My name is Jérôme Bétrisey. I did my studies in Switzerland, first at the ETH Zürich and then at the Geneva University, where I defended my PhD under the supervision of Prof. Georges Meynet and Dr. Gaël Buldgen. I am currently a postdoc in the Department of Physics and Astronomy at the Uppsala University in Sweden. I am specialising in seismic inferences of the internal stellar structure and I am presently mainly focusing on their applications to main-sequence solar-type stars.
My goal is to improve stellar characterisation in order to achieve a more precise and accurate stellar mass, radius and age, crucial for the preparation of the PLATO mission. I can then use these improved stellar parameters to do multidisciplinary applications in exoplanetology, where I can revise the planetary parameters, or in galactic archaeology. This goal is achieved by following two guidelines, improving the modelling strategies and improving the physics in stellar models. During the first years of my PhD, I followed these guidelines, therefore improving the current modelling strategies and developing new approaches to constrain the physics in stellar models. Hence, I refined my modelling strategy, that combines seismic inversions, and global and local minimisations, to achieve stellar parameters with an unprecedented robustness and accuracy. In particular, I demonstrated that my modelling strategy efficiently damps the so-called surface effects which are a recurrent issue in asteroseismology. I showed that my modelling strategy could easily meet the PLATO mission precision requirements for a benchmark target (Bétrisey et al. 2022), as well as for 10 Kepler targets with high-quality data (Bétrisey et al. 2023b), and is therefore compatible with a large-scale application (Bétrisey et al. 2024a). In addition, this benchmark target was also hosting an exoplanet of theoretical importance, for which I could revise the planetary parameters, achieving an unprecedented precision compared to the previous studies of this system. In parallel, I developed a new inversion based for the first time on frequency separation ratios instead of individual frequencies to constrain the properties of convective cores that is the main limiting factor of uncertainty on the determination of the stellar age (Bétrisey & Buldgen 2022). Moreover, I collaborated on a review for the prestigious journal Frontiers in Astronomy and Space Sciences about seismic inversions (Buldgen et al. 2022) to demonstrate the capabilities of such techniques and popularise their usage by the community. I also applied my modelling technique to constrain the transport of angular momentum in main-sequence F-type stars. I found that there exists a mass regime corresponding to stars around \( \sim 6000-6200\ \mathrm{K} \) for which it is difficult to constrain the AM transport processes, unlike for hotter, Gamma Dor stars or colder, less massive solar analogues, contrary to the expectations from the literature (Bétrisey et al. 2023a).
More recently, based on the analysis of 26.5 years of GOLF and BiSON observations, I demonstrated that magnetic activity has a non-negligible impact on the asteroseismic characterisation of the Sun-as-a-star, contrary to previous litrature expections (Bétrisey et al. 2024b). In particular, I found age variations up to 6.5% between solar minima and maxima, which is significant considering that the future PLATO mission requires 10% of precision for the age of a Sun-like star.