Research explores the turbulent lives of massive stars
Our understanding of the evolution of massive stars, spanning from their formation to their spectacular demise in supernova explosions, remains incomplete. The process is shaped by several key factors, which makes accurately predicting their life cycles scientifically challenging. Among the critical parameters that determine how a massive star evolves are its initial mass, rotation speed, chemical composition and internal mixing processes. However, mass loss through stellar winds and mass eruptions is one of the most decisive influences. All these factors are not static; they vary drastically as the star progresses through its life stages. Understanding these changes is essential for making accurate predictions about a supergiant star’s ultimate fate – whether it will collapse into a black hole or form a neutron star.
Complex states and interactions of massive stars
With the support of the Marie Sklodowska-Curie Actions (MSCA) programme, the POEMS project explored the winds and mass losses of massive stars, particularly during extreme evolutionary states. These states are often linked to luminous supergiants and hypergiants – massive stars undergoing brief transition stages. During these phases, stars exhibit violent mass ejection events over short periods. However, the exact mechanisms driving these eruptions remained elusive. “Our primary goal was to better understand these sudden mass ejection phases and quantify how much material is expelled,” notes Michaela Kraus, project coordinator. Researchers also analysed the chemical composition and physical structure of the material ejected during ejection phases. “This helps grasp how this ejected matter interacts with the surrounding interstellar environment as it significantly influences the dynamical and chemical evolution of the galaxies that host these stars,” explains Kraus. Another focus of POEMS was the role of stellar multiplicity, and specifically, how interactions within binary star systems or star mergers contribute to these extreme phases in massive stars.
New models of stellar winds, structures and instabilities
“A major development was the creation of new hydrodynamic models to simulate stellar winds and the formation of circumstellar structures such as arcs, rings, shells and discs. Using these models, new mass-loss prescriptions were found,” states Kraus. Another important contribution was the development of innovative radiative transfer codes to analyse the emissions from elements in circumstellar environments, including atomic gas, molecular gas and dust. These tools offered the flexibility to model emissions from various structures such as spherical shells and circumstellar discs. “We also explored pulsation instabilities, which were found to trigger sudden mass eruption events. Findings from this research allowed us to correlate observed characteristics of extreme massive stars, such as their quasi-periodic light variability and eruption behaviour, to theoretical predictions,” outlines Kraus.
Valuable insights from observational data
POEMS collected extensive data from renowned ground-based observatories like ESO and GEMINI, and space telescopes like TESS and GAIA. This observational data uncovered valuable details about massive stars – their winds, pulsation behaviour and surrounding environments on small and large scales. “Through this research, we discovered numerous previously unknown ejecta and nebulae, as well as many mass loss events preceding supernova explosions. These discoveries have prompted us to reevaluate the late evolutionary stages of massive stars,” notes Kraus. “We now better understand the instabilities affecting massive supergiants and hypergiants, which allows us to elucidate why they erupt so violently and the forces shaping their ejected material. Our work also examined the physical conditions in the environments surrounding many evolved massive stars within galaxies of the Local Group, a collection of over 80 galaxies, including the Milky Way,” concludes Kraus.
Keywords
POEMS, massive stars, mass loss, stellar wind, mass eruption, mass ejection