Stars on the asymptotic giant branch (AGB) lose a significant fraction of their mass to their surroundings through stellar winds. As a result, they are surrounded by circumstellar shells of gas and dust. This stellar mass loss replenishes and enriches the interstellar medium (ISM) with the products of stellar nucleosynthesis, progressively increasing its metallicity and thereby driving galactic chemical evolution. In this thesis I present a comprehensive study of oxygen-rich (O-rich) AGB stars and red supergiants (RSG) observed with the Spitzer Infrared Spectrograph and Infrared Space Observatory Short Wavelength Spectrometer in the Milky Way, the Large and Small Magellanic Clouds, and Galactic globular clusters; focusing on the composition of the dust in the circumstellar envelopes surrounding these stars. Combining spectroscopic and photometric observations with the GRAMS grid of radiative transfer models to derive (dust) mass-loss rates, I detect crystalline silicates in stars with dust mass-loss rates which span over a factor of 1000, down to rates of ~10^{-9} Msun/yr. Detections of crystalline silicates are more prevalent in higher mass-loss rate objects, and our results indicate that the dust mass-loss rate has a greater influence on the crystalline fraction than the gas mass-loss rate, suggesting that thermal annealing of amorphous silicate grains is the primary formation mechanism of crystalline silicates in such environments rather than the direct condensation of crystalline silicates from the gas phase. I also find that metallicity influences the composition of crystalline silicates, with enstatite seen increasingly at low metallicity, while forsterite becomes depleted at these metallicities due to the different chemical composition of the gas. To trace the evolution of alumina and silicate dust along the AGB, I present an alternative grid of MODUST radiative transfer-models for a range of dust compositions, mass-loss rates, dust shell inner radii and stellar parameters. Our analysis shows that the AKARI [11]-[15] versus [3.2]-[7] colour is a robust indicator of the fractional abundance of alumina in O-rich AGB stars. From the modelling, I show that a grain mixture consisting primarily of amorphous silicates, with contributions from amorphous alumina and metallic iron provides a good fit to the observed spectra of O-rich AGB stars in the LMC. In agreement with previous studies, we find a correlation between the dust composition and mass-loss rate; the lower the mass-loss rate the higher the percentage of alumina in the shell. Finally, I present mid-infrared observations of the Local Group dwarf elliptical galaxy M32; where I find a large population of dust-enshrouded stars. These observations will act as a pathfinder for future observations with the JWST and SPICA.

Evolved stars and planetary nebulae are rich and varied sites of molecule and dust formation. These objects undergo dramatic mass loss which ultimately enriches the interstellar medium. In this thesis, a number of studies, outlined below, have been undertaken to better understand the chemical and physical properties of these diverse objects. A molecular line survey of a sample of evolved stars and planetary nebulae has been carried out using the Mopra radio telescope, Australia. Transitions with hyperfine structure have been fitted to constrain optical depths. The population diagram method was applied to determine the rotation temperatures of molecules which had multiple transitions available. Column densities have been calculated for all detected species and isotopic ratios measured where possible. The results include the corroboration of the classification of II Lup as a J-type star. The 89.087 GHz HCN maser was detected in IRAS 15082-4808 for the first time from the aforementioned survey, bringing the total number of detections of this maser to ten. The velocity shift of this maser has been measured at −2.0+/-0.9 km/s. Drawing on literature data in addition to the survey data, the variation of maser intensity with pulsation phase has been investigated across all sources for the first time. Comparing these masers with model atmospheres constrains the formation region to between 2 and 4 stellar radii. CO in the circumstellar envelope of II Lup has been modelled using the radiative transfer codes GASTRoNOoM, and ComboCode. The models have demonstrated that a ‘standard’ smooth model does not satisfactorily reproduce the combined CO observations of PACS, JCMT, Mopra and APEX. Two potential solutions are proposed: a discontinuous temperature model, requiring the presence of an efficient cooling molecule that is most effective in the region 75-200 R*, or a variable mass loss model that requires a factor of ten increase inmass loss in the same region. Zinc abundances, a proxy for iron abundances, have been determined for a sample of Galactic planetary nebulae using the [Zn IV] 3.625 μm line. O++/O has been shown to be a reliable ionisation correction factor for Zn3+ from Cloudy photoionisation models. The majority of the sample are sub-solar in [Zn/H] and enriched in [O/Zn]. Zinc abundances as functions of Galactocentric distance have also been investigated and no evidence for a trend has been found.

Les mécanismes physiques de la perte de masse des étoiles évoluées sont encore largement inconnus. Ce processus essentiel est pourtant le moteur principal de l'évolution chimique du milieu interstellaire. Pour les supergéantes rouges (SGR), le déclenchement de l'éjection de la matière et les mécanismes de condensation de la poussière demeurent mal compris. La façon dont les géantes rouges parviennent à former des nébuleuses planétaires non-Sphériques est aussi inconnue. Au cours de ma thèse j'ai étudié des étoiles évoluées grâce à des techniques de haute résolution angulaire permettant de détailler leur surface et leur environnement proche où se trouve l'origine de la perte de masse. À partir d'observations interférométriques en infrarouge (IR), j'ai caractérisé l'enveloppe de vapeur d'eau et de monoxyde de carbone de la SGR Bételgeuse. J'ai également suivi l'évolution d'un point chaud à sa surface et analysé la structure de sa convection ainsi que celle d'Antarès (une autre SGR très proche) grâce à des simulations hydrodynamiques radiatives. L'imagerie à la limite de diffraction (optique adaptative en IR, télescope spatial en ultraviolet) m'a permis d'étudier l'évolution des inhomogénéités de l'enveloppe circumstellaire de Bételgeuse et découvrir un disque circumstellaire autour de L2 Puppis, une étoile de la branche asymptotique des géantes. Ces observations multi-Longueurs d'onde, répétées à plusieurs époques, m'ont permis d'initier un suivi temporel et d'apporter des informations sur la dynamique en jeu. Renouvelé sur un plus grand échantillon d'étoiles dans les années à venir, ce programme permettra de mieux appréhender la perte de masse des étoiles évoluées. / Mass loss of evolved stars is still largely mysterious, despite its importance as the main evolution engine for the chemical composition of the interstellar medium. For red supergiants (RSG), the triggering of the outflow and the mechanism of dust condensation remain unknown. Concerning red giant stars, we still do not know how their mass loss is able to form a bipolar planetary nebula. During my PhD thesis, I observed evolved stars with high angular resolution techniques. They allowed us to study the surface and the close environment of these stars, from where mass loss originates. With near-Infrared interferometric observations, I characterized the water vapor and carbon monoxide envelope of the nearby RSG Betelgeuse. I also monitored a hot spot on its surface and analyzed the structure of its convection, as well as that of Antares (another very nearby supergiant) thanks to radiative hydrodynamical simulations. Diffraction-Limited imaging techniques (near-Infrared adaptive optics, ultraviolet space telescope) allowed me to observe the evolution of inhomogeneities in the circumstellar envelope of Betelgeuse and to discover a circumstellar disk around L2 Puppis, an asymptotic giant branch star. These multi-Scale and multi-Wavelength observations obtained at several epochs allowed us to monitor the evolution of the structures and to derive information on the dynamics of the stellar environment. With a wider stellar sample expected in the next few years, this observing program will allow a better understanding of the mass loss of evolved stars.

More than 80% of the stars in the Universe are expected to have initial masses below eight to ten times the mass of our sun. These low mass stars, including our sun, become cool red giants during one of the final evolutionary stages of their life: the Asymptotic Giant Branch (or AGB) phase. AGB stars are among the main producers of carbon and heavy (s-process) elements in the Universe. These elements are synthesized inside the star and mixed to the stellar atmosphere where stellar winds are responsible for the loss of more than 50% of the stellar mass, hence, AGB stars are strong polluters of the interstellar medium. The ejected material can clump together into dusty particles which may serve as ingredients for the birth of new stars and planets. When most of the AGB stellar envelope is lost, the AGB star stops releasing nuclear energy from interior processes and swaps its giant face for a planetary nebulae look, whereafter it fades away as a white dwarf.<p><p>The dredge-up of carbon and s-process elements into the AGB atmosphere causes an important chemical anomaly among them: initial oxygen-rich stars (M stars) are transformed into carbon-rich stars (C stars). As a consequence, a group of oxygen-rich AGB stars exists which makes the transition between M and C stars. These transition stars are classified as S.<p><p>Although AGB stars are identified as producers of heavy elements, their nucleosynthesis and mixing processes are weakly constrained due to large uncertainties on their estimated temperature, gravity and chemical composition. Stronger constraints on the atmospheric parameter space, hence interior processes, of AGB stars can be obtained by investigating the atmosphere of S stars. Since they are transition objects on the AGB, they trace the rise of the s-process. S stars are less numerous than C stars, but their optical spectra are brighter making it easier to identify atomic and molecular lines. Therefore, S stars belong to the most interesting objects along the AGB to perform this task.<p><p><p><p>From a practical point of view, the spectra of S stars are extremely difficult to study since they are dominated by different, overlapping molecular bands, and the spectral shape may vary strongly from star to star due to their transition status. Therefore, tailored model atmospheres for S stars are of utmost importance to understand the spectroscopic, and even photometric, changes in terms of variations in the atmospheric parameters. A comparison between the models and observations aims not only at constraining the atmospheric parameter space of S stars, it will also test the reliability of 1D state-of-the-art model atmospheres for such complex stars.<p><p><p><p>From an evolutionary point of view, the S-star family is contaminated with stars who gained their atmospheric enrichment in heavy elements from a companion star. Evidences were found that these binary S stars are not at all located on the AGB, hence, they are labelled as extrinsic S stars while S stars on the AGB are labelled as intrinsic. The difference in evolutionary stages between intrinsic and extrinsic S stars was already found 20 years ago, however, a separation in terms of surface temperature, gravity and chemical composition is not well-established due to the lack of S-star model atmospheres. Such a distinction in atmospheric parameters will facilitate the discovery of these intruders and even help to calibrate stellar evolutionary models of single and binary stars.<p>To achieve these goals, the first step consists in the construction of a grid of model atmospheres for S stars. The grid will be used to quantify the influence of atmospheric parameters on the model structure and emergent flux. These results will be analyzed to derive precise atmospheric parameters of observed S stars, using a set of well-defined photometric and spectroscopic indices. Once the best model atmosphere has been selected for all observed S stars, their atmospheric parameters will be discussed in view of their evolutionary stage. The best-fitting model atmosphere will also be used to derive abundances from spectral syntheses. The abundance profiles are compared with stellar evolution model prediction to constrain nucleosynthesis and mixing processes inside S stars. Derived abundances of unstable elements will be used to estimate, for the first time, the age of AGB stars. Finally, their abundance profile will be discussed as a function of their time spent on the AGB. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished