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Publication

Observational imprints of binary evolution on B- and Be star populations

Book - Dissertation

Massive stars are among the most important cosmic engines in the Universe. They have a profound impact not only on their immediate surroundings, but on their entire host galaxies. It is therefore crucial to understand their life cycle, from their birth until their deaths which are often accompanied by energetic supernova explosions. Observations have shown that a majority of stars are members of binary systems, complicating our understanding of massive star evolution. The presence of a companion can strongly alter the life cycle of both stars in the system, leading to a multitude of different evolutionary pathways. In order to better understand these possible binary pathways, a profound understanding of the complex interaction physics is crucial. One possible way of characterizing the physics and the outcome of binary interactions is to study post-interaction systems, so-called binary interaction products. In this thesis, we aim to investigate the observational imprint of binary interactions on populations of stars in two different manners: First, we investigate whether classical Be stars are binary interaction products. Second, we study the occurrence and properties of binary interaction products in a population of stars, namely the open star cluster NGC 330 in the Small Magellanic Cloud. Different mechanisms have been proposed to explain the rapid rotation of classical Be stars, which are non-radially pulsating, early-type stars that are surrounded by a circumstellar decretion disk. One possible mechanism is that they are mass gainers in previous binary interactions, implying that there should be a lack of Be stars with close main-sequence companion. Search the available literature of a large sample of massive classical Be stars, we find no unambiguous report of a massive Be star in a close binary system with a main-sequence companion. This indicates that a majority of the massive classical Be stars are binary interaction products. In this context, we study the peculiar system HR 6819 and argue that HR 6819 is a post-mass transfer system containing a stripped star and a classical Be star. In order to benefit from large sample statistics, we study the massive star population of the 35 Myr-old cluster NGC 330. Given the total mass and age of NGC 330, the cluster is expected to contain a large number of binary interaction products. Using multi-epoch integral field spectroscopy from VLT/MUSE we investigate the entire massive star content of NGC 330. We characterize the stellar content, classify the binary status, and estimate physical properties, in particular the rotation rates. By qualitative comparison with predictions from binary population synthesis computations, we find several indications that the population of stars is strongly impacted by previous binary interactions, such as the large number of classical Be stars, the presence of slowly-rotating stars above the cluster turnoff, and the binary fraction and rotation rates as a function of evolutionary status. Putting it all together, this thesis shows that the observational imprint of binary interactions on populations of early-type stars is strong. On the one hand, this is supported by the interpretation of classical Be stars as binary interaction products. On the other hand, it is in agreement with the large number of binary interaction products observed in NGC 330. Comparing the obtained results with predictions from state-of-the-art single- and binary-star population synthesis predictions will provide new invaluable insights in our current understanding of massive star evolution and binary interaction physics.
Publication year:2021
Accessibility:Open