Circumstellar Disks Surrounding Massive Stars

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Many different types of disk-like distributions of gas, stars, and planets can be found in the observable universe. Our own Sun is located in a spiral arm within a disk-like distribution of stars, gas and dust in our Milky Way Galaxy. The Earth formed in a disk of material left over from the formation of the Sun. Black holes have disks of material swirling about them, and there are many more examples of the ubiquity of such disks. Consequently, knowing how disks form and appreciating the processes operating in them are critical to understanding many objects in astrophysics. ******My research program focuses on massive stars, in particular B-emission stars (Be stars), that are surrounded by disks. A variety of technologies are employed to observe these remarkably bright objects which provide an ideal environment for studying disks. Be stars can spew out one thousand times more energy per second than our Sun and thousands exist right in our own Galaxy. Since the disk is formed from gas launched from the star, it tends not to be shrouded in dust like disks in star forming regions which makes Be star disks easier to observe and the chemical makeup simpler. The star's fast rotation causes materials inside the star to be stirred up, ultimately affecting the production of energy and its lifetime. When a Be star reaches the end of its life and goes supernova, it can trigger the emergence of a new generation of stars, directly affecting the evolution of its parent galaxy. ******Despite decades of study, the fundamental question to be solved in this field of study is why the disk forms. The rapid rotation of the star certainly helps to propel material off the surface into the disk, but something else must help to eject the material perhaps gravity from an orbiting star, a wave or pulsation on the stellar surface or a push from the stellar radiation. ******When the light from the Be star goes through the disk, it is scattered, absorbed and re-emitted and this causes changes in the light emitted. I build detailed computer models that predict signatures in the emitted light over a large range of wavelength (colour). The temperature distribution in the disk is highly complex which causes different areas of the disk to produce different wavelengths. The idea is that if we get stellar parameters and disk conditions such as the chemical composition, temperature and density correct, we will be able to match the signatures in the light over this wide range. Then we can build models that reliably follow the disk structures over time to better understand how disks work. ******There is a long history of discipline-specific study and expertise in Canada, in particular at Western University, of Be star disks. The results of my program will not only provide key results for understanding this type of disk but will help researchers worldwide apply the insight to better understand other disk varieties, placing constraints on the formation of massive stars and on the evolution of stars and galaxies.*****