On June 3, at 14:00 Anni Kasikov will defend her doctoral thesis: “Evolution and variability of yellow hypergiant stars” for obtaining the degree of Doctor of Philosophy in Physics.
Supervisors
Associate Professor Indrek Kolka, University of Tartu
Associate Professor Anna Aret, University of Tartu
Opponent
Docent Erkki Kankare, University of Turku (Finland)
Summary
Massive stars are rare, unstable, and short-lived. Their stellar winds, eruptions, and, eventually, supernova explosions strongly affect their surrounding environments and the evolution of their host galaxy as a whole. The life of a massive star includes various evolutionary stages, some of which happen very quickly on cosmic timescales – in tens of thousands of years. Stars in these short transition stages are very rare, but understanding them is critical for constraining stellar evolutionary models.
This PhD thesis focuses on yellow hypergiant stars, which are in a highly unstable evolutionary stage. They experience significant variability, pulsations, and recurring mass-loss eruptions. We conducted spectroscopic and photometric studies of two hypergiants in the Milky Way: V509 Cas and RW Cep. The two stars are in different steps of
hypergiant evolution.
Long-term monitoring of the variability of V509 Cas revealed that the star has reached an unprecedented period of atmospheric stability. Additionally, there is a Keplerian disc-like structure surrounding the star. RW Cep, on the other hand, is only starting its journey as a yellow hypergiant and recently underwent a mass-loss outburst. Our analysis of atmospheric motions during the post‑outburst recovery shows its large-scale impact in different atmospheric layers. Additionally, spectral emission features reveal a shell of ejected matter surrounding RW Cep.
To place these individual cases in context, we conducted a study of the population of luminous yellow stars in the Milky Way. We studied their stellar environments – stellar clusters or associations – which allowed us to improve their distance and luminosity measurements. This work is a first step towards statistical population-level surveys.
