Investigando a rotação, o magnetismo e as abundâncias químicas de estrelas do tipo solar através de modelos de evolução e espectroscopia: da pré-sequência principal à gigante vermelha

Abstract

The study of the rotational evolution of the Sun and solar-type stars is undoubtedly one of the fields of stellar astrophysics in major evidence today. There are numerous issues still far from being fully understood, which makes research in this field intense and dynamic. Among these issues is the understanding of the phenomena that influence the angular momentum of stars during their formation, still in the Pre-Main Sequence phase; the understanding of the mechanisms that act in the stellar interiors throughout evolution, which end up determining, for example, the rotation profile of a solid body observed on the Sun; or which transport mechanisms act in evolved stars, which undergo profound changes in their structure, and manifest peculiar phenomena such as lithium enrichment and alterations in many chemical abundances. In all cases, we know that stellar magnetism plays a prominent role, although it is still far from being fully understood. In this thesis, we use models computed with the Toulouse-Geneva Stellar Evolution Code (TGEC) to study the rotational evolution of four samples of solar-type stars, in addition to revisiting studies with analogous stars from open clusters. In our analysis, we used two different magnetic brake prescriptions, which simulate the loss of angular momentum due to stellar winds. Additionally, we also studied a sample of giant solar-type stars that are lithium-rich, investigating the possible relationship of the enrichment with the magnetic character of some of these stars. As far as main sequence stars are concerned, we found some convergences and discrepancies between our models and the stellar samples studied. We found that the sample with seismological data, composed of stars of intermediate and older ages, is not well constrained for a study on the rotation and magnetic evolution of the Sun. The sample of lowactivity stars appears to be affected by a decrease in magnetic braking despite differences in metallicity, with targets with higher metallicity following closer our evolutionary tracks. Finally, we found a mismatch between our rotation evolution tracks and the position of the youngest stars. As for lithium-rich giant stars, we found that stars previously classified as red giant branch (RGB) may be in a different evolutionary state. Furthermore, we found that most stars in our sample with surface magnetic field detection show at least moderate rotation speeds. However, we were unable to detect a magnetic field in two rapidly rotating stars. Because of our small sample of magnetic giants, it is difficult to establish the linkage between the presence of a surface magnetic field and the Li-enrichment phenomena in giant stars. At last, we investigated questions regarding the radial velocity modulation for some stars of this sample, which could be indicative of them being binary or multiple systems.