Study of the inner region of the envelope of cool evolved stars

Intermediate-mass stars, like the Sun, will evolve into AGB (Asymptotic Giant Branch) stars, which have a much larger radius than the Sun and are a thousand times brighter than it. These evolved stars play a major role in stellar evolution. Evolved cool stars have a high mass loss rate, which contributes to the enrichment of the interstellar medium and so, to the recycling of matter in the Universe, through strong winds propagating in the star’s circumstellar envelope (CSE). The physics of these winds and the origin of these mass-loss phenomena are still poorly understood, but can be constrained by observations of various molecular transitions in the CSE. The material ejected in the interstellar medium depend heavily on whether the elemental C/O ratio at the stellar surface is below or above unity. Typically, molecules in circumstellar envelopes are formed at the stellar atmosphere under chemical equilibrium or in the outer expanding layers due to the action of photochemistry. The presence of most of the molecules detected is relatively well understood. However, there are a few molecules for which their origin is not clear at all. This is observed for example in O-rich stars, where the abundances of HCN, CS, and NH3 are more important than expected. We want to compare the abundance of a sample of molecules in C and O-rich stars, to reveal the differences between the equilibrium chemistry and the observations. Such study allows us to add more constraints on the models to reproduce those unexpected values. Moreover, it has recently been observed that most evolved stars have a close companion that impacts on the gas dynamics around the star and on the winds dynamics. This study focuses on the very inner region of the CSE where the molecules are formed but also where the magnetic field is detected. Its role is not fully understood but can be constrained thanks to observations of various molecular transitions in the CSE. I observed the SiO maser line emission from an AGB star, U Herculis, to detect the magnetic field in the inner gas region of the envelope and to understand it origin, and its impact in this region.