Exploring S stars: stellar parameters, abundances and constraints on the s-process from a new grid of model atmospheres

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.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.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 alo
Doctorat en Sciences
info:eu-repo/semantics/nonPublished