Jon Bjorkman
(Astrophyics: Stellar Winds)

Research Professor of Astronomy

Ph.D., 1992 - University of Wisconsin

Recent Hubble Space Telescope images (such as those of Eta Carinae, the Hour Glass Nebula, and the Egg Nebulae), show that circumstellar disks are quite common, and that they play a central role in shaping the stellar ejecta that form these nebulae. J. Bjorkman is trying to determine how and why such disks occur. Some stars are so luminous that the light they emit pushes their outer atmosphere away from the star, forming an outflowing "stellar wind".

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


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