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Silicon monoxide maser emission has been detected in many evolved stars in circumstellar envelopes in different vibrationally-excited rotational transitions. It is considered a good tracer to study the dynamics in a region close to the photosphere of the star. We present multi-epoch, total intensity, high-resolution images of 43 GHz, v=1, J=1-0 SiO maser emission toward the Mira variable R Cas. In total we have 23 epochs of data for R Cas at approximate monthly intervals over an optical pulsation phase range of φ = 0.158 to φ = 1.782. These maps show a ring-like distribution of the maser features in a shell, which is assumed to be centred on the star at average radius of 1.6 → 2.3 times the radius of star, R⋆. It is clear from these images that the maser emission is significantly extended around the star. At some epochs a faint outer arc can be seen at about 4 R⋆. The intensity of the emission waxes and wanes during the stellar phase. Some maser features are seen infalling as well as outflowing. We have made initial comparisons of our data with models by Gray et al. (2009). We have investigated the polarization morphology by mapping the linear and circular polarization of SiO masers in the v=1, J=1-0 transition. We found that some of the polarization vectors are either tangential or radial, which indicate a bimodal structure of the linear polarization morphology. Other angles can be seen as well. This is consistent with a radial, stellar-centred magnetic field in the SiO maser shell. We found in some isolated features the fractional linear polarization exceeds 100%. In other features, the polarization angle abruptly flips by 90◦. We found that our data are in the regime that the Zeeman splitting rate g is much greater than the stimulated emission rate R which in turn is greater than the decay rate , which indicates that the solution of Goldreich et al. (1973) can be applied.

M dwarf stars make up over 70% of stars by number in the Milky Way Galaxy and are known to host at least two exoplanets per star on average. Using mutually eclipsing double-lined spectroscopic binary stars (SB2 EBs), astronomers can empirically measure stellar properties of M dwarf stars including mass and radius. However, empirical measurements systematically differ from the predictions of stellar evolutionary models and show large scatter. Some M dwarf stars are outliers, with radii that are a factor of 2-to-3 larger than model predictions, assuming they were measured accurately. In this dissertation, I investigated whether the outliers, systematic offset, and the scatter seen in the mass-radius diagram are physical, using SB2 EBs with photometry from NASA's Kepler Mission and high-resolution near-infrared ground-based spectroscopy. Empirical measurements using space-based photometry and high-resolution near-infrared ground-based spectroscopy, together with Bayesian model-fitting techniques, provide significant advancements over previous measurements. For this dissertation work, a sample of Kepler EBs were carefully chosen to be detached and non-interacting. I conducted a radial velocity survey of the sample using Immersion GRating INfrared Spectrometer (IGRINS) with the Discovery Channel Telescope (DCT) and iSHELL with NASA's Infrared Telescope Facility (IRTF). Combined with high-precision Kepler data, I determined the masses and radii of the component stars in the sample. I also determined a new mass-radius relationship of M dwarf stars using the sample of Kepler EB systems. My investigation showed that the outliers in the mass-radius diagram of M dwarf stars are not physical and they are due to the quality of data and from analysis using different pipelines. I also showed that the offset and scatter in the mass-radius diagram are persistent, which are not from the measurement uncertainties. This suggests the need for an extra degree of freedom to accurately capture the discrepancies between the empirical measurements and model predictions. Lastly, I showed that reduced convective heat flow due to enhanced magnetic fields from rapid stellar rotation can account for the offset and scatter in the measurements.

<p>The information about the chemical compositions of stars is encoded in their spectra. Accurate determinations of these compositions are crucial for our understanding of stellar nucleosynthesis and Galactic chemical evolution. The determination of elemental abundances in stars requires models for the stellar atmospheres and the processes of line formation. Nearly all spectroscopic analyses of late-type stars carried out today are based on one-dimensional (1D), hydrostatic model atmospheres and on the assumption of local thermodynamic equilibrium (LTE). This approach can lead to large systematic errors in the predicted stellar atmospheric structures and line-strengths, and, hence, in the derived stellar abundances. In this thesis, examples of departures from LTE and from hydrostatic equilibrium are explored. The effects of background line opacities (line-blocking) due to atomic lines on the statistical equilibrium of Fe are investigated in late-type stars. Accounting for this line opacity is important at solar metallicity, where line-blocking significantly reduces the rates of radiatively induced ionizations of Fe. On the contrary, the effects of line-blocking in metal-poor stars are insignificant. In metal-poor stars, the dominant uncertainty in the statistical equilibrium of Fe is the treatment of inelastic H+Fe collisions. Substantial departures of Fe abundances from LTE are found at low metallicities: about 0.3 dex with efficient H+Fe collisions and about 0.5 dex without. The impact of three-dimensional (3D) hydrodynamical model atmospheres on line formation in red giant stars is also investigated. Inhomogeneities and correlated velocity fields in 3D models and differences between the mean 3D stratifications and corresponding 1D model atmospheres can significantly affect the predicted line strengths and derived abundances, in particular at very low metallicities. In LTE, the differences between 3D and 1D abundances of C, N, and O derived from CH, NH, and OH weak low-excitation lines are in the range -0.5 dex to -1.0 dex at [Fe/H]=-3. Large negative corrections (about -0.8 dex) are also found in LTE for weak low-excitation neutral Fe lines. We also investigate the impact of 3D hydrodynamical model stellar atmospheres on the determination of elemental abundances in the carbon-rich, hyper iron-poor stars HE 0107-5240 and HE 1327-2326. The lower temperatures of the line-forming regions of the 3D models compared with 1D models cause changes in the predicted spectral line strengths. In particular we find the 3D abundances of C, N, and O to be lower by about -0.8 dex (or more) than estimated from a 1D analysis. The 3D abundance of Fe is decreased but only by -0.2 dex. Departures from LTE for Fe might actually be very large for these stars and dominate over the effects due to granulation.</p>

Stellare Magnetfelder spielen eine wichtige Rolle bei der Entstehung und Entwicklung von Sternen. Leider entziehen sie sich aber, aufgrund ihrer großen Entfernung zur Erde, einer direkten Beobachtung. Dies gilt zumindest für derzeitige und in naher Zukunft zur Verfügung stehende Instrumente. Um aber beispielsweise zu verstehen, ob Magnetfelder durch einen Dynamoprozess generiert werden oder Überbleibsel der Sternentstehung sind, ist es zwingend erforderlich, die Oberflächenstruktur und die zeitliche Entwicklung von stellaren Feldern zu untersuchen. Glücklicherweise haben wir mit der Dopplerverschiebung sowie der Polarisation von Licht Mittel zur Verfügung, um indirekt die Magnetfeldtopologie entfernter Sternen zu rekonstruieren, wenn auch nur die schnell rotierender. Die auf den beiden genannten Effekten basierende Rekonstruktionsmethode ist unter dem Namen Zeeman-Doppler Imaging (ZDI) bekannt. Sie stellt eine leistungsfähige Methode dar, um aus rotationsverbreiterten Stokes Profilen schnell rotierender Sterne Oberflächenkartierungen der Temperatur und Magnetfeldverteilung zu erstellen. Durch das ZDI konnten in den vergangenen Jahren die Magnetfeldverteilungen zahlreicher Sterne rekonstruiert werden. Diese Methode stellt allerdings sehr hohe Anforderungen sowohl an die Instrumentierung als auch an die Rechenleistung und ist deshalb häufig mit zahlreichen Annahmen und Näherungen verbunden. Ziel dieser Arbeit war es, Methoden für ein ZDI zu entwickeln, das darauf ausgelegt ist, zeitaufgelöste spektropolarimetrische Daten von aktiven späten Sternen zu invertieren. Es sollte also insbesondere den komplexen und lokalen Magnetfeldstrukturen dieser Sterne Rechnung getragen werden. Um die Orientierung und Stärke solcher Felder zuverlässig rekonstruieren zu können, sollte die Inversion im Stande sein, alle vier Stokes-Komponenten einzubeziehen. Ferner war vorgesehen auf vollständigen polarisierten Strahlungstransportmodellierungen aufzubauen. Dies ermöglicht eine simultane und selbstkonsistente Temperatur- und Magnetfeld-Inversion, die damit dem komplexen Zusammenspiel zwischen Temperatur und Magnetfeld gerecht wird. Schließlich sollte die Anwendung eines neu zu entwickelnden ZDI Programms auf Stokes I und V Beobachtungen von II Pegasi (kurz: II Peg) erste Magnefeldkarten dieses sehr aktiven Sterns liefern. Um den hohen Rechenaufwand, der mit der Inversionsmethode einhergeht, besser bewältigen zu können, wurde zunächst eine schnelle Approximationsmethode für den polarisierten Strahlungstransport entwickelt. Sie basiert auf einer Hauptkomponentenanalyse (PCA) sowie auf künstlichen Neuronalen Netzen. Letztere approximieren den funktionalen Zusammenhang zwischen atmosphärischen Parametern und den zugehörigen lokalen Stokes Profilen. Inverse Probleme sind potentiell schlecht gestellt und erfordern in der Regel eine Regularisierung. Der entwickelte Ansatz verwendet eine lokale Entropie, die auf die Besonderheiten bei der Rekonstruktion lokalisierter Magnetfeder eingeht. Ein weiterer neuartiger Ansatz befasst sich mit der Rauschreduktion polarimetrischer Beobachtungsdaten. Er macht sich die Hauptkomponentenanalyse zu Nutze, um mit Hilfe einer Vielzahl beobachteter Spektrallinien, einzelne Linien mit drastisch vergrößertem Signal-zu-Rausch-Verhältnis wieder zu geben. Diese Methode hat gegenüber anderen Multi-Spektrallinien-Verfahren den Vorteil, nach wie vor eine Inversion auf der Basis einzelner Spektrallinien durchführen zu können. Schließlich wurde das Inversionsprogramm iMap entwickelt, das die zuvor genannten Methoden implementiert. Detaillierte Testrechnungen demonstrieren die Funktionsfähigkeit und Genauigkeit der schnellen Synthese-Methode und weisen einen Zeitgewinn von nahezu drei Größenordnungen gegenüber der konventionellen Strahlungstransportberechnung auf. Desweiteren untersuchen wir den Einfluss der verschiedenen Stokes Komponenten (IV bzw. IVQU) auf die Zuverlässigkeit, ein bekanntes Magnetfeld zu rekonstruieren. Damit belegen wir die Zuverlässigkeit unseres Inversionsprogrammes und zeigen darüber hinaus auch Einschränkungen von Magnetfeldinversionen im allgemeinen auf. Eine erste Inversion von Stokes I und V Profilen von II Peg liefert zum ersten Mal für diesen Stern simultan Temperatur- und Magnetfeldverteilungen. / Stellar magnetic fields, as a crucial component of star formation and evolution, evade direct observation at least with current and near future instruments. However investigating whether magnetic fields are generated by a dynamo process or represent relics from the formation process, or whether they show a behavior similar to the sun or something very different, it is essential to investigate their structure and temporal evolution. Fortunately nature provides us with the possibility to indirectly observe surface topologies on distant stars by means of Doppler shift and polarization of light, though not without its challenges. Based on the mentioned effects, the so called Zeeman-Doppler Imaging technique is a powerful method to retrieve magnetic fields from rapid rotating stars based on measurements of spectropolarimetric observations in terms of Stokes profiles. In recent years, a large number of stellar magnetic field distributions could be reconstructed by Zeeman-Doppler Imaging (ZDI). However, the implementation of this method often relies on many approximations because, as an inversion method, it entails enormous computational requirements. The aim of this thesis is to develop methods for a ZDI, designed to invert time-resolved spectropolarimetric data of active late type stars, and to account for the expected complex and small scale magnetic fields on these stars. In order to reliably reconstruct the detailed field orientation and strength, the inversion method is employed to be able to use of all four Stokes components. Furthermore it is based on fully polarized radiative transfer calculations to account for the intricate interplay between temperature and magnetic field. Finally, the application of a newly developed ZDI code to Stokes I and V observations of II Pegasi (short: II Peg) was supposed to deliver the first magnetic surface maps for this highly active star. To accomplish the high computational burden of a radiative transfer based ZDI, we developed a novel approximation method to speed up the inversion process. It is based on Principal Component Analysis and Artificial Neural Networks. The latter approximate the functional mapping between atmospheric parameters and the corresponding local Stokes profiles. Inverse problems, as we are dealing with, are potentially ill-posed and require a regularization method. We propose a new regularization scheme, which implements a local entropy function that accounts for the peculiarities of the reconstruction of localized magnetic fields. To deal with the relatively large noise that is always present in polarimetric data, we developed a multi-line denoising technique based on Principal Component Analysis. In contrast to other multi-line techniques that extract from a large number of spectral lines a sort of mean profile, this method allows to extract individual spectral lines and thus allows for an inversion on the basis of specific lines. All these methods are incorporated in our newly developed ZDI code iMap, which is based on a conjugated gradient method. An in depth validation of our new synthesis method demonstrates the reliability and accuracy of this approach as well as a gain in computation time by almost three orders of magnitude relative to the conventional radiative transfer calculations. We investigated the influence of the different Stokes components (IV / IVQU) on the ability to reconstruct a known synthetic field configuration. In doing so we validate the capability of our inversion code, and we also assess limitations of magnetic field inversions in general. In a first application to II Peg, a K2 IV subgiant, we derived temperature and magnetic field surface distributions from spectropolarimetric data obtained in 2004 and 2007. It gives for the first time simultaneously the temporal evolution of the surface temperature and magnetic field distribution on II Peg.

The information about the chemical compositions of stars is encoded in their spectra. Accurate determinations of these compositions are crucial for our understanding of stellar nucleosynthesis and Galactic chemical evolution. The determination of elemental abundances in stars requires models for the stellar atmospheres and the processes of line formation. Nearly all spectroscopic analyses of late-type stars carried out today are based on one-dimensional (1D), hydrostatic model atmospheres and on the assumption of local thermodynamic equilibrium (LTE). This approach can lead to large systematic errors in the predicted stellar atmospheric structures and line-strengths, and, hence, in the derived stellar abundances. In this thesis, examples of departures from LTE and from hydrostatic equilibrium are explored. The effects of background line opacities (line-blocking) due to atomic lines on the statistical equilibrium of Fe are investigated in late-type stars. Accounting for this line opacity is important at solar metallicity, where line-blocking significantly reduces the rates of radiatively induced ionizations of Fe. On the contrary, the effects of line-blocking in metal-poor stars are insignificant. In metal-poor stars, the dominant uncertainty in the statistical equilibrium of Fe is the treatment of inelastic H+Fe collisions. Substantial departures of Fe abundances from LTE are found at low metallicities: about 0.3 dex with efficient H+Fe collisions and about 0.5 dex without. The impact of three-dimensional (3D) hydrodynamical model atmospheres on line formation in red giant stars is also investigated. Inhomogeneities and correlated velocity fields in 3D models and differences between the mean 3D stratifications and corresponding 1D model atmospheres can significantly affect the predicted line strengths and derived abundances, in particular at very low metallicities. In LTE, the differences between 3D and 1D abundances of C, N, and O derived from CH, NH, and OH weak low-excitation lines are in the range -0.5 dex to -1.0 dex at [Fe/H]=-3. Large negative corrections (about -0.8 dex) are also found in LTE for weak low-excitation neutral Fe lines. We also investigate the impact of 3D hydrodynamical model stellar atmospheres on the determination of elemental abundances in the carbon-rich, hyper iron-poor stars HE 0107-5240 and HE 1327-2326. The lower temperatures of the line-forming regions of the 3D models compared with 1D models cause changes in the predicted spectral line strengths. In particular we find the 3D abundances of C, N, and O to be lower by about -0.8 dex (or more) than estimated from a 1D analysis. The 3D abundance of Fe is decreased but only by -0.2 dex. Departures from LTE for Fe might actually be very large for these stars and dominate over the effects due to granulation.

Mass loss, in the form of slow stellar winds, is a decisive factor for the evolution of cool luminous giants, eventually turning them into white dwarfs. These dense outflows are also a key factor in the enrichment of the interstellar medium with newly produced elements from the interior of these stars. There are strong indications that these winds are accelerated by radiation pressure on dust grains, but the actual grain species responsible for driving the outflows in M-type Asymptotic Giant Branch stars are still a matter of debate. Observations of dust features in the circumstellar environment of these stars suggest that magnesium-iron silicates are possible wind-drivers. However, the optical properties of these silicate grains are strongly influenced by the Fe-content. Fe-bearing condensates heat up strongly when interacting with the radiation field and therefore cannot form close enough to the star to trigger outflows. Fe-free condensates, on the other hand, have a low absorption cross-section at near-IR wavelengths where AGB stars emit most of their flux.  To solve this conundrum, it has been suggested that winds of M-type AGB stars may be driven by photon scattering on Fe-free silicate grains with sizes comparable to the wavelength of the flux maximum, rather than by true absorption. In this thesis we investigate dynamical models of M-type AGB stars, using Fe-free silicates as the wind-driving dust species. According to our findings these models produce both dynamic and photometric properties consistent with observations. Especially noteworthy are the large photometric variations in the visual band during a pulsation cycle, seen both in the observed and synthetic fluxes. A closer examination of the models reveals that these variations are caused by changes in the molecular layers, and not by changes in the dust. This is a strong indication that stellar winds of M-type AGB stars are driven by dust materials that are very transparent in the visual and near-infrared wavelength regions, otherwise these molecular effects would not be visible.

Dans cette thèse, j'analyse des observations spectropolarimétriques d'étoiles froides évoluées dans tous les paramètres de Stokes ($I$, $Q$, $U$ et $V$).L'étude des spectres polarisés circulairement de trois étoiles supergéantes rouges (RSG) m'a permis de détecter un faible champ magnétique (de l'ordre de 1 Gauss) à leur surface ainsi que de mettre en évidence l'influence de la forte polarisation linéaire de ces étoiles sur la mesure du champ magnétique.Cette forte polarisation linéaire est d'origine non magnétique et est reliée à la polarisation du continu dont l'observabilité résulte de la présence de structures brisants la symétrie cylindrique du disque stellaire.Après une analyse en profondeur de la polarisation du continu des étoiles RSG, j'utilise un modèle analytique spectropolarimétrique permettant de remonter à la position sur le disque de ces structures, en bon accord avec des observations interférométriques.Les étoiles variables pulsantes montrent elles aussi des signaux polarisés linéairement intenses dont l'origine est la polarisation intrinsèque (où de résonance) associée à certaines raies spectrales.Pour ces étoiles, les gradients de vitesses, introduits par les ondes de chocs se propageants dans leur atmosphère, amplifient la polarisation intrinsèque des raies, un mécanisme d'éclaircissement Doppler, bien connu dans le cas du Soleil, qui offre de nouvelles perspectives dans l'étude de ces objets. / In this thesis work, full Stokes ($I$, $Q$, $U$ et $V$) spectropolarimetric observations of cool evolved stars have been analysed.I have measured a weak magnetic field at the surface of three well known red supergiant stars. Furthermore, I have shown that the strong level of linear polarisation, measured for these stars, can lead to some ambiguity in the measurement of magnetic fields.This strong, unmagnetic, linear polarisation originate from depolarisation of the continuum which, in turn, is only detectable because of symmetry-breaking effects on the stellar disk.I study in some details the continuum polarisation of RSG stars and I present a way of mapping symmetry-breaking strucutres through the mean of a spectropolarimetric model. Good agreement with interferometric observations is reached.From their side, pulsating variable stars also show strong linearly polarised features in their spectra. These features are indeed due to resonant scattering polarisation associated to metallic lines, a phenomenon reminiscent of the second solar spectrum.For these stars, the velocity gradients, pertaining to the shock waves propagating through their atmosphere, enhance the intrinsic polarisation of the lines. This Doppler brightening effect, well known in the solar case, offer a new innovative method for the analysis of these stars.

Stellar chemical abundances provide astronomers with vital information about the production of chemical elements. Some stars preserve the composition of the environment in which they were born on their surfaces. By analysing the light from a star, the abundances of elements, its age and its path in space can be derived, and translated into the language of galactic history. The spallative history of boron in the early Galaxy was reinvestigated by observations of an ultraviolet spectral line in the old star HD 140283 with the Hubble Space Telescope. The line was barely detected and the upper limit abundance derived was lower than expected, which calls for further observations of this line in halo stars. Stars evolved into subgiants were observed with the ESO CAT, La Silla, and NOT, La Palma, to deduce their usefulness for galactic evolution studies. The high resolution spectroscopy study of the 26 objects showed that these stars are indeed useful for such studies. They are more luminous than dwarf stars and their ages can be accurately derived. They do not seem to have changed their surface abundances due to their evolution into giants. Subgiants can successfully be used to observationally reach regions further from the Earth, which can remove local biases that may appear when only observing nearby dwarf stars. A NLTE investigation of neutral Ca showed that cool metal rich dwarf stars did not deviate significantly from LTE, as had earlier been suggested. By an LTE analysis of a sample of 17 such dwarfs, using recent MARCS atmospheres, synthetic spectroscopy and modern atomic line data, cool metal rich dwarfs were shown not to deviate significantly from the expected abundance patterns in a number of elements. This increases the number of potential targets for studies of galactic chemical evolution in the metal rich regime since most stars are cool.

No description available.