The role of surface fossil magnetic fields in massive star evolution

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Keszthelyi, Zsolt
massive star , stellar evolution , fossil fields , surface magnetic fields , magnetic braking , mass-loss rate
Models describing the evolution of massive stars have advanced significantly due to understanding and including key physical phenomena in the calculations, such as mass loss and rotation. As stellar magnetometry has progressed very rapidly in the past decade, recent efforts have begun to focus on characterizing magnetic properties of massive stars and including their effects in evolutionary model calculations. By implementing established scaling relations of surface fossil magnetic fields, we account for magnetic braking, mass-loss quenching, and time evolution; using a simple, coherent prescription. In our study, we use two well-known hydrodynamical stellar evolution codes: the Modules for Experiments in Stellar Astrophysics (MESA) software instrument and the Geneva stellar evolution code (GENEC). We show that the incorporation of the effects of surface fossil magnetic fields impacts the model predictions, and, as a consequence, implies that the previous analysis of observed magnetic massive stars, which utilized non-magnetic evolutionary models for comparison, may need to be revised. We reinforce that magnetic braking and rotational mixing produce stars that can be identified as Group~2 (slowly-rotating, nitrogen-enriched) stars on the Hunter diagram, and we place constraints on their observable magnetic field strengths. We identify a unique feature of initially fast-rotating magnetic stars: following their initial blueward evolution, their rapid spin-down leads to apparently similar evolutionary tracks as if the star initiated its evolution as a slow rotator. This can, amongst others, affect the age determination of magnetic massive stars. Metallicity affects the stellar mass-loss rates, which are reduced if the star is in a low-metallicity environment. We show that surface fossil magnetic fields can also greatly reduce the mass-loss rate of the star. Therefore, even in high-metallicity environments, magnetic massive stars may follow an evolutionary path that resembles that of stars in a low-metallicity environment. As a consequence, astrophysical phenomena that are canonically attributed to massive stars in distant galaxies could potentially be produced by magnetic progenitors in the Milky Way. We conclude that the inclusion of new physical ingredients advances state-of-the-art model predictions, and therefore these models provide a basis for a platform to further investigate the nature of magnetic massive stars.
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