Investigating stars with asteroseismology
The main limitation to testing the theory of stellar evolution is that classical astronomical observations only give access to properties of the stars' surfaces, such as luminosity and chemical composition. This is clearly insufficient in order to describe the stars' interiors, where complex processes take place. The only way to improve our knowledge of stellar evolution is to 'see' inside stars and learn about their internal structure. For this purpose, astronomers working on the 'Stellar astrophysics, helioseismology, asteroseismology and nucleosynthesis' (SAHA-NUC) project exploited measurements by recent photometric missions. The Kepler and particularly the Convection, rotation and planetary transits (CoROT) mission have capitalised on the conditions in space to measure small-amplitude oscillations of stars. Astronomers could probe stars' interiors as different oscillation modes penetrate to different depths. The Sun was the study's starting point because it allows the 'calibration' of poorly understood physics that is applied to the modelling of distant stars. Determined based on helioseismic data of high accuracy, the solar core composition was compared with the solar surface composition to gain insights into the evolution of the early solar system. This advanced our knowledge of the Sun and improved the solar model, and by extension the stellar models. SAHA-NUC scientists reviewed the standard solar model, which includes all the physics considered standard in stellar models. In addition, the natural extension of helioseismology involved applying the same techniques to stars that present oscillations with properties similar to those observed on the Sun. Team members observed pulsations in star members of stellar clusters, and estimated key parameters like mass and age independently of asteroseismic measurements. The ultimate aim was to use these types of measurements to construct a more detailed picture of stellar interiors and the physical processes taking place therein. Among the results are constraints set for the first time on the mass loss of red giant stars in open stellar clusters. Overall, the SAHA-NUC project removed significant limitations to allow discriminating between different scenarios of the formation and evolution of our galaxy's components.