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Physics of Extreme Massive Stars

Periodic Reporting for period 1 - POEMS (Physics of Extreme Massive Stars)

Reporting period: 2019-01-01 to 2022-05-31

Massive stars are powerful cosmic engines. Throughout their life-time they enrich their environment with huge amounts of energy and with chemically processed material. The energy released is so powerful that it can trigger the formation of new generations of stars and planets, and the chemical elements produced inside these stars and released to their environment via winds and eruptions are the building blocks of all life as we know it on Earth.

Despite the importance of massive stars for cosmic evolution, their aging process from the cradle up to their death in spectacular supernova explosions is most uncertain. This is due to the lack of precise knowledge of the physical mechanisms behind mass eruptions that occasionally occur during various late phases in the evolution of massive stars, and due to the high uncertainties in reliable values of the total amount of mass the star loses during each epoch of its life.

This project aims to remove these uncertainties so that reliable predictions for the entire evolution and fate of massive stars can be made. The acquired knowledge will boost our understanding of physical and chemical processes taking place in the universe and their mutual interactions, which have relevance not only on galactic and intergalactic scales, but also on Earth and in every-day life. The results of our research will make the society more aware of our current position and our transiency in the universe. The new methodologies of calculation, image processing, data management, numerical methods, etc., created by our team will be used according to the needs of business, industry and for teaching at different levels of education. The newly acquired knowledge has the potential to drive the development of new devices to carry out future research projects and of new technologies of which society as a whole will benefit.
We have established a multidisciplinary, international network of researchers from Europe, Asia, and South America with expertise and background in both theory and observations and in a variety of disciplines such as astronomy and astrophysics, fluid dynamics, nonlinear physics, molecular physics, astrochemistry, statistics and applied mathematics.

To achieve our goals our research focuses on diverse aspects, which are investigated in parallel over the course of the entire project. The final results are then combined to form the overall picture of the puzzle. The individual research pieces are devoted to (i) developing reliable physical descriptions of how the stellar wind emerges, and what kind of structures (disks, nebulae) it may form, (ii) investigating the influence of pulsations on stellar mass loss and mass eruptions, (iii) studying how the mass lost in each stage of life can alter the star's evolutionary path, (iv) examining the chemical process within the ejected matter, (v) elaborating how the wind material interacts with the interstellar medium, and (vi) exploring the role of companions in altering the evolution of the stars and in shaping the material of the wind and the ejecta.

The work carried out has produced a number of valuable results:

• We have gained new insights into the structure of stellar winds and mass loss from massive stars. The newly developed recipes for the wind descriptions have major impact on the evolution of massive stars.

• We have performed first numerical simulations for the formation of circumstellar disks, implementing the improved mass-loss prescriptions (Figure 1).

• We have achieved much better understanding of the instabilities at work in evolved massive supergiants and hypergiants.

• We have discovered numerous new ejecta and nebulae (Figure 2) and recorded multiple mass-loss events in yellow hypergiants, requiring a rethinking of the late evolutionary stages of massive stars.

• We have detected hot water vapor from the environment of a yellow hypergiant, making this object unique in its class and requiring new approaches to interpretation.

• We have revealed that classical Be stars have the potential to create environments in which carbon monoxide molecules can form. This discovery provides clues of the emergence of the B[e] phenomenon.

• We have found a high binary fraction for the class of luminous blue variable stars, posing new constraints to binary evolution in massive stars.

• We have completed the first database of synthetic spectra and provided it to the community for exploitation.

Our results have been published in 20 articles (+2 in press) in world renowned journals, 12 conference proceedings (+6 in press), 1 PhD and 3 Master theses. We also published the book "Pulsations along stellar evolution" consisting of 12 chapters, 9 of which have been written by POEMS team members.
For the the next period we envision organizing a summer school on "Stellar winds and ejecta" to educate students and the next generation of young scientists on the state-of-the-art and our newly acquired knowledge in this research field. We also plan to organize an international workshop to disseminate our results to the community and attract potential new colleagues to the study of extreme massive stars. We expect the completion of several ongoing doctoral theses presenting valuable research results. Moreover, we are going to act as Guest Editors in Special Issues of magazines with topics related to our project.

The research carried out by our team is at the forefront of science in several research areas, such as stellar wind modeling, modeling of oscillations in extreme massive stars, and observing and modeling the environment of evolved massive stars in particular with respect to B[e] stars and supergiants. Our team develops cutting-edge models and software and is extremely efficient in getting assigned observing time with most modern observing facilities. A particularly important role play hereby the multiple, unique facilities at Gemini Observatory which our team use intensively. Access to these facilities is granted solely to our South-American partners, and especially our Argentinian colleagues are at the forefront in obtaining observing time with them. This makes the collaboration between Europe and South-America extremely beneficial and fruitful and allows us to collect the best possible data (and often the first of their kind) that are shared among POEMS members, placing our team ahead. With the final results from POEMS we expect to maintain and consolidate this leading position in massive star research.

POEMS helps to enhance the Master and PhD theses of the students and to promote the obtaining of their degrees. Students and young researchers further benefit from the exchange visits within the large international collaboration that allows them to set-up and tighten their scientific network, and the acquired knowledge and training they receive helps them for being promoted to new academic positions. Also, young female participants from Azerbaijan have received high recognition in their home country for their participation in POEMS. We expect that these trends, including the promotion of young females in science, will continue.

Moreover, POEMS members have achieved recognition by policy makers for their engagement in promoting and consolidating cooperation in science, technology and innovation, for their efforts in training human resources, and for the huge impact of their scientific work and results at the international level.
Supplementary Figure 1
Supplementary Figure 2
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