2017817, har astrofysiker försökt analysera de insamlade elektromagnetiska spektrumen för att försöka identifiera r-processämnen. Denna analys har dock hindrats på grund av en brist på fundamental atomisk data. Motiverat av spektralmodellering av kilonovor utanför ekvilibrium, syftar detta projekt på att utveckla ett verktyg för att utifrån spektroskopiskt noggranna linjelistor från Vienna Atomic Line Database (VALD) gererera listor över energinivåer automatiskt för godtycklig jon. VALD används på grund av att den har våglänger som är noggranna nog för linjeidentifiering. De resulterande nivålistorna jämförs med motsvarande nivålistor från databasen som sköts av National Institute for Standards and Technology (NIST), detta för att dels säkerställa att verktyget fungerade, dels identifiera skillnader mellan databaserna. Ett antal problem med VALD identifierades, vilka oftast resulterade i dubbletter eller avsaknad av energinivåer. Slutligen testades datan i beräkningar av kilonova-expansionsopaciteter i en komplett solär r-process-ämnesblandning. Vidare arbete krävs för att evaluera hur skadliga dessa problem är för modellering av kilonovor, och vid behov åtgärda problemen.

<p>The astrophysical ³⁰P(<em>p</em>,<em>γ</em>)³¹S reaction rate is a key quantity used in both classical nova and type I X-ray burst models that predict isotopic abundances produced during nucleosynthesis in the outburst. Currently, uncertainties in ³¹S structure parameters lead to a variation in the reaction rate by a factor of 20 at nova temperatures causing predicted isotopic abundance ratios in the Si-Ar mass region to vary by factors of up to 4. The ³⁰P(<em>p</em>,<em>γ</em>)³¹S reaction rate can be determined indirectly by measuring transfer reactions populating excited states in ³¹S. Nuclear structure information for ³¹S resonant states above the proton threshold of 6131 keV and within the Gamow window that contribute most significantly to the reaction rate can be used to re-evaluate the rate for nova and type I X-ray burst temperatures and reduce current uncertainties. We have performed an experiment in order to study the level structure of ³¹S via the ³²S(<em>d</em>,<em>t</em>)³¹S single-nucleon transfer reaction using the MP tandem accelerator and Q3D magnetic spectrograph at MLL in Munich, Germany. Excited states of ³¹S in the 6-7 MeV region were observed and spin-parity constraints have been suggested. In this work we describe the experimental setup, data analysis and results for both experiments and provide recommendations for further investigation of the ³⁰P(<em>p</em>,<em>γ</em>)³¹S astrophysical reaction rate.</p> / Master of Science (MSc)

<p>This thesis describes a new experimental approach that has been designed to determine the alpha-branching ratio of the 4.03-MeV excited state of Ne-19. A precise measurement of this quantity is needed to reduce reaction rate uncertainties in Type I x-ray bursts models. The alpha-branching will be measured by the detection of the beta+ activity of the associated alpha-decay product. This activity method has been modelled using two separate simulations. The first, a Monte Carlo code to simulate the reaction process and energy distributions of the decay products. Secondly, a GEANT4 simulation was created to predict the detector response to the O-15 beta+ activity. Along with the simulations two NaI(Tl) detectors, which were customized to this experiment's geometric constraints, have been tested and their response and resolution have been determined. The results of this work will be used to refine the experimental setup such that the proposed test run and eventual alpha-branching ratio measurement of the 4.03-MeV state will be successful. With the results of the simulations and subsequent yield calculations, it has been found that reasonable statistical significance in the O-15 yield from the 4.03-MeV excited state in Ne-19 can be achieved within 10 days of beamtime.</p> / Master of Science (MSc)

Carbon is the basis for life, as we know it, but its origin is still largely unclear. Carbon-rich Asymptotic Giant Branch (AGB) stars (carbon stars) play an important rôle in the cosmic matter cycle and may contribute most of the carbon in the Galaxy. In this thesis it is explored how the dust-driven mass loss of these stars depends on the basic stellar parameters by computing a large grid of wind models. The existence of a critical wind regime and mass-loss thresholds for dust-driven winds are confirmed. Furthermore, a steep dependence of mass loss on carbon excess is found. Exploratory work on the effects of different stellar metallicities and the sizes of dust grains shows that strong dust-driven winds develop also at moderately low metallicities, and that typical sizes of dust grains affect the wind properties near a mass-loss threshold. It is demonstrated that the mass-loss rates obtained with the wind models have dramatic consequences when used in models of carbon-star evolution. A pronounced superwind develops soon after the star becomes carbon rich, and it therefore experiences only a few thermal pulses as a carbon star before the envelope is lost. The number of dredge-up events and the thermal pulses is limited by a self-regulating mechanism: each thermal pulse dredges up carbon, which increases the carbon excess and hence also the mass-loss rate. In turn, this limits the number of thermal pulses. The mass-loss evolution during a thermal pulse (He-shell flash) is considered as an explanation of the observations of so-called detached shells around carbon stars. By combining models of dust-driven winds with a stellar evolution model, and a simple hydrodynamic model of the circumstellar envelope, it is shown that wind properties change character during a He-shell flash such that a thin detached gas shell can form by wind-wind interaction. Finally, it is suggested that carbon stars are responsible for much of the carbon in the interstellar medium, but a scenario where high-mass stars are major carbon producers cannot be excluded. In either case, however, the carbon abundances of the outer Galactic disc are relatively low, and most of the carbon has been released quite recently. Thus, there may neither be enough carbon, nor enough time, for more advanced carbon-based life to emerge in the outer Galaxy. This lends some support to the idea that only the mid-part of the Galactic disc can be a “Galactic habitable zone”, since the inner parts of the Galaxy are plagued by frequent supernova events that are presumably harmful to all forms of life.

Observations from the 1930s until the present have established the existence of dark matter with an abundance that is much larger than that of luminous matter. Because none of the known particles of nature have the correct properties to be identified as the dark matter, various exotic candidates have been proposed. The neutralino of supersymmetric theories is the most promising example. Such cold dark matter candidates, however, lead to a conflict between the standard simulations of the evolution of cosmic structure and observations. Simulations predict excessive structure formation on small scales, including density cusps at the centers of galaxies, that is not observed. This conflict still persists in early 2007, and it has not yet been convincingly resolved by attempted explanations that invoke astrophysical phenomena, which would destroy or broaden all small scale structure. We have investigated another candidate that is perhaps more exotic: Lorentz-violating dark matter, which was originally motivated by an unconventional fundamental theory, but which in this dissertation is defined as matter which has a nonzero minimum velocity. Furthermore, the present investigation evolved into the broader goal of exploring the properties of Lorentz-violating matter and the astrophysical consequences – a subject which to our knowledge has not been previously studied. Our preliminary investigations indicated that this form of matter might have less tendency to form small-scale structure. These preliminary calculations certainly established that Lorentz-violating matter which always moves at an appreciable fraction of the speed of light will bind less strongly. However, the much more thorough set of studies reported here lead to the conclusion that, although the binding energy is reduced, the small-scale structure problem is not solved by Lorentz-violating dark matter. On the other hand, when we compare the predictions of Lorentz-violating dynamics with those of classical special relativity and general relativity, we find that differences might be observable in the orbital motions of galaxies in a cluster. For example, galaxies – which are composed almost entirely of dark matter – observed to have enlarged orbits about the cluster center of mass may be an indication of Lorentz violation.