THEORETICAL AND OBSERVATIONAL CONSEQUENCES OF STELLAR ROTATION AND MAGNETIC FIELDS FOR STELLAR WINDS

Abstract

ABSTRACT My dissertation concerns the study of stellar wind structure from (1) the theoretical modeling of stellar rotation and magnetic fields and (2) 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 using principles from the Wind Compressed Disk (WCD) model. Relative to a spherical wind, the effect of rotation is to increase the wind density at the equator while decreasing the density near the poles. An investigation of mild equatorial density enhancements has led to the development of the Wind Compressed Zone (WCZ) model. The WCZ model predicts that equatorial wind compressions are most likely to occur for stellar winds with low terminal speeds and/or radial velocity distributions that increase gradually from the base of the wind. In favorable cases stellar rotation can produce significant equatorial density enhancements in the winds of Wolf-Rayet stars, B~supergiants, Asymptotic Giant Branch stars, and even some novae at moderate stellar rotations of 20-30% break-up. A spectral line diagnostic based on multiline observations at Infrared wavelengths has been developed to measure the velocity law in the Wolf-Rayet winds, to determine whether they are likely susceptible to wind compression effects. The second major part of the thesis relates to the result that the WCZ model predicts the magnetic field structure in the wind, if the field strength is relatively weak. However, there are generally no good spectral diagnostics of sub-kiloGauss stellar magnetic fields. Magnetic fields at these low strengths are difficult to measure with the Zeeman effect, because the Zeeman line splitting is significantly smaller than the Doppler broadening. Thus, I have explored applications of the Hanle effect, that is sensitive to weaker magnetic fields than is the Zeeman effect, for probing the magnetic properties of stellar winds. The Hanle effect concerns the modification of resonance line polarization by the magnetic field. It has been used in studies of the Solar atmosphere, but not in other stars. 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 total line polarization. Consequences of the Hanle effect for the polarization of line profiles is also investigated, and analytic results are presented for a few special cases. A magnetic field can cause strong variations of the polarized line profile shape as compared to the case without a magnetic field. With multiline observations the Hanle effect will be a valuable diagnostic of stellar magnetic fields in the range 1-1000 Gauss.