Asymptotic Giant Branch (AGB) stars are stars with low- to intermediate mass in a late stage in their stellar evolution. An important feature of stellar evolution is the ongoing nucleosynthesis, the creation of heavier elements. Unlike main sequence stars, the AGB stars have a thick convective envelope which makes it possible to dredge-up the heavier fused elements from the stellar core to its surface. AGB stars are also pulsating variable stars, meaning the interior expands and contracts, causing the brightness to fluctuate. These pulsations will also play a major role in the mass loss observed in these stars. The mass loss is caused by stellar winds that accelerate gas and dust from the surface of these stars and thereby chemical enrich the interstellar medium. It is important to understand the properties of these pulsations since they play a key role in how stellar winds are produced and then enrich the galaxy with heavier synthesized elements. These pulsation periods can be observed with their corresponding Light-Curves, where the periodic motion of the brightness can be clearly seen. The main goal with this project is to calculate these pulsation periods for different AGB stars and compare these values with the periods listed in the General Catalogue of Variable Stars (GCVS). The comparison between these values gives a better understanding of methods of determining these periods and the uncertainties that follow. / Asymptotiska jättegrenen är en del av slutstadiet för låg- till medelmassiva stjärnor (AGB stjärnor). Ett viktigt kännetecken hos stjärnutvecklingen är den pågående nukleosyntesen, sammanslagningen av tyngre ämnen i stjärnans inre. Till skillnad mot stjärnor på huvudserien har AGB stjärnor ett tjockt konvektivt lager som gör det möjligt att dra upp dessa nybildade ämnen till stjärnans yta. AGB stjärnor är pulserande variabla stjärnor där variationer i stjärnans radie gör att ljusstyrkan varierar. Dessa pulsationer kommer även att spela en viktig roll för den massförlust som observeras hos dessa stjärnor. Massförlusten orsakas av stjärnvindar som accelererar gas och stoft från stjärnans yta och därmed kemiskt berikar det interstellära mediet. Det är viktigt att förstå dessa pulsationer eftersom de är en viktig komponent för hur stjärnvindar uppstår och sedan berikar galaxer med tyngre ämnen. Dessa pulsationsperioder kan studeras genom att observera stjärnornas ljuskurvor, där man tydligt ser det periodiska beteendet hos ljusstyrkan. Det huvudsakliga målet med detta projekt är att beräkna dessa perioder för olika AGB stjärnor och att sedan jämföra dem med värden från General Catalogue of Variable Stars (GCVS). Jämförelsen mellan dessa värden ger en bättre förståelse för metoderna som används för att bestämma dessa perioder och hur osäkra dessa värden är.

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Thesis (Ph. D.)--University of Washington, 2006. / Vita. Includes bibliographical references (p. 139-145).

Despite being observed since the XVIIIth century, Algol systems and related objects are<p>still rather poorly understood. We know that they are composed by a generally B-A main sequence<p>star and a lighter but more evolved companion star. This paradox is explained by the transfer of mass<p>between the two stars, but new problems arose. In particular, I studied the mass-transfer driven spin-<p>up of the accreting star that drives the star to critical rotation and the puzzling, indirectly observed, non-conservative evolution. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

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Some of the brightest and most energetic events in the Universe are associated with the death of stars. These stellar deaths power transient electromagnetic emission which are routinely observed on Earth. This dissertation presents our research on various such transients. Its topics includes, supernova remnants, kilonovae, gamma-ray bursts (GRBs): The "long'' type produced from core-collapse supernovae and the "short'' type associated with neutron star merger events. It also focuses on the disruption of stars by the tidal forces of supermassive black holes i.e., tidal disruption events (TDEs). We model the emission from these transients and compare them to observations in order to draw a number of conclusions and make predictions for future detections. For example, we find that the non-thermal emission from supernovae and kilonovae associated with GRBs can produce long term emission which may be detected as a re-brightening in the overall emission. The sharp cut off observed in some TDE flares can be caused by a pre-existing accretion disk present around a supermassive black hole, which is expected in active galactic nuclei. Our work successfully predicted the nature of the very first electromagnetic detection from a neutron star merger, and was able to reproduce the emission that had been observed for more than one hundred days after the merger. This dissertation also provides frameworks on how the observable features of these transients can be leveraged to probe the properties of the progenitor system and their environment. <br>

El trabajo que presentamos en esta tesis tiene como finalidad la operación, calibración y análisis de datos ópticos del instrumento OMC (Optical Monitoring Camera) a bordo del satélite INTEGRAL (INTErnational Gamma-Ray Astrophysics Laboratory). La cámara OMC ofrece, por primera vez, la oportunidad de realizar observaciones fotométricas de larga duración en el rango óptico, simultáneamente con las de rayos X y rayos gamma. En la primera parte de la tesis describimos el proyecto INTEGRAL/OMC y sus operaciones científicas, detallando el proceso que hemos seguido para la construcción del catálogo de entrada de la OMC. Definimos la estrategia y requisitos que debe satisfacer el software de apuntado encargado de generar los telecomandos que controlan el funcionamiento de la cámara. La combinación del catálogo y el software de apuntado, junto con el buen funcionamiento del algoritmo de centrado de a bordo, han permitido la determinación del apuntado de la OMC, a bordo y en tiempo real, con una precisión mejor que 1 píxel desde el inicio mismo de las operaciones. La segunda parte se centra en el procedimiento que hemos elaborado para el análisis de los datos. Describimos el flujo de datos desde que llegan a tierra, pasando por los distintos sistemas de procesamiento, hasta que, finalmente, se convierten en datos científicos listos para ser utilizados por la comunidad astrofísica. Detallamos todos los aspectos relacionados con la calibración del instrumento. Explicamos los distintos algoritmos desarrollados para el análisis científico de los datos OMC, haciendo especial énfasis en las soluciones adoptadas para mejorar los resultados fotométricos y astrométricos del instrumento. En la última parte presentamos los archivos y catálogos resultantes de las observaciones realizadas con la cámara OMC. Describimos los archivos de datos procesados en tiempo real y de datos consolidados, así como el portal Web de acceso a los datos OMC. Avanzamos lo que será el catálogo final de la OMC, que contendrá todas las fuentes observadas y detectadas por este instrumento, una vez finalizada la misión INTEGRAL. Presentamos también dos de los subcatálogos preliminares obtenidos tras analizar determinados grupos de fuentes: "Contrapartidas ópticas de fuentes de altas energías del 4º Catálogo IBIS/ISGRI" y "Primer catálogo de fuentes variables observadas con la OMC". / The work presented in this thesis describes the system that allows the operation, calibration and data analysis of the OMC instrument (Optical Monitoring Camera) on board the ESA's INTEGRAL spacecraft (INTErnational Gamma-Ray Astrophysics Laboratory). The OMC camera offers, for the first time, the opportunity to perform long photometric observations in the optical range simultaneously to those in the X-ray and gamma-ray bands. In the first part of the thesis we describe the INTEGRAL/OMC project and its scientific operations, giving some details about the building process of the OMC Input Catalogue. We define the strategy and requirements of the OMC Pointing Software which generates the telecommands to control the camera operations. The combination of both the Input Catalogue and the Pointing Software, as well as the excellent behaviour of the on-board centring algorithm, allow us the on-board determination in real time of the OMC pointing direction with an accuracy better than 1 pixel from the beginning of the mission operations. The second part is focused on the development involved in the data analysis procedures. We describe the data flow starting when the telemetry data are received at Earth, followed by the different data processing stages present in the systems, until they are finally converted into scientific data which can be used by the astrophysics community. We explain all aspects related to the instrument calibration and the algorithms developed for the OMC scientific analysis, making special emphasis in the adopted solutions that allowed us to improve the photometric and astrometric results. In the last part we present the archives and catalogues compiled with the OMC data obtained during INTEGRAL observations. We describe the consolidated and near real time data archives as well as the OMC Archive Web Portal. We summarize the contents of the final OMC Catalogue which will be published at the end of the INTEGRAL mission including all sources observed and detected by OMC. In addition we present two preliminary sub-catalogues obtained by analysing some specific objects groups: "OMC optical monitoring of sources in the 4th IBIS/ISGRI catalogue" and "The first INTEGRAL/OMC Catalogue of optically variable sources".

Stars form inside clouds of molecular gas and dust. In the early stages of stellar evolution the remainders of the initial cloud form a circumstellar disk. For the next few million years the disk will slowly dissipate via accretion, outflows, photoevaporation and planet growth while the star makes its way onto the Main Sequence. This stage of a star's life is referred to as the T Tauri phase and is characterised by high-level spectrophotometric variability. This thesis aims to study and map out the environments of T Tauri stars down to the very low mass regime by the means of time-domain monitoring. Different physical processes in the system manifest themselves as variability on different time- scales as well as produce characteristic spectroscopic and photometric features at various wave- lengths. In order to study young stellar objects in depth, the observing campaigns presented in this work were designed to cover a large range of time-scales - minutes, hours, days and months. Combining all the data, this thesis establishes a baseline of over a decade for some objects. The observations also cover a wide range of wavelengths from the optical to the mid-infrared part of the spectrum. The star RW Aur experienced two long-lasting dimming events in 2010 and 2014. This thesis presents a large collection of spectral and photometric measurements carried out just before and during the 2014 event. Spectral accretion signatures indicate no change in the accretion activity of the system. Photometry indicates that parallel to the dimming in the optical the star becomes brighter in the mid-infrared. The observations in this work combined with literature data suggest that the origin of the 2014 event is most likely obscuration of the star by hot dust from the disk being lifted into the disk wind. Very low mass stars (< 0.4 M⊙) are the most common type of star in the Galaxy. In order to understand the early stages of stellar evolution we must study young very low mass stars. This work investigates the photometric and spectroscopic variability of seven brown dwarfs in star forming regions near σ Ori and ε Ori. All targets exhibit optical photometric variability between from 0.1 to over 1.0 magnitude that persists on a time-scale of at least one decade. Despite the photometric variability no change in the spectral type is measured. In the cases where the stars are accreting, modelling of the spectral changes suggest the accretion flow is more homogeneous and less funnelled compared to Sun-like T Tauri stars. The non-accreting variables are more plausibly explained by obscuration by circumstellar material, possibly a ring made out of multiple clouds of dust grains and pebbles with varying optical depths. The star-disk systems studied in this thesis have some broader implications for star and planet formation theory. The case-study of RW Aur has unambiguously demonstrated that the planet- forming environment is very dynamic and can change dramatically on short time-scales, which in turn would have implications for the diversity of planetary systems found in the Galaxy. The Orion stars have shown that the current theory for the T Tauri stage of stellar evolution is valid down to the very low mass regime. The seven dwarfs are a good example for the evolutionary path of circumstellar disks, showing the transition from gas-high, flared accretion disks (σ Ori) to dust-dominated, depleted, structured debris disks (ε Ori).

Este trabalho tem como objetivo o ensino dos fundamentos da Astrofísica Estelar moderna a alunos do Ensino Médio através de uma abordagem empírica que usa como ferramentas principais da metodologia a observação, o registro, a análise e a interpretação das mudanças de brilho das estrelas variáveis ao longo do tempo. Mediante técnicas simples (fotometria visual) para a estimativa dos valores dos brilhos aparentes (magnitudes) dessas estrelas no decorrer de vários meses, o aluno coleta seus próprios dados observacionais. Em seguida, esses dados serão colocados em gráficos (curvas de luz), cuja análise permitirá a medida de seus períodos e amplitudes de variação, bem como a determinação de uma série de parâmetros que caracterizam as propriedades físicas das estrelas. O estudo é conduzido de uma forma que permite apresentar aos alunos vários dos grandes temas da Astrofísica contemporânea sem que eles percam o vínculo sensorial com a observação direta dos fenômenos celestes, seja com auxílio de instrumental modesto (binóculos ou pequenos telescópios), seja até mesmo a olho nu. O produto final de nosso trabalho é um Guia de Estudos intitulado Astrofísica Estelar para o Ensino Médio, em doze capítulos e mais de trezentas páginas, que está incluído em sua íntegra nesta monografia. O Guia foi desenvolvido tendo como alvos principais os alunos e professores do Ensino Médio formal. Não obstante, ele pode ser também utilizado por qualquer pessoa que tenha interesse no assunto e que possua os conceitos necessários de Matemática e Física do Ensino Médio; ou ainda em cursos do ensino não-formal que sejam ministrados, por exemplo, por planetários, museus de ciência e astrônomos amadores (situação essa testada com ótimos resultados pelo autor). Por fim, vale mencionar que, neste tipo de aprendizado (conhecido como research-based education, que poderíamos traduzir livremente como aprendizado vinculado à pesquisa), utilizamos intensivamente diversos recursos tecnológicos disponibilizados gratuitamente na internet pelas grandes entidades internacionais que se dedicam à pesquisa e ao estudo das estrelas variáveis tais como softwares específicos, bancos de dados, simulações e cartas para a estimativa das magnitudes. O uso dessa metodologia vinculada à pesquisa, em nossa experiência, se constitui em um fator valioso para desenvolver nos alunos a criatividade, o raciocínio crítico, a familiarização com o método científico e as habilidades para a pesquisa. / This work aims at teaching the fundamental aspects of Stellar Astrophysics to high school students through an empirical approach which uses variable stars observations, data collection, and the analysis and interpretation of brightness changes as key methodological tools. By using some simple techniques (visual photometry) to estimate the values of apparent brightness (magnitudes) of these stars over several months, the student collects his/her own observational data. Subsequently, that data will be plotted in graphs (light curves), whose analysis will in turn allow the determination of its periods and amplitudes of variation, as well as of a series of physical parameters that characterize the physical properties of the stars. The whole study is conducted in a way that shows students several of the major themes of contemporary Astrophysics without losing their sensory link with the direct observation of celestial phenomena - whether with the aid of modest instruments (binoculars or small telescopes) or even naked eye. The final product of our work is a Study Guide called \"Stellar Astrophysics for High School\" in twelve chapters and more than three hundred pages, which is included in their entirety in this monograph. The Guide was developed with the main targets being students and teachers of the formal high school level. Nevertheless, it can also be used by anyone who has an interest in the subject and who possesses the required concepts of high school Mathematics and Physics; or even in non-formal teaching courses that are taught, for example, by planetariums, science museums and amateur astronomers (a situation that has been tested with great results by the author). Last but not least, it is worth mentioning that in this type of learning (known as \"research-based education\") we intensively use various technological resources freely available on the internet from the great international entities that are dedicated to the research and study of variable stars such as specific softwares, databases, simulations and charts for estimating magnitudes. The use of research-based education, in our experience, is a valuable tool to foster and develop students creativity, critical thinking, and the familiarization with the scientific method and research skills.