Thesis (M.S.)--University of Missouri-Columbia, 2006. / The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file viewed on (February 6, 2007) Includes bibliographical references.

No description available.

We report on a search for short -period binary systems composed of pairs of evolved stars. The search is being carried out concurrently with a program to characterize the kinematical properties of two different samples of stars. Each sample has produced one close binary candidate for which further spectroscopic observations are planned. We also recapitulate the discovery of a close detached binary system composed of two cool DA white dwarfs, and we discuss the null results of Ha observations of the suspected white dwarf /brown dwarf system G 29-38.

In this study we present photometric predictions for C-type Asymptotic Giant Branch Stars (AGB) stars from Eriksson et al. (2014) for the James Webb Space Telescope (JWST) and the Wide-field Infrared Survey Explorer (WISE) instruments. The photometric predictions we have done are for JWST’s general purpose wide-band filters on NIRCam and MIRI covering wavelengths of 0.7 — 21 microns. AGB stars contribute substantially to the integrated light of intermediate-age stellar popula- tions and is a substantial source of the metals (especially carbon) in galaxies. Studies of AGB stars are (among other reasons) important for the understanding of the chemical evolution and dust cycle of galaxies. Since the JWST is scheduled for launch in 2018 it should be a high priority to prepare observing strategies. With these predictions we hope it will be possible to optimize observing strategies of AGB stars and maximize the science return of JWST. By testing our method on Whitelock et al. (2006) objects from the WISE catalog and comparing them with our photometric results based on Eriksson et al. (2014) we have been able to fit 20 objects with models. The photometric data set can be accessed at: http://www.astro.uu.se/AGBmodels/ / I den här studien har jag gjort fotometriska förutsägelser för asymptotis- ka jättegrensstjärnor (AGB-stjärnor) av C typ från Eriksson et al. (2014) modifierade för instrument ombord på James Webb Space Telescope (JWST) och Wide-field Infrared Survey Explorer (WISE). AGB-stjärnor bidrar kraftigt till det totala ljuset av stjärnor av intermediär ålder och är också en stor källa till metaller (speciellt kol) i galaxer. Studier av AGB stjärnor är viktiga av flera anledningar, däribland för att förstå den kemiska evolutionen och stoftcykler i galaxer. JWST är planerad att skjutas upp 2018 och fram till dess bör det vara en hög prioritet att förbereda observeringsstrategier. Med den fotometriska datan i den här studien hoppas vi att användare av JWST kommer kunna optimera sina observeringsstrategier av AGB-stjärnor och få ut så mycket som möjligt av sin obseravtionstid med teleskopet. Vi har testat metoden genom att titta på objekt från Whitelock et al. (2006) i WISE-katalogen och jämföra dem med de fotometriska resultaten baserade på modellerna från Eriksson et al. (2014). På detta sett har vi lyckats matcha 20 objekt med modeller. Den fotometriska datan går att ladda ner ifrån: http://www.astro.uu.se/AGBmodels/

Cool giant and supergiant stars are among the largest and most luminous stars in the Universe and, therefore, dominate the integrated light of their host galaxies. These stars were extensively studied during last few decades, however their relevant properties like photometric variability and mass loss are still poorly constrained. Understanding of these properties is crucial in the context of a broad range of astrophysical questions including chemical enrichment of the Universe, supernova progenitors, and the extragalactic distance scale. Atmospheres of giant and supergiant stars are characterized by complex dynamics due to different interacting processes, such as convection, pulsation, formation of molecules and dust, and the development of mass loss. Current 1D/3D dynamical model atmospeheres are able to simulate these processes and produce a good agreement with the observed spectral features of evolved stars. However, the models lack constraints and need to be confronted to observables. Dynamical processes in stellar atmospheres impact the formation of spectral lines producing their asymmetries and Doppler shifts. Thus, by studying the line-profile variations on spatial and temporal scales it is possible to reconstruct atmospheric motions in evolved stars. As will be shown in this thesis, a tomographic method is an ideal technique for this purpose. The tomographic method is based on construction and cross-correlation of spectral templates (masks) with observed or synthetic stellar spectra in order to recover velocity fields at different optical depths in the stellar atmosphere.The first part of the thesis further improves the original implementation of the tomographic method. This improvement involves the computation of the contribution function in order to correctly determine an optical depth of formation of spectral lines. The tomographic method is, then, fully validated by applying it to a stellar convection simulation of a red supergiant star and correctly recovering its velocity field throughout the atmosphere. The second part of the thesis applies the tomographic method to the red supergiant star μ Cep in order to constrain its atmospheric motions and relate them to photometric variability. A phase lag (hysteresis) between the effective temperature and the radial velocity variations is revealed with timescales of a few hundred days, similar to photometric ones. A comparison to a stellar convection simulation of a red supergiant star indicates that hysteresis loops are linked to the stochastic shocks generated and shaped by the underlying large-scale convection and may be responsible for photometric variations in μ Cep. The third part of the thesis applies the tomographic method to spectro-interferometric observations of the Mira-type star S Ori. The uniform-disk angular diameters measured at wavelengths contributing to the tomographic masks increase with decrease of an optical depth probed by the masks. This validates the capability of the tomographic method to probe distinct geometrical depths in the stellar atmosphere. The last part of the thesis applies the tomograhic method to the Mira-type star RY Cep and compares the results to those obtained for μ Cep in this thesis. The comparison reveals differences in their behavior in the temperature-velocity plane pointing to the posibility to differentiate between Mira-type and red supergiant stars from their spectroscopic signatures. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

Abstract not available

The 17;18O(p,α)14;15N nuclear reactions play an important role in several astrophysical scenarios, and in Asymptotic Giant Branch (AGB) stars in particular. These stars are the site of several mixing and recirculating processes that transport matter from their hot cores to their cooler surfaces, and vice versa. Some of these mixing processes are still not well understood. Constraining them would improve our knowledge of stars that are in, or will enter, the AGB phase, including our own Sun. An ideal way to trace these poorly understood mixing processes are provided by the rare, stable 17;18O isotopes. Their abundances are strongly sensitive to the 17;18O(p,α)14;15N reactions. At temperatures of astrophysical interest, the 17O(p,α)14N reaction is dominated by a narrow, isolated resonance at Eproton=70 keV. This resonance has been studied several times in the past, using both direct and indirect methods. However, the picture painted in the literature is still not completely satisfying. The situation is more complex for the 18O(p,α)15N, for which an interference pattern between at least three resonances dominates the reaction rate at the temperatures of interest. This thesis work concerns an experimental campaign aimed at measuring both reactions at energies of astrophysical interest. These challenging measurements were performed by exploiting the low radiation background at the underground LUNA accelerator in Gran Sasso Laboratories, Italy. The two reactions were investigated in direct kinematics. A proton beam was accelerated onto solid Ta2O5 targets and the alpha particles produced were detected at backward angles using an array of silicon detectors mounted in a purpose-built scattering chamber. Our results indicate that the 17O(p,α)14N reaction rate at temperatures of astrophysical interest is approximately a factor of two higher than previously reported, solving a long standing puzzle on the origin of some pre-solar grains. For the 18O(p,α)15N reaction, we find a reaction rate largely in agreement with previous investigations, but with a significantly reduced uncertainty which could help improve the accuracy of stellar models of a number of stellar sites.

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.

Si l'évolution des étoiles simples est relativement bien comprise, l'étude des étoiles binaires, qui représentent la majorité des étoiles, nécessite encore des progrès majeurs, particulièrement en ce qui concerne leurs différents modes d'interactions. Dans ces systèmes, la composition de surface d'une étoile peut être altérée non seulement par l'accrétion d'éléments synthétisés au sein de l'étoile compagnon, mais également par des processus de mélanges internes induits par les forces de marées et d'un transport du moment angulaire. Plusieurs classes d'étoiles post-transfert de masse (les étoiles à baryum, CH et S) possèdent effectivement des compositions de surface caractérisées par la présence d'éléments lourds, tel que le baryum. Ces systèmes sont présumés se former au sein de systèmes binaires incluant une étoile de la branche asymptotique des géantes (appelé étoile AGB). Ces dernières sont des étoiles remarquables qui représentent l'unique site d'une nucléosynthèse particulière. En effet, elles constituent les contributeurs essentiels de la production de fluor ou de baryum. Les étoiles AGB sont également caractérisées par une importante perte de masse par vent qui éjecte progressivement leur enveloppe enrichie en ces éléments. Au sein d'un système binaire, une partie de ce vent est accréditée par l'étoile compagnon et pollue ainsi sa surface, laissant une signature spectrale distincte qui subsistera longtemps après que l'étoile AGB ait disparu. Ce scénario est suggéré comme étant responsable de la formation d'une grande variété d'étoiles chimiquement particulières, tels que les étoiles à baryum.<p>Cependant, plusieurs propriétés clés de ces systèmes, en particulier leurs distributions de périodes orbitales et d'excentricités, demeurent inexpliquées depuis des décennies. L'incapacité de nos modèles à reproduire ces propriétés orbitales met en évidence notre compréhension limitée des mécanismes d'interaction qui gouvernent l'évolution des systèmes binaires. Plus particulièrement, des mécanismes qui génèrent des orbites excentriques au sein des étoiles à baryum et des systèmes analogues sont requis. Nous examinons ainsi la possibilité qu'à sa naissance l'étoile naine blanche subisse un kick ou que la présence d'un disque entourant le système binaire soit à l'origine des fortes excentricités observées chez les étoiles à baryum. Ces deux mécanismes permettent pour la première fois depuis l'étude de ces systèmes d'apporter une solution à ces problèmes. Il est montré comment comprendre les signatures induites par un compagnon étoile AGB et les corréler avec les propriétés orbitales du système binaire est essentiel pour tester et améliorer notre connaissance de l'évolution des étoiles binaires; l'objectif de ce travail.<p> / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

Context. Theoretical studies predict that the length scale of convection in stellar atmospheres isproportional to the pressure scale height, which implies that giant and supergiant stars should have convection granules of sizes comparable to their radii. Numerical simulations and the observation of anisotropies on stellar discs agree well with this prediction. Aims. To measure the characteristic sizes of convection structures of models simulated with the CO5BOLD code, to look at how they vary between models and to study their limitations due to numerical resolution. Methods. Fourier analyses are performed to frames from the models to achieve spatial spectral power distributions which are averaged over time. The position of the main peak and the averagevalue of the wavevector are taken as indicators of these sizes. The general shape of the intensity map of the disc in the frame is fitted and subtracted so that it does not contaminate the Fourier analysis. Results. A general relationship of the convection granule size being more or less ten times larger than the pressure length scale is found. The expected wavevector value of the time-averaged spectral power distributions is higher than the position of the main peak. Loose increasing trends with the characteristic sizes by the pressure scale height increasing against stellar mass, radius, luminosity,temperature and gravity are found, while a decreasing trends are found with the radius and modelresolution. Bad resolution subtracts signals on the slope at the side of the main peak towards larger wavevector values and in extreme cases it creates spurious signal towards the end of the spectrum due to artifacts appearing on the frames. Conclusions. The wavevector position of the absolute maximum in the time-averaged spectral power distribution is the best measure of the most prominent sizes in the stellar surfaces. The proportionality constant between granule size and pressure length scale is of the same order ofmagnitude as the one in the literature, however, models present sizes larger than the ones expected, likely because the of prominent features do not correspond to convection granules but to larger features hovering above them. Further studies on models with higher resolution will help in drawing more conclusive results. Appendix. The SIA pipeline takes a set of time-dependent pictures of stellar disks and uses a Fourier Analysis to measure the characteristic sizes of their features and other useful quantities, such as standard deviations or the spatial power distributions of features. The main core of the pipeline consists in identifying the stellar disc in the frames and subtracting their signal from the spatial power distributions through a general fit of the disc intensity. To analyze a time sequence, the SIA pipeline requires at least two commands from the user. The first commandorders the SIA pipeline to read the .sav IDL data structure file where the frame sequence is stored and to produce another .sav file with information on the spectral power distributions, the second command orders the reading of such file to produce two more .sav files, one containing time-averaged size measurements and their deviations while the other breaking down time-dependant information and other arrays used for the calculations. The SIA pipeline has been entirely written in Interactive Data Language (IDL). Most of the procedures used here are original from the SIA pipeline, but a small handfull like ima3_distancetransform.pro, power2d1d.pro, extremum.pro and smooth2d.pro from Bernd Freytag and peaks.pro and compile opt.pro amongst others are actually external. / <p>The report consists in two parts:</p><p>1.- The main project, where we apply our pipeline and get scientific results.</p><p>2.- The appendix, where a technical description of the pipeline is given.</p>