The Role of Circumstellar Disks in Massive Star Formation and Evolution

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Stars with masses greater than eight times the mass of our Sun play a critical role in the Milky Way Galaxy. Their high luminosities and winds are important sources of energy and matter input into the interstellar gas and dust between the stars. Supernova explosions, which occur at the end of the lives of these massive stars, inject even larger amounts of energy and matter into the interstellar medium, including newly synthesized heavy elements or "metals." This material is incorporated into new generations of stars and planetary systems, which become more metal rich over time. The remnants left behind after supernova explosions, neutron stars and black holes, give rise to some of the most exotic phenomena in the Universe, such as gamma-ray bursts and gravitational waves. Understanding the life cycles of these massive stars underlies much of our current understanding of the Universe, from the evolution of galaxies, to the formation of the first generation of stars. Many massive stars rotate very rapidly, in just a day or two, and the theoretical treatment of rapid rotation has been identified as a key uncertainty in our understanding of how these stars evolve and die. Rapid rotation can mix material and angular momentum within a star and, in some cases, it can lead to the ejection of gas from the star's atmosphere into a circumstellar disk. The current treatment of both of these processes is highly uncertain, and new observational constraints will play an important role is making the predictions of stellar evolution with rotation more accurate. The research proposed in this Discovery Grant will focus on rapidly rotating, massive stars that have ejected circumstellar disks, using them as "astrophysical laboratories" in which to better understand how rotation affects their lives. Spectroscopic observations will be analyzed for the signature of elements produced in stellar cores by fusion and transported to the surface by rotational mixing. Interferometric observations will be used to determine the masses of the circumstellar disks ejected by these stars. Spectroscopic analysis will be extended to massive stars in the Large and Small Magellanic Clouds, satellite galaxies of our Milky Way with stars that are metal poor. As larger ground-based and space-based telescopes extend observations to ever more distant, early, and metal-poor epochs in the Universe, it is critical to improve our understanding of stellar evolution in rapidly-rotating, metal-poor stellar systems in order to properly take advantage of the flood of new observational data. Students are an integral part of the proposed research program, and they will receive training in radiative transfer, high performance computation, data modelling, and scientific communication. These students will also have access to inter-disciplinary training through Western's collaborative graduate programs in Planetary Science and Scientific Computing.