Evolution and nucleosynthesis in 1 to 12 Msun stars -

Abstract

Physical aspects of the evolution and nucleosynthesis in stars in the mass range (1-12)Mʘ, where Mʘ denotes the solar mass, of solar-like initial composition are thoroughly investigated in this work. Stars in this mass range are very numerous in the Galaxy. They evolve after reaching the red giant branch (RGB) to the peculiar phase of asymptotic giant branch (AGB), where they exhibit thermal pulsations. These pulsations facilitate the physical conditions for the formation of about half of the trans-iron elements and their isotopes in the Galaxy via the s-process nucleosynthesis. Light elements like carbon and fluorine are also partially produced in this stellar site. Stars in the above mass range are evolved using a well-tested hydrodynamic stellar evolution code with up-to-date physics input. The investigations were performed along different directions as follows. (a) On the RGB phase, stars experience the first envelope mixing event that changes the surface abundances. Basic stellar parameters and surface abundances are compared with observations. This approach has improved the estimation of the stellar masses of an observed sample of red giants and helped constrain the amount of convective mixing in low mass stars. (b) Stars of M ≥ 4Mʘ exhibit excursion in the Hertzsprung-Russell diagram to higher effective temperatures in what is called the blue loop , which characterizes the evolution through the Cepheid instability strip. The impact of key nuclear reactions and the treatment of mixing on the characteristics of blue loops is investigated. (c) Modeling AGB stars is challenged by the complex behavior of convective mixing during thermal pulsations. This work aims to understand the physical conditions for the operation of s-process nucleosynthesis, which requires the activation of the main neutron source via ¹³C(α,n)¹⁶O that feeds the s-process calculation. The role of convective mixing in the formation of this neutron source is investigated and an extended