When these stars also rotate rapidly, the outflow does not occur uniformly. Instead the outflow tends to move toward the star's equator where it forms a dense disk, much like the rings around Saturn. By modeling the physical forces acting on the stellar wind, we can calculate in detail how the wind flows away from the star, as well as how rotational forces naturally lead to the formation of a circumstellar disk. From these calculations we can make theoretical predictions of the disk structure that we can compare with observations.
Because the stars are so far away, we cannot see these disks
directly. Instead we must infer their presence by their effect on the
starlight passing through the disk. By modeling this radiation transfer
through the disk, we can infer the physical properties and chemical
composition of the disk. For example, these disks are gaseous and emit
light at certain wavelengths that indicate they are composed mostly of
hydrogen, with traces of other elements like iron. The strength of this
emission tells us how much material is in the disk, and the amount of
starlight intercepted by the disk tells us that they are quite thin -
like Saturn's rings. By comparing these observationally deduced
properties with his theoretical predictions, Bjorkman is attempting to
uncover which physical mechanisms are responsible for producing
circumstellar disks in stellar outflows.
Bjorkman, J. E. 1997, "Circumstellar Disks" in Stellar Atmospheres: Theory and Observations, European Astrophysical Doctoral Network IXth Predoctoral School, ed. J.-P. deGreve (Berlin: Springer), in press
Quirrenbach, A., Bjorkman, K. S., Bjorkman, J. E., Hummel, C. A.,Buscher, D. F., Armstrong, J. T., Mozurkewich, D., Elias, N. M., & Babler, B.L. 1997, "Constraints on the Geometry of Circumstellar Envelopes: Optical Interferometric and Spectropolarimetric Observations of Seven Be Stars",ApJ, 479, 477
Wood, K., Bjorkman, K. S., & Bjorkman, J. E. 1997, "Deriving the Geometry of Be Star Circumstellar Envelopes from Continuum Spectropolarimetry. I. The Case of Zeta Tauri", ApJ, 477, 926
Wood, K., Bjorkman, J. E., Whitney, B. A., & Code, A. D. 1996, "The Effect of Multiple Scattering on the Polarization from Axisymmteric Circumstellar Envelopes. II. Thomson Scattering in the Presence of Absorptive Opacity Sources", ApJ, 461, 847
Wood, K., Bjorkman, J. E., Whitney, B. A., & Code, A. D. 1996, "The Effect of Multiple Scattering on the Polarization from Axisymmteric Circumstellar Envelopes. I. Pure Thomson Scattering Envelope", ApJ, 461, 828
Ignace, R., Cassinelli, J. P., & Bjorkman, J. E. 1996, "Equatorial Wind Compression Effects across the H-R Diagram", ApJ, 459, 671
Bjorkman, J. E. 1995, "The Solution Topology of Radiation-driven Winds. I. The X-Type Nature of the CAK Critical Point", ApJ, 453, 369
Bjorkman, J. E., & Bjorkman, K. S. 1994, "The Effects of Gravity Darkening on the Ultraviolet Continuum Polarization Produced by Circumstellar Disks", ApJ, 436, 818
Bjorkman, J. E., Ignace, R., Tripp, T. M., & Cassinelli, J. P. 1994, "Evidence for a Disk in the Wind of HD 93521: UV Line Profiles from an Axisymmetric Model", ApJ, 435, 416
Bjorkman, J. E., & Cassinelli, J. P. 1993, "Equatorial Disk Formation Around Rotating Stars Due to Ram Pressure Confinement by the Stellar Wind "", ApJ, 409, 429
E-mail: jon@physics.utoledo.edu