Massive hot stars lose material through a stellar wind, which is driven by radiation pressure from the starlight. The material lost in this way creates an envelope of matter around the star. If these stars are also rotating rapidly, this envelope may develop into a disk of material around the equator of the star. Evidence of such circumstellar disks is seen in many types of stars, from the earliest young stars to older planetary nebulae, and the presence of disk-like geometries seems to be ubiquitous in the stellar zoo. Studying such systems can help to shed light on the ways that stars are formed, how they evolve and lose mass throughout their lives, their effect on the environment around them, and on the development of possible protoplanetary systems around other stars. The interaction between massive stars and the interstellar medium can also be explored by examining the nature of stellar winds and circumstellar envelopes.
Some of the questions we ask about winds, disks and circumstellar envelopes are these: What causes disks to form around massive stars? Are there interactions between stellar winds and circumstellar disks, and if so what are their consequences? What are the physical characteristics of disks? (are they thick or thin? hot or cool? high or low density?) How does the presence of disks affect the evolution of the star and the interstellar medium around it? Are disks that form around main-sequence stars partially responsible for the bipolar geometries observed in some planetary nebulae and evolved systems (such as luminous blue variables and asymptotic giant branch stars)? Are there similarities or connections between the disks observed in pre-main-sequence (young) stars, which may be the precursors to planetary systems, and those observed in main-sequence (older) stars? Can massive stars form protoplanetary disks similar to those suspected in lower mass, cooler stars like our sun? If so, are there differences in the types of planets that can form from massive star disks compared to those around cooler stars? What are the observational signatures of disks and bipolar geometries in circumstellar envelopes?
To try and answer these questions, I use several observational techniques (spectroscopy, spectropolarimetry, and imaging) at multiple wavelengths (optical, ultraviolet, and infrared). Combining these different types of observations can help to develop a more consistent picture of the nature of circumstellar envelopes and disks. These data are also used for comparison with physical models, which allows us to distinguish between different ideas of how disks form and what their physical characteristics are.
Much of my research is done in a collaborative environment, where I may work with many different astronomers (in some cases from all over the world), each contributing a particular piece to the puzzle solution. Particularly in multi-wavelength studies, or in space-based observations where instrument development and operations can take a number of years, working with teams of astronomers is a normal part of the process.
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Last updated 14 May 1997
If you have comments, questions, or suggestions regarding these pages, please send email. Thanks!
Karen S. Bjorkman (karen@astro.utoledo.edu)