Thèse de doctorat
Résumé : Man’s interest in stars and stardust roots not only to better understand the Universewe live in but also to understand the human evolution and its origin. Stars play animportant role as building blocks of the Universe. The study of stellar interiors andevolution has been an important field in modern astrophysics for decades.Of particular interest in this thesis are the Asymptotic Giant Branch (AGB) stars.AGB stars are low- to intermediate-mass stars in the late stages of stellar evolution.Due to their huge mass-loss and sheer number, these stars are major contributors ofheavy (s-process) elements in the interstellar medium. AGB stars are ideal testbedsfor understanding the mixing processes that take place in the stellar interiors. Despiteits importance, the AGB is one of the least understood phases of stellar evolution.Constraints on the AGB models from systematic observational studies is thereforemandatory. Such investigations will shed light not only on the future of our Sun, butalso on the heavy element production and thus on the chemical evolution of our Galaxy.Among evolved stars, S-type stars are especially interesting in this thesis, because theycome in two flavours: the intrinsic ones (genuine AGB stars) and the extrinsic ones(binaries which have been polluted in the past by an AGB, now an extinct white dwarf).The atmospheric parameter determination of these stars is complex, because theirspectra are dominated by molecules. Also, a large-scale, systematic mapping of derivedabundances in S stars which would be able to better constrain the AGB nucleosyntheticmodelling is still lacking.To contribute to bridge the gap between observations and models of AGB starswe initiated a detailed investigation of S stars. This study started with devising amethodology to constrain the atmospheric parameters of S stars. Our iterative methodmakes use of the recently released Gaia parallaxes and the high-resolution HERMESspectra. The derivation of the atmospheric parameters made it possible to expand ourstudy to the abundance analysis of S stars, to get insights into AGB nucleosynthesis.We located the intrinsic and extrinsic S stars in the Hertzsprung-Russell (HR) diagramto constrain their evolutionary status. We confirm that intrinsic S stars are indeed purethermally-pulsing AGB (TP-AGB) stars, undergoing third dredge-up events which areresponsible for transporting the nuclear burning products to the stellar surface. Onthe other hand, extrinsic S stars are less evolved counterparts, this can be establishednot only from their locations in the HR diagram but also from their heavy elementabundances. A important result of our HR diagram of S stars was the discovery oflow-mass AGB (initial mass∼1 M) stars. Such an evidence for third dredge-upoccurrence at low-mass and solar metallicity was not accounted for by most AGBmodels.The abundances of S stars reveal their rich nucleosynthetic history. The zirconium-niobium pair helps distinguishing extrinsic from intrinsic stars and is as such, adiagnostic as reliable as technetium detection. We also discovered a special class of’trinsic’ S stars, which are new TP-AGB stars that also show signatures of previousbinary interaction with a former AGB companion. The heavy element abundancestogether with the infrared excess are complient with the evolution of TP-AGB stars inthe HR diagram. We also compared the measured elemental abundances with thosefrom the nucleosynthesis predictions.The evolutionary and chemical study of the S stars shed light on several importantaspects of AGB stars like the third dredge-up and s-process nucleosynthesis. We tackledseveral challenges while determining the parameters and abundances of S stars. Ourmethodology and atomic list of ’gold’ lines can be used in further investigations. Webuild-up a systematic set of observational constraints on the onset of the third dredge-upand the associated nucleosynthesis. These observational constraints are essential cluesto better understand AGB nucleosynthesis.