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Session 18 - Stellar Winds and Rotation.
Oral session, Monday, June 10

[18.03] Theoretical and Observational Consequences of Rotation and Magnetic Fields in Stellar Winds

R. Ignace (U. of Wisconsin)

My dissertation concerns the study of stellar winds from theoretical modeling of the wind structure and the development of observational diagnostics. First, I have investigated the effects of stellar rotation for the wind structure of stars across the H-R Diagram. The effect of rotation is to increase the wind density at the equator while decreasing the density near the poles. The model, known as the Wind-Compressed Zone (WCZ) model, predicts that equatorial wind compressions are most likely to occur for stars with rapid rotation, low terminal speeds, and/or radial velocity distributions that increase gradually from the base of the wind. It is found that in favorable cases, stellar rotation can play a significant role in shaping the winds of Wolf Rayet stars, B supergiants, Asymptotic Giant Branch stars, and even some novae. The second major part of my thesis relates to the fact that the WCZ model will predict the magnetic field structure in the wind, if the field strength is relatively weak. However, there are generally no good diagnostics of stellar magnetic fields in the weak field limit, where Zeeman splitting is smaller than Doppler broadening. Thus, I have explored applications of the Hanle effect for probing magnetic fields in stellar winds. This effect (which has been used in studies of the solar atmosphere) deals with the modification of resonance line scattering polarization by a magnetic field. Solutions for the Hanle effect in optically thin axisymmetric extended stellar envelopes have been derived. Relative to the zero field case, the Hanle effect can result in significant changes of the line polarization, in some cases causing a position angle flip of 90^\circ. With multiline observations the Hanle effect is a viable diagnostic of stellar magnetic fields in the range 1-1000 Gauss.

This thesis work was completed under the supervision of Joseph Cassinelli and in collaboration with Kenneth Nordsieck and Jon Bjorkman.

Program listing for Monday